Publications

A measurement of the top-quark mass (mt) in the tt¯ → lepton + jets channel is presented, with an experimental technique which exploits semileptonic decays of b-hadrons produced in the top-quark decay chain. The distribution of the invariant mass mℓμ of the lepton, ℓ (with ℓ = e, μ), from the W-boson decay and the muon, μ, originating from the b-hadron decay is reconstructed, and a binned-template profile likelihood fit is performed to extract mt. The measurement is based on data corresponding to an integrated luminosity of 36.1 fb−1 of √s = 13 TeV pp collisions provided by the Large Hadron Collider and recorded by the ATLAS detector. The measured value of the top-quark mass is mt = 174.41 ± 0.39 (stat.) ± 0.66 (syst.) ± 0.25 (recoil) GeV, where the third uncertainty arises from changing the Pythia8 parton shower gluon-recoil scheme, used in top-quark decays, to a recently developed setup. [Figure not available: see fulltext.].

This article presents a search for new resonances decaying into a Z or W boson and a 125 GeV Higgs boson h, and it targets the νν¯ bb¯ , ℓ+ℓ−bb¯ , or ℓ±νbb¯ final states, where ℓ = e or μ, in proton-proton collisions at √s = 13 TeV. The data used correspond to a total integrated luminosity of 139 fb−1 collected by the ATLAS detector during Run 2 of the LHC at CERN. The search is conducted by examining the reconstructed invariant or transverse mass distributions of Zh or Wh candidates for evidence of a localised excess in the mass range from 220 GeV to 5 TeV. No significant excess is observed and 95% confidence-level upper limits between 1.3 pb and 0.3 fb are placed on the production cross section times branching fraction of neutral and charged spin-1 resonances and CP-odd scalar bosons. These limits are converted into constraints on the parameter space of the Heavy Vector Triplet model and the two-Higgs-doublet model. [Figure not available: see fulltext.].

The electroweak production of Z(νν¯) γ in association with two jets is studied in a regime with a photon of high transverse momentum above 150 GeV using proton–proton collisions at a centre-of-mass energy of 13 TeV at the Large Hadron Collider. The analysis uses a data sample with an integrated luminosity of 139 fb−1 collected by the ATLAS detector during the 2015–2018 LHC data-taking period. This process is an important probe of the electroweak symmetry breaking mechanism in the Standard Model and is sensitive to quartic gauge boson couplings via vector-boson scattering. The fiducial Z(νν¯) γjj cross section for electroweak production is measured to be 0.77−0.30+0.34 fb and is consistent with the Standard Model prediction. Evidence of electroweak Z(νν¯) γjj production is found with an observed significance of 3.2σ for the background-only hypothesis, compared with an expected significance of 3.7σ. The combination of this result with the previously published ATLAS observation of electroweak Z(νν¯) γjj production yields an observed (expected) signal significance of 6.3σ (6.6σ). Limits on anomalous quartic gauge boson couplings are obtained in the framework of effective field theory with dimension-8 operators. 

Measurements of single-, double-, and triple-differential cross-sections are presented for boosted top-quark pair-production in 13 TeV proton–proton collisions recorded by the ATLAS detector at the LHC. The top quarks are observed through their hadronic decay and reconstructed as large-radius jets with the leading jet having transverse momentum (pT) greater than 500 GeV. The observed data are unfolded to remove detector effects. The particle-level cross-section, multiplied by the tt¯ → WWbb¯ branching fraction and measured in a fiducial phase space defined by requiring the leading and second-leading jets to have pT> 500 GeV and pT> 350 GeV, respectively, is 331 ± 3(stat.) ± 39(syst.) fb. This is approximately 20% lower than the prediction of 398−49+48 fb by Powheg+Pythia 8 with next-to-leading-order (NLO) accuracy but consistent within the theoretical uncertainties. Results are also presented at the parton level, where the effects of top-quark decay, parton showering, and hadronization are removed such that they can be compared with fixed-order next-to-next-to-leading-order (NNLO) calculations. The parton-level cross-section, measured in a fiducial phase space similar to that at particle level, is 1.94 ± 0.02(stat.) ± 0.25(syst.) pb. This agrees with the NNLO prediction of 1.96−0.17+0.02 pb. Reasonable agreement with the differential cross-sections is found for most NLO models, while the NNLO calculations are generally in better agreement with the data. The differential cross-sections are interpreted using a Standard Model effective field-theory formalism and limits are set on Wilson coefficients of several four-fermion operators. 

A search for flavour-changing neutral current (FCNC) tqH interactions involving a top quark, another up-type quark (q = u, c), and a Standard Model (SM) Higgs boson decaying into a τ-lepton pair (H → τ + τ −) is presented. The search is based on a dataset of pp collisions at √s = 13 TeV that corresponds to an integrated luminosity of 139 fb −1 recorded with the ATLAS detector at the Large Hadron Collider. Two processes are considered: single top quark FCNC production in association with a Higgs boson (pp → tH), and top quark pair production in which one of top quarks decays into Wb and the other decays into qH through the FCNC interactions. The search selects events with two hadronically decaying τ-lepton candidates (τ had) or at least one τ had with an additional lepton (e, μ), as well as multiple jets. Event kinematics is used to separate signal from the background through a multivariate discriminant. A slight excess of data is observed with a significance of 2.3σ above the expected SM background, and 95% CL upper limits on the t → qH branching ratios are derived. The observed (expected) 95% CL upper limits set on the t → cH and t → uH branching ratios are 9.4×10−4(4.8−1.4+2.2×10−4) and 6.9×10−4(3.5−1.0+1.5×10−4) , respectively. The corresponding combined observed (expected) upper limits on the dimension-6 operator Wilson coefficients in the effective tqH couplings are Ccφ < 1.35 (0.97) and Cuφ < 1.16 (0.82). [Figure not available: see fulltext.]

A low-energy electronic recoil calibration of XENON1T, a dual-phase xenon time projection chamber, with an internal 37 Ar source was performed. This calibration source features a 35-day half-life and provides two mono-energetic lines at 2.82 keV and 0.27 keV. The photon yield and electron yield at 2.82 keV are measured to be (32.3±0.3) photons/keV and (40.6±0.5) electrons/keV, respectively, in agreement with other measurements and with NEST predictions. The electron yield at 0.27 keV is also measured and it is (68.0-3.7+6.3) electrons/keV. The 37 Ar calibration confirms that the detector is well-understood in the energy region close to the detection threshold, with the 2.82 keV line reconstructed at (2.83±0.02) keV, which further validates the model used to interpret the low-energy electronic recoil excess previously reported by XENON1T. The ability to efficiently remove argon with cryogenic distillation after the calibration proves that 37 Ar can be considered as a regular calibration source for multi-tonne xenon detectors.

A search for the leptonic charge asymmetry (Acℓ) of top-quark-antiquark pair production in association with a W boson (tt¯ W) is presented. The search is performed using final states with exactly three charged light leptons (electrons or muons) and is based on √s = 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider at CERN during the years 2015–2018, corresponding to an integrated luminosity of 139 fb −1. A profile-likelihood fit to the event yields in multiple regions corresponding to positive and negative differences between the pseudorapidities of the charged leptons from top-quark and top-antiquark decays is used to extract the charge asymmetry. At reconstruction level, the asymmetry is found to be −0.12 ± 0.14 (stat.) ± 0.05 (syst.). An unfolding procedure is applied to convert the result at reconstruction level into a charge-asymmetry value in a fiducial volume at particle level with the result of −0.11 ± 0.17 (stat.) ± 0.05 (syst.). The Standard Model expectations for these two observables are calculated using Monte Carlo simulations with next-to-leading-order plus parton shower precision in quantum chromodynamics and including next-to-leading-order electroweak corrections. They are −0.084−0.003+0.005 (scale) ± 0.006 (MC stat.) and −0.063−0.004+0.007 (scale) ± 0.004 (MC stat.) respectively, and in agreement with the measurements. 

A search for new phenomena has been performed in final states with at least one isolated high-momentum photon, jets and missing transverse momentum in proton–proton collisions at a centre-of-mass energy of s = 13 TeV. The data, collected by the ATLAS experiment at the CERN LHC, correspond to an integrated luminosity of 139 fb−1. The experimental results are interpreted in a supersymmetric model in which pair-produced gluinos decay into neutralinos, which in turn decay into a gravitino, at least one photon, and jets. No significant deviations from the predictions of the Standard Model are observed. Upper limits are set on the visible cross section due to physics beyond the Standard Model, and lower limits are set on the masses of the gluinos and neutralinos, all at 95% confidence level. Visible cross sections greater than 0.022 fb are excluded and pair-produced gluinos with masses up to 2200 GeV are excluded for most of the NLSP masses investigated.

The selection of low-radioactive construction materials is of the utmost importance for rare-event searches and thus critical to the XENONnT experiment. Results of an extensive radioassay program are reported, in which material samples have been screened with gamma-ray spectroscopy, mass spectrometry, and 222 Rn emanation measurements. Furthermore, the cleanliness procedures applied to remove or mitigate surface contamination of detector materials are described. Screening results, used as inputs for a XENONnT Monte Carlo simulation, predict a reduction of materials background ( ∼ 17%) with respect to its predecessor XENON1T. Through radon emanation measurements, the expected 222 Rn activity concentration in XENONnT is determined to be 4.2 ( - 0.7 + 0.5 )  μ Bq/kg, a factor three lower with respect to XENON1T. This radon concentration will be further suppressed by means of the novel radon distillation system.

A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton–proton collision data at a centre-of-mass energy of 13 TeV, corresponding to 139fb-1 of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on final states with three or four electrons or muons from the possible decays of new heavy leptons via intermediate electroweak bosons. No significant deviations above the Standard Model expectation are observed; upper and lower limits on the heavy lepton production cross-section and masses are derived respectively. These results are then combined for the first time with the ones already published by ATLAS using the channel with two leptons in the final state. The observed lower limit on the mass of the type-III seesaw heavy leptons combining two, three and four lepton channels together is 910 GeV at the 95% confidence level.

Searches are performed for nonresonant and resonant di-Higgs boson production in the bb¯γγ final state. The dataset used corresponds to an integrated luminosity of 139 fb-1 of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No excess above the expected background is found and upper limits on the di-Higgs boson production cross sections are set. A 95% confidence-level upper limit of 4.2 times the cross section predicted by the Standard Model is set on pp→HH nonresonant production, where the expected limit is 5.7 times the Standard Model predicted value. The expected constraints are obtained for a background hypothesis excluding pp→HH production. The observed (expected) constraints on the Higgs boson trilinear coupling modifier κλ are determined to be [-1.5,6.7] ([-2.4,7.7]) at 95% confidence level, where the expected constraints on κλ are obtained excluding pp→HH production from the background hypothesis. For resonant production of a new hypothetical scalar particle X (X→HH→bb¯γγ), limits on the cross section for pp→X→HH are presented in the narrow-width approximation as a function of mX in the range 251 GeV≤mX≤1000 GeV. The observed (expected) limits on the cross section for pp→X→HH range from 640 fb to 44 fb (391 fb to 46 fb) over the considered mass range.

Several observables sensitive to the fragmentation of b quarks into b hadrons are measured using 36 fb-1 of √s=13 TeV proton-proton collision data collected with the ATLAS detector at the LHC. Jets containing b hadrons are obtained from a sample of dileptonic t¯t events, and the associated set of charged-particle tracks is separated into those from the primary pp interaction vertex and those from the displaced b-decay secondary vertex. This division is used to construct observables that characterize the longitudinal and transverse momentum distributions of the b hadron within the jet. The measurements have been corrected for detector effects and provide a test of heavy-quark-fragmentation modeling at the LHC in a system where the top-quark decay products are color connected to the proton beam remnants. The unfolded distributions are compared with the predictions of several modern Monte Carlo parton-shower generators and generator tunes, and a wide range of agreement with the data is observed, with p values varying from 5×10-4 to 0.98. These measurements complement similar measurements from e+e- collider experiments in which the b quarks originate from a color singlet Z/γ∗.

We report on a blinded analysis of low-energy electronic recoil data from the first science run of the XENONnT dark matter experiment. Novel subsystems and the increased 5.9 ton liquid xenon target reduced the background in the (1, 30) keV search region to (15.8±1.3) events/(ton×year×keV), the lowest ever achieved in a dark matter detector and ∼5 times lower than in XENON1T. With an exposure of 1.16 ton-years, we observe no excess above background and set stringent new limits on solar axions, an enhanced neutrino magnetic moment, and bosonic dark matter.

A direct search for Higgs bosons produced via vector-boson fusion and subsequently decaying into invisible particles is reported. The analysis uses 139 fb−1 of pp collision data at a centre-of-mass energy of √s = 13 TeV recorded by the ATLAS detector at the LHC. The observed numbers of events are found to be in agreement with the background expectation from Standard Model processes. For a scalar Higgs boson with a mass of 125 GeV and a Standard Model production cross section, an observed upper limit of 0.145 is placed on the branching fraction of its decay into invisible particles at 95% confidence level, with an expected limit of 0.103. These results are interpreted in the context of models where the Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross section of weakly interacting massive particles and nucleons. Invisible decays of additional scalar bosons with masses from 50 GeV to 2 TeV are also studied, and the derived upper limits on the cross section times branching fraction decrease with increasing mass from 1.0 pb for a scalar boson mass of 50 GeV to 0.1 pb at a mass of 2 TeV. [Figure not available: see fulltext.]

A search for events with two displaced vertices from long-lived particle (LLP) pairs using data collected by the ATLAS detector at the LHC is presented. This analysis uses 139 fb-1 of proton-proton collision data at √s=13 TeV recorded in 2015-2018. The search employs techniques for reconstructing vertices of LLPs decaying to jets in the muon spectrometer displaced between 3 and 14 m with respect to the primary interaction vertex. The observed numbers of events are consistent with the expected background and limits for several benchmark signals are determined. For the Higgs boson with a mass of 125 GeV, the paper reports the first exclusion limits for branching fractions into neutral long-lived particles below 0.1%, while branching fractions above 10% are excluded at 95% confidence level for LLP proper lifetimes ranging from 4 cm to 72.4 m. In addition, the paper present the first results for the decay of LLPs into tt¯ in the ATLAS muon spectrometer.

We present results on the search for two-neutrino double-electron capture (2νECEC) of 124Xe and neutrinoless double-β decay (0νββ) of 136Xe in XENON1T. We consider captures from the K shell up to the N shell in the 2νECEC signal model and measure a total half-life of T1/22νECEC=(1.1±0.2stat±0.1sys)×1022yr with a 0.87kgyr isotope exposure. The statistical significance of the signal is 7.0σ. We use XENON1T data with 36.16kgyr of 136Xe exposure to search for 0νββ. We find no evidence of a signal and set a lower limit on the half-life of T1/20νββ>1.2×1024 yr at 90% CL. This is the best result from a dark matter detector without an enriched target to date. We also report projections on the sensitivity of XENONnT to 0νββ. Assuming a 275 kg yr 136Xe exposure, the expected sensitivity is T1/20νββ>2.1×1025 yr at 90% CL, corresponding to an effective Majorana mass range of ⟨mββ⟩

The XENON collaboration has published stringent limits on specific dark matter – nucleon recoil spectra from dark matter recoiling on the liquid xenon detector target. In this paper, we present an approximate likelihood for the XENON1T 1 t-year nuclear recoil search applicable to any nuclear recoil spectrum. Alongside this paper, we publish data and code to compute upper limits using the method we present. The approximate likelihood is constructed in bins of reconstructed energy, profiled along the signal expectation in each bin. This approach can be used to compute an approximate likelihood and therefore most statistical results for any nuclear recoil spectrum. Computing approximate results with this method is approximately three orders of magnitude faster than the likelihood used in the original publications of XENON1T, where limits were set for specific families of recoil spectra. Using this same method, we include toy Monte Carlo simulation-derived binwise likelihoods for the upcoming XENONnT experiment that can similarly be used to assess the sensitivity to arbitrary nuclear recoil signatures in its eventual 20 t-year exposure.

This paper presents updated Monte Carlo configurations used to model the production of single electroweak vector bosons (W, Z/γ∗) in association with jets in proton-proton collisions for the ATLAS experiment at the Large Hadron Collider. Improvements pertaining to the electroweak input scheme, parton-shower splitting kernels and scale-setting scheme are shown for multi-jet merged configurations accurate to next-to-leading order in the strong and electroweak couplings. The computational resources required for these set-ups are assessed, and approximations are introduced resulting in a factor three reduction of the per-event CPU time without affecting the physics modelling performance. Continuous statistical enhancement techniques are introduced by ATLAS in order to populate low cross-section regions of phase space and are shown to match or exceed the generated effective luminosity. This, together with the lower per-event CPU time, results in a 50% reduction in the required computing resources compared to a legacy set-up previously used by the ATLAS collaboration. The set-ups described in this paper will be used for future ATLAS analyses and lay the foundation for the next generation of Monte Carlo predictions for single vector-boson plus jets production. 

The associated production of a Higgs boson and a top-quark pair is measured in events characterised by the presence of one or two electrons or muons. The Higgs boson decay into a b-quark pair is used. The analysed data, corresponding to an integrated luminosity of 139 fb−1, were collected in proton-proton collisions at the Large Hadron Collider between 2015 and 2018 at a centre-of-mass energy of √s = 13 TeV. The measured signal strength, defined as the ratio of the measured signal yield to that predicted by the Standard Model, is 0.35−0.34+0.36. This result is compatible with the Standard Model prediction and corresponds to an observed (expected) significance of 1.0 (2.7) standard deviations. The signal strength is also measured differentially in bins of the Higgs boson transverse momentum in the simplified template cross-section framework, including a bin for specially selected boosted Higgs bosons with transverse momentum above 300 GeV.

This search, a type not previously performed at ATLAS, uses a comparison of the production cross sections for e+μ− and e−μ+ pairs to constrain physics processes beyond the Standard Model. It uses 139fb−1 of proton–proton collision data recorded at √s=13 TeV at the LHC. Targeting sources of new physics which prefer final states containing e+μ− to e−μ+, the search contains two broad signal regions which are used to provide model-independent constraints on the ratio of cross sections at the 2% level. The search also has two special selections targeting supersymmetric models and leptoquark signatures. Observations using one of these selections are able to exclude, at 95% confidence level, singly produced smuons with masses up to 640 GeV in a model in which the only other light sparticle is a neutralino when the R-parity-violating coupling λ′231 is close to unity. Observations using the other selection exclude scalar leptoquarks with masses below 1880 GeV when g1Reu=g1Rμc=1, at 95% confidence level. The limit on the coupling reduces to g1Reu=g1Rμc=0.46 for a mass of 1420 GeV.

The ATLAS experiment at the Large Hadron Collider has a broad physics programme ranging from precision measurements to direct searches for new particles and new interactions, requiring ever larger and ever more accurate datasets of simulated Monte Carlo events. Detector simulation with Geant4 is accurate but requires significant CPU resources. Over the past decade, ATLAS has developed and utilized tools that replace the most CPU-intensive component of the simulation—the calorimeter shower simulation—with faster simulation methods. Here, AtlFast3, the next generation of high-accuracy fast simulation in ATLAS, is introduced. AtlFast3 combines parameterized approaches with machine-learning techniques and is deployed to meet current and future computing challenges, and simulation needs of the ATLAS experiment. With highly accurate performance and significantly improved modelling of substructure within jets, AtlFast3 can simulate large numbers of events for a wide range of physics processes.

The accurate simulation of additional interactions at the ATLAS experiment for the analysis of proton–proton collisions delivered by the Large Hadron Collider presents a significant challenge to the computing resources. During the LHC Run 2 (2015–2018), there were up to 70 inelastic interactions per bunch crossing, which need to be accounted for in Monte Carlo (MC) production. In this document, a new method to account for these additional interactions in the simulation chain is described. Instead of sampling the inelastic interactions and adding their energy deposits to a hard-scatter interaction one-by-one, the inelastic interactions are presampled, independent of the hard scatter, and stored as combined events. Consequently, for each hard-scatter interaction, only one such presampled event needs to be added as part of the simulation chain. For the Run 2 simulation chain, with an average of 35 interactions per bunch crossing, this new method provides a substantial reduction in MC production CPU needs of around 20%, while reproducing the properties of the reconstructed quantities relevant for physics analyses with good accuracy.

A search for the Higgs boson decaying into a pair of charm quarks is presented. The analysis uses proton–proton collisions to target the production of a Higgs boson in association with a leptonically decaying W or Z boson. The dataset delivered by the LHC at a centre-of-mass energy of √s=13TeV and recorded by the ATLAS detector corresponds to an integrated luminosity of 139 fb-1. Flavour-tagging algorithms are used to identify jets originating from the hadronisation of charm quarks. The analysis method is validated with the simultaneous measurement of WW, WZ and ZZ production, with observed (expected) significances of 2.6 (2.2) standard deviations above the background-only prediction for the (W/ Z) Z(→ cc¯) process and 3.8 (4.6) standard deviations for the (W/ Z) W(→ cq) process. The (W/ Z) H(→ cc¯) search yields an observed (expected) upper limit of 26 (31) times the predicted Standard Model cross-section times branching fraction for a Higgs boson with a mass of 125GeV, corresponding to an observed (expected) constraint on the charm Yukawa coupling modifier |κc|

A search for invisible decays of the Higgs boson as well as searches for dark matter candidates, produced together with a leptonically decaying Z boson, are presented. The analysis is performed using proton−proton collisions at a centre-of-mass energy of 13 TeV, delivered by the LHC, corresponding to an integrated luminosity of 139 fb−1 and recorded by the ATLAS experiment. Assuming Standard Model cross-sections for ZH production, the observed (expected) upper limit on the branching ratio of the Higgs boson to invisible particles is found to be 19% (19%) at the 95% confidence level. Exclusion limits are also set for simplified dark matter models and two-Higgs-doublet models with an additional pseudoscalar mediator.

Delayed single- and few-electron emissions plague dual-phase time projection chambers, limiting their potential to search for light-mass dark matter. This paper examines the origins of these events in the XENON1T experiment. Characterization of the intensity of delayed electron backgrounds shows that the resulting emissions are correlated, in time and position, with high-energy events and can effectively be vetoed. In this work we extend previous S2-only analyses down to a single electron. From this analysis, after removing the correlated backgrounds, we observe rates

A search for the dimuon decay of the Standard Model (SM) Higgs boson is performed using data corresponding to an integrated luminosity of 139 fb(-1) collected with the ATLAS detector in Run 2 pp collisions at root s = 13 TeV at the Large Hadron Collider. The observed (expected) significance over the background-only hypothesis for a Higgs boson with a mass of 125.09 GeV is 2.0 sigma (1.7 sigma). The observed upper limit on the cross section times branching ratio for pp -> H -> mu mu is 2.2 times the SM prediction at 95% confidence level, while the expected limit on a H -> mu mu signal assuming the absence (presence) of a SM signal is 1.1(2.0). The best-fit value of the signal strength parameter, defined as the ratio of the observed signal yield to the one expected in the SM, is mu = 1.2 +/- 0.6. 

We report on a search for nuclear recoil signals from solar 8B neutrinos elastically scattering off xenon nuclei in XENON1T data, lowering the energy threshold from 2.6 to 1.6 keV. We develop a variety of novel techniques to limit the resulting increase in backgrounds near the threshold. No significant 8B neutrinolike excess is found in an exposure of 0.6 t × y. For the first time, we use the nondetection of solar neutrinos to constrain the light yield from 1–2 keV nuclear recoils in liquid xenon, as well as nonstandard neutrino-quark interactions. Finally, we improve upon world-leading constraints on dark matter-nucleus interactions for dark matter masses between 3 and 11 GeV c−2 by as much as an order of magnitude.

FASER nu at the CERN Large Hadron Collider (LHC) is designed to directly detect collider neutrinos for the first time and study their cross sections at TeV energies, where no such measurements currently exist. In 2018, a pilot detector employing emulsion films was installed in the far-forward region of ATLAS, 480 m from the interaction point, and collected 12.2 fb(-1) of proton-proton collision data at a center-of-mass energy of 13 TeV. We describe the analysis of this pilot run data and the observation of the first neutrino interaction candidates at the LHC. This milestone paves the way for high-energy neutrino measurements at current and future colliders.

The integrated fiducial cross-section and unfolded differential jet mass spectrum of high transverse momentum Z -> b (b) over bar decays are measured in Z gamma events in proton-proton collisions at root s = 13 TeV. The data analysed were collected between 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 fb(-1). Photons are required to have a transverse momentum p(T) > 175 GeV. The Z -> b (b) over bar decay is reconstructed using a jet with p(T) > 200 GeV, found with the anti-k(t) R = 1.0 jet algorithm, and groomed to remove soft and wide-angle radiation and to mitigate contributions from the underlying event and additional proton-proton collisions. Two different but related measurements are performed using two jet grooming definitions for reconstructing the Z -> b (b) over bar decay: trimming and soft drop. These algorithms differ in their experimental and phenomenological implications regarding jet mass reconstruction and theoretical precision. To identify Zbosons, b-tagged R = 0.2 track-jets matched to the groomed large-R calorimeter jet are used as a proxy for the b-quarks. The signal yield is determined from fits of the data-driven background templates to the different jet mass distributions for the two grooming methods. Integrated fiducial cross-sections and unfolded jet mass spectra for each grooming method are compared with leading-order theoretical predictions. The results are found to be in good agreement with Standard Model expectations within the current statistical and systematic uncertainties.

A measurement of event-shape variables in proton-proton collisions at large momentum transfer is presented using data collected at s = 13 TeV with the ATLAS detector at the Large Hadron Collider. Six event-shape variables calculated using hadronic jets are studied in inclusive multijet events using data corresponding to an integrated luminosity of 139 fb−1. Measurements are performed in bins of jet multiplicity and in different ranges of the scalar sum of the transverse momenta of the two leading jets, reaching scales beyond 2 TeV. These measurements are compared with predictions from Monte Carlo event generators containing leading-order or next-to-leading order matrix elements matched to parton showers simulated to leading-logarithm accuracy. At low jet multiplicities, shape discrepancies between the measurements and the Monte Carlo predictions are observed. At high jet multiplicities, the shapes are better described but discrepancies in the normalisation are observed. [Figure not available: see fulltext.]

A search is presented for pair production of long-lived neutral particles using 33 fb-1 of s=13 TeV proton-proton collision data, collected during 2016 by the ATLAS detector at the LHC. This search focuses on a topology in which one long-lived particle decays in the ATLAS inner detector and the other decays in the muon spectrometer. Special techniques are employed to reconstruct the displaced tracks and vertices in the inner detector and in the muon spectrometer. One event is observed that passes the full event selection, which is consistent with the estimated background. Limits are placed on scalar boson propagators with masses from 125 GeV to 1000 GeV decaying into pairs of long-lived hidden-sector scalars with masses from 8 GeV to 400 GeV. The limits placed on several low-mass scalars extend previous exclusion limits in the range of proper lifetimes cτ from 5 cm to 1 m.

The ATLAS collaboration E-mail: atlas.publications@cern.ch Erratum to: JHEP02(2020)042 ArXiv ePrint: 1911.00453 Figure 5b of the paper [1] contained a misinterpretation in the comparison between the reported new ATLAS measurement of the process pp ! Xp and previously published CMS data [2]. The ATLAS measurement corresponds to cases where either proton dissociates. In the comparison, the CMS measurement was assumed to be defined similarly, whereas in fact it includes the dissociation of only one of the protons. On further investigation, the comparison between the ATLAS and CMS measurements is far from straightforward. The double dissociation (pp → XY ) contribution in the CMS data is not well constrained and indications from the CMS analysis suggest it may be significantly larger than was thought originally. Fuerthermore, the potentially sizeable central diffraction (pp → pXp) contribution was subtracted in the ATLAS measurement, but included by CMS. There is also a small effect associated with the different centre of mass energies (7 TeV for CMS and 8 TeV for ATLAS). In view of these complications, the figure has now been withdrawn from the publication [1].

This Letter presents direct searches for lepton flavour violation in Higgs boson decays, H→eτ and H→μτ, performed with the ATLAS detector at the LHC. The searches are based on a data sample of proton–proton collisions at a centre-of-mass energy s=13 TeV, corresponding to an integrated luminosity of 36.1 fb−1. No significant excess is observed above the expected background from Standard Model processes. The observed (median expected) 95% confidence-level upper limits on the lepton-flavour-violating branching ratios are 0.47% (0.34−0.10 +0.13%) and 0.28% (0.37−0.10 +0.14%) for H→eτ and H→μτ, respectively.

Neutrinos are copiously produced at particle colliders, but no collider neutrino has ever been detected. Colliders produce both neutrinos and anti-neutrinos of all flavors at very high energies, and they are therefore highly complementary to those from other sources. FASER, the Forward Search Experiment at the LHC, is ideally located to provide the first detection and study of collider neutrinos. We investigate the prospects for neutrino studies with FASERν, a proposed component of FASER, consisting of emulsion films interleaved with tungsten plates with a total target mass of 1.2 t, to be placed on-axis at the front of FASER. We estimate the neutrino fluxes and interaction rates, describe the FASERν detector, and analyze the characteristics of the signals and primary backgrounds. For an integrated luminosity of 150fb-1 to be collected during Run 3 of the 14 TeV LHC in 2021–23, approximately 1300 electron neutrinos, 20,000 muon neutrinos, and 20 tau neutrinos will interact in FASERν, with mean energies of 600 GeV to 1 TeV. With such rates and energies, FASER will measure neutrino cross sections at energies where they are currently unconstrained, will bound models of forward particle production, and could open a new window on physics beyond the standard model.

Xenon dual-phase time projection chambers designed to search for weakly interacting massive particles have so far shown a relative energy resolutionwhich degrades with energy above similar to 200 keV due to the saturation effects. This has limited their sensitivity in the search for rare events like the neutrinoless double-beta decay of Xe-136 at its Q value, Q(beta beta) similar or equal to 2.46 MeV. For the XENON1T dual-phase time projection chamber, we demonstrate that the relative energy resolution at 1 sigma/mu is as low as (0.80 +/- 0.02)% in its one-ton fiducial mass, and for single-site interactions at Q(beta beta). We also present a new signal correction method to rectify the saturation effects of the signal readout system, resulting in more accurate position reconstruction and indirectly improving the energy resolution. The very good result achieved in XENON1T opens up new windows for the xenon dual-phase dark matter detectors to simultaneously search for other rare events.

XENONnT is a dark matter direct detection experiment, utilizing 5.9 t of instrumented liquid xenon, located at the INFN Laboratori Nazionali del Gran Sasso. In this work, we predict the experimental background and project the sensitivity of XENONnT to the detection of weakly interacting massive particles (WIMPs). The expected average differential background rate in the energy region of interest, corresponding to (1, 13) keV and (4, 50) keV for electronic and nuclear recoils, amounts to 12.3 ± 0.6 (keV t y)-1 and (2.2± 0.5)× 10-3 (keV t y)-1, respectively, in a 4 t fiducial mass. We compute unified confidence intervals using the profile construction method, in order to ensure proper coverage. With the exposure goal of 20 t y, the expected sensitivity to spin-independent WIMP-nucleon interactions reaches a cross-section of 1.4×10-48 cm2 for a 50 GeV/c2 mass WIMP at 90% confidence level, more than one order of magnitude beyond the current best limit, set by XENON1T . In addition, we show that for a 50 GeV/c2 WIMP with cross-sections above 2.6×10-48 cm2 (5.0×10-48 cm2) the median XENONnT discovery significance exceeds 3σ (5σ). The expected sensitivity to the spin-dependent WIMP coupling to neutrons (protons) reaches 2.2×10-43 cm2 (6.0×10-42 cm2).

We report results from searches for new physics with low-energy electronic recoil data recorded with the XENON1T detector. With an exposure of 0.65 tonne-years and an unprecedentedly low background rate of 76±2stat events/(tonne×year×keV) between 1 and 30 keV, the data enable one of the most sensitive searches for solar axions, an enhanced neutrino magnetic moment using solar neutrinos, and bosonic dark matter. An excess over known backgrounds is observed at low energies and most prominent between 2 and 3 keV. The solar axion model has a 3.4σ significance, and a three-dimensional 90% confidence surface is reported for axion couplings to electrons, photons, and nucleons. This surface is inscribed in the cuboid defined by gae

Azimuthal anisotropies of muons from charm and bottom hadron decays are measured in Pb+Pb collisions at sNN=5.02TeV. The data were collected with the ATLAS detector at the Large Hadron Collider in 2015 and 2018 with integrated luminosities of 0.5nb−1 and 1.4nb−1, respectively. The kinematic selection for heavy-flavor muons requires transverse momentum 4

A search for direct pair production of scalar partners of the top quark (top squarks or scalar third-generation up-type leptoquarks) in the all-hadronic tt¯ plus missing transverse momentum final state is presented. The analysis of 139 fb - 1 of s=13 TeV proton–proton collision data collected using the ATLAS detector at the LHC yields no significant excess over the Standard Model background expectation. To interpret the results, a supersymmetric model is used where the top squark decays via t~→t(∗)χ~10, with t(∗) denoting an on-shell (off-shell) top quark and χ~10 the lightest neutralino. Three specific event selections are optimised for the following scenarios. In the scenario where mt~>mt+mχ~10, top squark masses are excluded in the range 400–1250 GeV for χ~10 masses below 200 GeV at 95% confidence level. In the situation where mt~∼mt+mχ~10, top squark masses in the range 300–630 GeV are excluded, while in the case where mt~

A search for Higgs boson pair production via vector-boson fusion (VBF) in the bbbb final state is carried out with the ATLAS experiment using 126 fb(-1) of proton- proton collision data delivered at s = 13 TeV by the Large Hadron Collider. This search is sensitive to VBF production of additional heavy bosons that may decay into Higgs boson pairs, and in a non-resonant topology it can constrain the quartic coupling between the Higgs bosons and vector bosons. No significant excess relative to the Standard Model expectation is observed, and limits on the production cross-section are set at the 95% confidence level for a heavy scalar resonance in the context of an extended Higgs sector, and for non-resonant Higgs boson pair production. Interpretation in terms of the coupling between a Higgs boson pair and two vector bosons is also provided: coupling values normalised to the Standard Model expectation of kappa (2V) 2.90 are excluded at the 95% confidence level in data.

The top anti-top quark production cross-section is measured in the lepton+jets channel using proton–proton collision data at a centre-of-mass energy of s=13 TeV collected with the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 139 fb−1. Events with exactly one charged lepton and four or more jets in the final state, with at least one jet containing b-hadrons, are used to determine the tt¯ production cross-section through a profile-likelihood fit. The inclusive cross-section is measured to be σinc=830±0.4(stat.)±36(syst.)±14(lumi.)pb with a relative uncertainty of 4.6%. The result is consistent with theoretical calculations at next-to-next-to-leading order in perturbative QCD. The fiducial tt¯ cross-section within the experimental acceptance is also measured.

Results are presented from the measurement by ATLAS of long-range (| Δ η| > 2) dihadron angular correlations in s=8 and 13 TeV pp collisions containing a Z boson. The analysis is performed using 19.4 fb - 1 of s=8 TeV data recorded during Run 1 of the LHC and 36.1 fb - 1 of s=13 TeV data recorded during Run 2. Two-particle correlation functions are measured as a function of relative azimuthal angle over the relative pseudorapidity range 2 < | Δ η| < 5 for different intervals of charged-particle multiplicity and transverse momentum. The measurements are corrected for the presence of background charged particles generated by collisions that occur during one passage of two colliding proton bunches in the LHC. Contributions to the two-particle correlation functions from hard processes are removed using a template-fitting procedure. Sinusoidal modulation in the correlation functions is observed and quantified by the second Fourier coefficient of the correlation function, v2 , 2, which in turn is used to obtain the single-particle anisotropy coefficient v2. The v2 values in the Z-tagged events, integrated over 0.5 < pT< 5 GeV, are found to be independent of multiplicity and s, and consistent within uncertainties with previous measurements in inclusive pp collisions. As a function of charged-particle pT, the Z-tagged and inclusive v2 values are consistent within uncertainties for pT< 3 GeV.

Combined measurements of Higgs boson production cross sections and branching fractions are presented. The combination is based on the analyses of the Higgs boson decay modes H → γγ, ZZ∗, WW∗, τ τ, bb, μ μ, searches for decays into invisible final states, and on measurements of off-shell Higgs boson production. Up to 79.8 fb-1 of proton-proton collision data collected at √ = 13 TeV with the ATLAS detector are used. Results are presented for the gluon-gluon fusion and vector-boson fusion processes, and for associated production with vector bosons or top-quarks. The global signal strength is determined to be μ = 1.11-0.08+0.09. The combined measurement yields an observed (expected) significance for the vector-boson fusion production process of 6.5δ (5.3δ). Measurements in kinematic regions defined within the simplified template cross section framework are also shown. The results are interpreted in terms of modifiers applied to the Standard Model couplings of the Higgs boson to other particles, and are used to set exclusion limits on parameters in two-Higgs-doublet models and in the simplified minimal supersymmetric Standard Model. No significant deviations from Standard Model predictions are observed.

A search for flavour-changing neutral current (FCNC) events via the coupling of a top quark, a photon, and an up or charm quark is presented using 81 fb−1 of proton–proton collision data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Events with a photon, an electron or muon, a b-tagged jet, and missing transverse momentum are selected. A neural network based on kinematic variables differentiates between events from signal and background processes. The data are consistent with the background-only hypothesis, and limits are set on the strength of the tqγ coupling in an effective field theory. These are also interpreted as 95% CL upper limits on the cross section for FCNC tγ production via a left-handed (right-handed) tuγ coupling of 36 fb (78 fb) and on the branching ratio for t→γu of 2.8×10−5 (6.1×10−5). In addition, they are interpreted as 95% CL upper limits on the cross section for FCNC tγ production via a left-handed (right-handed) tcγ coupling of 40 fb (33 fb) and on the branching ratio for t→γc of 22×10−5 (18×10−5).

A search for the direct production of the supersymmetric partners of τ-leptons (staus) in final states with two hadronically decaying τ-leptons is presented. The analysis uses a dataset of pp collisions corresponding to an integrated luminosity of 139 fb-1, recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy of 13 TeV. No significant deviation from the expected Standard Model background is observed. Limits are derived in scenarios of direct production of stau pairs with each stau decaying into the stable lightest neutralino and one τ-lepton in simplified models where the two stau mass eigenstates are degenerate. Stau masses from 120 GeV to 390 GeV are excluded at 95% confidence level for a massless lightest neutralino.

A search for new resonances decaying into a pair of jets is reported using the dataset of proton-proton collisions recorded at s = 13 TeV with the ATLAS detector at the Large Hadron Collider between 2015 and 2018, corresponding to an integrated luminosity of 139 fb−1. The distribution of the invariant mass of the two leading jets is examined for local excesses above a data-derived estimate of the Standard Model background. In addition to an inclusive dijet search, events with jets identified as containing b-hadrons are examined specifically. No significant excess of events above the smoothly falling background spectra is observed. The results are used to set cross-section upper limits at 95% confidence level on a range of new physics scenarios. Model-independent limits on Gaussian-shaped signals are also reported. The analysis looking at jets containing b-hadrons benefits from improvements in the jet flavour identification at high transverse momentum, which increases its sensitivity relative to the previous analysis beyond that expected from the higher integrated luminosity. 

The production yield of Z bosons is measured in the electron and muon decay channels in Pb+Pb collisions at √Snn = 5.02 TeV with the ATLAS detector. Data from the 2015 LHC run corresponding to an integrated luminosity of 0.49 nb-1 are used for the analysis. The Z boson yield, normalised by the total number of minimum-bias events and the mean nuclear thickness function, is measured as a function of dilepton rapidity and event centrality. The measurements in Pb+Pb collisions are compared with similar measurements made in proton-proton collisions at the same centre-of-mass energy. The nuclear modification factor is found to be consistent with unity for all centrality intervals. The results are compared with theoretical predictions obtained at next-to-leading order using nucleon and nuclear parton distribution functions. The normalised Z boson yields in Pb+Pb collisions lie 1-3σ above the predictions. The nuclear modification factor measured as a function of rapidity agrees with unity and is consistent with a next-to-leading-order QCD calculation including the isospin effect.

A mistake was identified for the paper [1] in the treatment of the radion [2] cross-sections, which resulted in multiple changes.

The combination of measurements of the W boson polarization in top quark decays performed by the ATLAS and CMS collaborations is presented. The measurements are based on proton-proton collision data produced at the LHC at a centre-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of about 20 fb−1 for each experiment. The measurements used events containing one lepton and having different jet multiplicities in the final state. The results are quoted as fractions of W bosons with longitudinal (F0), left-handed (FL), or right-handed (FR) polarizations. The resulting combined measurements of the polarization fractions are F0 = 0.693 ± 0.014 and FL = 0.315 ± 0.011. The fraction FR is calculated from the unitarity constraint to be FR = −0.008 ± 0.007. These results are in agreement with the standard model predictions at next-to-next-to-leading order in perturbative quantum chromodynamics and represent an improvement in precision of 25 (29)% for F0 (FL) with respect to the most precise single measurement. A limit on anomalous right-handed vector (VR), and left- and right-handed tensor (gL, gR) tWb couplings is set while fixing all others to their standard model values. The allowed regions are [−0.11, 0.16] for VR, [−0.08, 0.05] for gL, and [−0.04, 0.02] for gR, at 95% confidence level. Limits on the corresponding Wilson coefficients are also derived. [Figure not available: see fulltext.]

A search for the Zγ decay of the Higgs boson, with Z boson decays into pairs of electrons or muons is presented. The analysis uses proton–proton collision data at s = 13 TeV corresponding to an integrated luminosity of 139 fb−1 recorded by the ATLAS detector at the Large Hadron Collider. The observed data are consistent with the expected background with a p-value of 1.3%. An upper limit at 95% confidence level on the production cross-section times the branching ratio for pp→H→Zγ is set at 3.6 times the Standard Model prediction while 2.6 times is expected in the presence of the Standard Model Higgs boson. The best-fit value for the signal yield normalised to the Standard Model prediction is 2.0−0.9+1.0 where the statistical component of the uncertainty is dominant.

Inclusive and differential fiducial cross sections of the Higgs boson are measured in the H→ ZZ∗→ 4 ℓ (ℓ= e, μ) decay channel. The results are based on proton−proton collision data produced at the Large Hadron Collider at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector from 2015 to 2018, equivalent to an integrated luminosity of 139 fb - 1. The inclusive fiducial cross section for the H→ ZZ∗→ 4 ℓ process is measured to be σfid= 3.28 ± 0.32 fb, in agreement with the Standard Model prediction of σfid , SM= 3.41 ± 0.18 fb. Differential fiducial cross sections are measured for a variety of observables which are sensitive to the production and decay of the Higgs boson. All measurements are in agreement with the Standard Model predictions. The results are used to constrain anomalous Higgs boson interactions with Standard Model particles.

Higgs boson properties are studied in the four-lepton decay channel (where lepton = e, μ) using 139 fb - 1 of proton–proton collision data recorded at s=13 TeV by the ATLAS experiment at the Large Hadron Collider. The inclusive cross-section times branching ratio for H→ ZZ∗ decay is measured to be 1.34 ± 0.12 pb for a Higgs boson with absolute rapidity below 2.5, in good agreement with the Standard Model prediction of 1.33 ± 0.08 pb. Cross-sections times branching ratio are measured for the main Higgs boson production modes in several exclusive phase-space regions. The measurements are interpreted in terms of coupling modifiers and of the tensor structure of Higgs boson interactions using an effective field theory approach. Exclusion limits are set on the CP-even and CP-odd ‘beyond the Standard Model’ couplings of the Higgs boson to vector bosons, gluons and top quarks.

A search for a chargino-neutralino pair decaying via the 125 GeV Higgs boson into photons is presented. The study is based on the data collected between 2015 and 2018 with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 139 fb−1 of pp collisions at a centre-of-mass energy of 13 TeV. No significant excess over the expected background is observed. Upper limits at 95% confidence level for a massless χ˜10 are set on several electroweakino production cross-sections and the visible cross-section for beyond the Standard Model processes. In the context of simplified supersymmetric models, 95% confidence-level limits of up to 310 GeV in m(χ˜1±/χ˜20), where m(χ˜10) = 0.5 GeV, are set. Limits at 95% confidence level are also set on the χ˜1±χ˜20 cross-section in the mass plane of m(χ˜1±/χ˜20) and m(χ˜10), and on scenarios with gravitino as the lightest supersymmetric particle. Upper limits at the 95% confidence-level are set on the higgsino production cross-section. Higgsino masses below 380 GeV are excluded for the case of the higgsino fully decaying into a Higgs boson and a gravitino.

This paper presents a search for new heavy particles decaying into a pair of top quarks using 139 fb−1 of proton-proton collision data recorded at a centre-of-mass energy of s = 13 TeV with the ATLAS detector at the Large Hadron Collider. The search is performed using events consistent with pair production of high-transverse-momentum top quarks and their subsequent decays into the fully hadronic final states. The analysis is optimized for resonances decaying into a tt¯ pair with mass above 1.4 TeV, exploiting a dedicated multivariate technique with jet substructure to identify hadronically decaying top quarks using large-radius jets and evaluating the background expectation from data. No significant deviation from the background prediction is observed. Limits are set on the production cross-section times branching fraction for the new Z′ boson in a topcolor-assisted-technicolor model. The Z′ boson masses below 3.9 and 4.7 TeV are excluded at 95% confidence level for the decay widths of 1% and 3%, respectively.

A search for new-physics resonances decaying into a lepton and a jet performed by the ATLAS experiment is presented. Scalar leptoquarks pair-produced in pp collisions at s = 13 TeV at the Large Hadron Collider are considered using an integrated luminosity of 139 fb−1, corresponding to the full Run 2 dataset. They are searched for in events with two electrons or two muons and two or more jets, including jets identified as arising from the fragmentation of c- or b-quarks. The observed yield in each channel is consistent with the Standard Model background expectation. Leptoquarks with masses below 1.8 TeV and 1.7 TeV are excluded in the electron and muon channels, respectively, assuming a branching ratio into a charged lepton and a quark of 100%, with minimal dependence on the quark flavour. Upper limits on the aforementioned branching ratio are also given as a function of the leptoquark mass. 

Inclusive and differential cross-sections for the production of top quarks in association with a photon are measured with proton-proton collision data corresponding to an integrated luminosity of 139 fb−1. The data were collected by the ATLAS detector at the LHC during Run 2 between 2015 and 2018 at a centre-of-mass energy of 13 TeV. The measurements are performed in a fiducial volume defined at parton level. Events with exactly one photon, one electron and one muon of opposite sign, and at least two jets, of which at least one is b-tagged, are selected. The fiducial cross-section is measured to be 39.6−2.3+2.7 fb. Differential cross-sections as functions of several observables are compared with state-of-the-art Monte Carlo simulations and next-to-leading-order theoretical calculations. These include cross-sections as functions of photon kinematic variables, angular variables related to the photon and the leptons, and angular separations between the two leptons in the event. All measurements are in agreement with the predictions from the Standard Model.

Two additions impacting tables 3 and 4 in ref. [1] are presented in the following. No significant impact is found for other results or figures in ref. [1].

The production of a prompt photon in association with a Z boson is studied in proton-proton collisions at a centre-of-mass energy s = 13 TeV. The analysis uses a data sample with an integrated luminosity of 139 fb −1 collected by the ATLAS detector at the LHC from 2015 to 2018. The production cross-section for the process pp → ℓ +ℓ −γ + X (ℓ = e, μ) is measured within a fiducial phase-space region defined by kinematic requirements on the photon and the leptons, and by isolation requirements on the photon. An experimental precision of 2.9% is achieved for the fiducial cross-section. Differential cross-sections are measured as a function of each of six kinematic variables characterising the ℓ +ℓ −γ system. The data are compared with theoretical predictions based on next-to-leading-order and next-to-next-to-leading-order perturbative QCD calculations. The impact of next-to-leading-order electroweak corrections is also considered. 

Evidence for electroweak production of two jets in association with a Zγ pair in s=13 TeV proton–proton collisions at the Large Hadron Collider is presented. The analysis uses data collected by the ATLAS detector in 2015 and 2016 that corresponds to an integrated luminosity of 36.1fb −1. Events that contain a Z boson candidate decaying leptonically into either e +e − or μ +μ −, a photon, and two jets are selected. The electroweak component is measured with observed and expected significances of 4.1 standard deviations. The fiducial cross-section for electroweak production is measured to be σ Zγjj−EW=7.8±2.0fb, in good agreement with the Standard Model prediction.

The XENON1T liquid xenon time projection chamber is the most sensitive detector built to date for the measurement of direct interactions of weakly interacting massive particles with normal matter. The data acquisition system (DAQ) is constructed from commercial, open source, and custom components to digitize signals from the detector and store them for later analysis. The system achieves an extremely low signal threshold by triggering each channel independently, achieving a single photoelectron acceptance of (93 +/- 3)%, and deferring the global trigger to a later, software stage. The event identification is based on MongoDB database queries and has over 98% efficiency at recognizing interactions at the analysis threshold in the center of the target. A readout bandwidth over 300 MB/s is reached in calibration modes and is further expandable via parallelization. This DAQ system was successfully used during three years of operation of XENON1T.

Inclusive and differential cross-sections for the production of a top-quark pair in association with a photon are measured with proton-proton collision data corresponding to an integrated luminosity of 36.1 fb −1, collected by the ATLAS detector at the LHC in 2015 and 2016 at a centre-of-mass energy of 13 TeV. The measurements are performed in single-lepton and dilepton final states in a fiducial volume. Events with exactly one photon, one or two leptons, a channel-dependent minimum number of jets, and at least one b-jet are selected. Neural network algorithms are used to separate the signal from the backgrounds. The fiducial cross-sections are measured to be 521±9(stat.)±41(sys.) fb and 69±3(stat.)±4(sys.) fb for the single-lepton and dilepton channels, respectively. The differential cross-sections are measured as a function of photon transverse momentum, photon absolute pseudorapidity, and angular distance between the photon and its closest lepton in both channels, as well as azimuthal opening angle and absolute pseudorapidity difference between the two leptons in the dilepton channel. All measurements are in agreement with the theoretical predictions.

This paper presents measurements of charged-particle distributions sensitive to the properties of the underlying event in events containing a Z boson decaying into a muon pair. The data were obtained using the ATLAS detector at the LHC in proton–proton collisions at a centre-of-mass energy of 13 TeV with an integrated luminosity of 3.2 fb −1. Distributions of the charged-particle multiplicity and of the charged-particle transverse momentum are measured in regions of the azimuth defined relative to the Z boson direction. The measured distributions are compared with the predictions of various Monte Carlo generators which implement different underyling event models. The Monte Carlo model predictions qualitatively describe the data well, but with some significant discrepancies.

The momentum measurement capability of the ATLAS muon spectrometer relies fundamentally on the intrinsic single-hit spatial resolution of the monitored drift tube precision tracking chambers. Optimal resolution is achieved with a dedicated calibration program that addresses the specific operating conditions of the 354 000 high-pressure drift tubes in the spectrometer. The calibrations consist of a set of timing offsets and drift time to drift distance transfer relations, and result in chamber resolution functions. This paper describes novel algorithms to obtain precision calibrations from data collected by ATLAS in LHC Run 2 and from a gas monitoring chamber, deployed in a dedicated gas facility. The algorithm output consists of a pair of correction constants per chamber which are applied to baseline calibrations, and determined to be valid for the entire ATLAS Run 2. The final single-hit spatial resolution, averaged over 1172 monitored drift tube chambers, is 81.7 ± 2.2 μm.

Measurements of isolated prompt photon and massive electroweak (W and Z) boson production in different collision systems are of great interest to understand the partonic structure of heavy nuclei, and serve as a constraint on the initial state in larger collision systems. These channels are sensitive to a variety of effects such as the modification of the parton densities in nuclei in certain kinematic regions, and the energy loss of partons as they undergo multiple interactions in the nucleus before the hard parton-parton scattering. High-statistics samples of lead-lead and proton-lead collision data at root s(NN)=5.02 TeV and 8.16 TeV, respectively, taken by the ATLAS experiment at the LHC, as well as proton-proton comparison data at analogous collision energies, allow for a detailed study of these phenomena in data and comprehensive comparisons to the predictions of a variety of theoretical approaches. This paper presents the latest ATLAS results in these topics, including updated results on inclusive prompt photon production in proton-lead collisions over a broad kinematic range and high-precision W boson results in lead lead collisions.

The ratio of the cross sections for inclusive isolated-photon production in pp collisions at centre-of-mass energies of 13 and 8 TeV is measured using the ATLAS detector at the LHC. The integrated luminosities of the 13 TeV and 8 TeV datasets are 3.2 fb(-1) and 20.2 fb(-1), respectively. The ratio is measured as a function of the photon transverse energy in different regions of the photon pseudorapidity. The predictions from next-to-leading-order perturbative QCD calculations are compared with the measured ratio. The experimental systematic uncertainties as well as the uncertainties affecting the predictions are evaluated taking into account the correlations between the two centre-of-mass energies, resulting in a reduction of up to a factor of 2.5 (5) in the experimental (theoretical) systematic uncertainties. The predictions based on several parameterisations of the proton parton distribution functions agree with the data within the reduced experimental and theoretical uncertainties. In addition, this ratio to that of the fiducial cross sections for Z boson production at 13 and 8 TeV using the decay channels Z e(+)e(-) and Z (+-) is made and compared with the theoretical predictions. In this double ratio, a further reduction of the experimental uncertainty is obtained because the uncertainties arising from the luminosity measurement cancel out. The predictions describe the measurements of the double ratio within the theoretical and experimental uncertainties.

The performance of identification algorithms (taggers) for hadronically decaying top quarks and W bosons in pp collisions at = 13TeV recorded by the ATLAS experiment at the Large Hadron Collider is presented. A set of techniques based on jet shape observables are studied to determine a set of optimal cut-based taggers for use in physics analyses. The studies are extended to assess the utility of combinations of substructure observables as a multivariate tagger using boosted decision trees or deep neural networks in comparison with taggers based on two-variable combinations. In addition, for highly boosted top-quark tagging, a deep neural network based on jet constituent inputs as well as a re-optimisation of the shower deconstruction technique is presented. The performance of these taggers is studied in data collected during 2015 and 2016 corresponding to 36.1fb-1 for the tt and +jet and 36.7-1 for the dijet event topologies.

A search for new particles decaying into a pair of top quarks is performed using proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy of ffiffi s p ¼ 13 TeV corresponding to an integrated luminosity of 36.1 fb−1. Events consistent with top-quark pair production and the fully hadronic decay mode of the top quarks are selected by requiring multiple high transverse momentum jets including those containing b-hadrons. Two analysis techniques, exploiting dedicated top-quark pair reconstruction in different kinematic regimes, are used to optimize the search sensitivity to new hypothetical particles over a wide mass range. The invariant mass distribution of the two reconstructed top-quark candidates is examined for resonant production of new particles with various spins and decay widths. No significant deviation from the Standard Model prediction is observed and limits are set on the production cross-section times branching fraction for new hypothetical Z0 bosons, dark-matter mediators, Kaluza-Klein gravitons and KaluzaKlein gluons. By comparing with the predicted production cross sections, the Z0 boson in the topcolorassisted-technicolor model is excluded for masses up to 3.1 3.6 TeV, the dark-matter mediators in a simplified framework are excluded in the mass ranges from 0.8 to 0.9 TeV and from 2.0 to 2.2 TeV, and the Kaluza-Klein gluon is excluded for masses up to 3.4 TeV, depending on the decay widths of the particles.

This Letter describes the observation of the light-by-light scattering process, γγ → γγ, in Pb þ Pb collisions at sNN p ¼ 5.02 TeV. The analysis is conducted using a data sample corresponding to an integrated luminosity of 1.73 nb−1, collected in November 2018 by the ATLAS experiment at the LHC. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy Eγ T > 3 GeV and pseudorapidity jηγ j < 2.4, diphoton invariant mass above 6 GeV, and small diphoton transverse momentum and acoplanarity. After applying all selection criteria, 59 candidate events are observed for a background expectation of 12 3 events. The observed excess of events over the expected background has a significance of 8.2 standard deviations. The measured fiducial cross section is 78 13ðstatÞ 7ðsystÞ 3ðlumiÞ nb.

This paper presents a measurement of forward-forward and forward-central dijet azimuthal angular correlations and conditional yields in proton-proton (pp) and proton-lead (p+Pb) collisions as a probe of the nuclear gluon density in regions where the fraction of the average momentum per nucleon carried by the parton entering the hard scattering is low. In these regions, gluon saturation can modify the rapidly increasing parton distribution function of the gluon. The analysis utilizes 25pb-1 of pp data and 360μb-1 of p+Pb data, both at sNN=5.02 TeV, collected in 2015 and 2016, respectively, with the ATLAS detector at the Large Hadron Collider. The measurement is performed in the center-of-mass frame of the nucleon-nucleon system in the rapidity range between -4.0 and 4.0 using the two highest transverse-momentum jets in each event, with the highest transverse-momentum jet restricted to the forward rapidity range. No significant broadening of azimuthal angular correlations is observed for forward-forward or forward-central dijets in p+Pb compared to pp collisions. For forward-forward jet pairs in the proton-going direction, the ratio of conditional yields in p+Pb collisions to those in pp collisions is suppressed by approximately 20%, with no significant dependence on the transverse momentum of the dijet system. No modification of conditional yields is observed for forward-central dijets.

We present first results on the scalar coupling of weakly interacting massive particles (WIMPs) to pions from 1 t yr of exposure with the XENON1T experiment. This interaction is generated when the WIMP couples to a virtual pion exchanged between the nucleons in a nucleus. In contrast to most nonrelativistic operators, these pion-exchange currents can be coherently enhanced by the total number of nucleons and therefore may dominate in scenarios where spin-independent WIMP-nucleon interactions are suppressed. Moreover, for natural values of the couplings, they dominate over the spin-dependent channel due to their coherence in the nucleus. Using the signal model of this new WIMP-pion channel, no significant excess is found, leading to an upper limit cross section of 6.4 x 10(-46) cm(2) (90% confidence level) at 30 GeV/c(2) WIMP mass.

The XENON1T experiment searches for dark matter particles through their scattering off xenon atoms in a 2 metric ton liquid xenon target. The detector is a dual-phase time projection chamber, which measures simultaneously the scintillation and ionization signals produced by interactions in target volume, to reconstruct energy and position, as well as the type of the interaction. The background rate in the central volume of XENON1T detector is the lowest achieved so far with a liquid xenon-based direct detection experiment. In this work we describe the response model of the detector, the background and signal models, and the statistical inference procedures used in the dark matter searches with a 1 metric ton x year exposure of XENON1T data, that leads to the best limit to date on WIMP-nucleon spin-independent elastic scatter cross section for WIMP masses above 6 GeV/c(2).

The result of a search for the pair production of the lightest supersymmetric partner of the bottom quark (b~1) using 139 fb−1 of proton-proton data collected at s√ = 13 TeV by the ATLAS detector is reported. In the supersymmetric scenarios considered both of the bottom-squarks decay into a b-quark and the second-lightest neutralino, b~1→b+χ~02. Each χ~02 is assumed to subsequently decay with 100% branching ratio into a Higgs boson (h) like the one in the Standard Model and the lightest neutralino: χ~02→h+χ~01. The χ~01 is assumed to be the lightest supersymmetric particle (LSP) and is stable. Two signal mass configurations are targeted: the first has a constant LSP mass of 60 GeV; and the second has a constant mass difference between the χ~02 and χ~01 of 130 GeV. The final states considered contain no charged leptons, three or more b-jets, and large missing transverse momentum. No significant excess of events over the Standard Model background expectation is observed in any of the signal regions considered. Limits at the 95% confidence level are placed in the supersymmetric models considered, and bottom-squarks with mass up to 1.5 TeV are excluded.

The ForwArd Search ExpeRiment (FASER) is an approved experiment dedicated to searching for light, extremely weakly interacting particles at the LHC. Such particles may be produced in the LHC's high-energy collisions and travel long distances through concrete and rock without interacting. They may then decay to visible particles in FASER, which is placed 480 m downstream of the ATLAS interaction point. In this work we briefly describe the FASER detector layout and the status of potential backgrounds. We then present the sensitivity reach for FASER for a large number of long-lived particle models, updating previous results to a uniform set of detector assumptions, and analyzing new models. In particular, we consider all of the renormalizable portal interactions, leading to dark photons, dark Higgs bosons, and heavy neutral leptons; light B - L and L-i - L-j gauge bosons; axion like particles that are coupled dominantly to photons, fermions, and gluons through nonrenormalizable operators; and pseudoscalars with Yukawa-like couplings. We find that FASER and its follow-up, FASER 2, have a full physics program, with discovery sensitivity in all of these models and potentially far-reaching implications for particle physics and cosmology.

Two-neutrino double electron capture (2 nu ECEC) is a second-order weak-interaction process with a predicted half-life that surpasses the age of the Universe by many orders of magnitude(1). Until now, indications of 2 nu ECEC decays have only been seen for two isotopes(2-5), Kr-78 and Ba-130, and instruments with very low background levels are needed to detect them directly with high statistical significance(6,7). The 2 nu ECEC half-life is an important observable for nuclear structure models(8-14) and its measurement represents a meaningful step in the search for neutrinoless double electron capture-the detection of which would establish the Majorana nature of the neutrino and would give access to the absolute neutrino mass(15-17). Here we report the direct observation of 2 nu ECEC in Xe-124 with the XENON1T dark-matter detector. The significance of the signal is 4.4 standard deviations and the corresponding half-life of 1.8 x 10(22) years (statistical uncertainty, 0.5 x 10(22) years; systematic uncertainty, 0.1 x 10(22) years) is the longest measured directly so far. This study demonstrates that the low background and large target mass of xenon-based dark-matter detectors make them well suited for measuring rare processes and highlights the broad physics reach of larger next-generation experiments(18-20).

A search for high-mass dielectron and dimuon resonances in the mass range of 250 GeV to 6TeV is presented. The data were recorded by the ATLAS experiment in proton-proton collisions at a centre-ofmass energy of root s = 13 TeV during Run 2 of the Large Hadron Collider and correspond to an integrated luminosity of 139 fb(-1). A functional form is fitted to the dilepton invariant-mass distribution to model the contribution from background processes, and a generic signal shape is used to determine the significance of observed deviations from this background estimate. No significant deviation is observed and upper limits are placed at the 95% confidence level on the fiducial cross-section times branching ratio for various resonance width hypotheses. The derived limits are shown to be applicable to spin-0, spin-1 and spin-2 signal hypotheses. For a set of benchmark models, the limits are converted into lower limits on the resonance mass and reach 4.5 TeV for the E-6-motivated Z(psi)' boson. Also presented are limits on Heavy Vector Triplet model couplings.

This paper reports on a search for electroweak diboson (WW/WZ/ZZ) production in association with a high-mass dijet system, using data from proton-proton collisions at a center-of-mass energy of s=13 TeV. The data, corresponding to an integrated luminosity of 35.5 fb-1, were recorded with the ATLAS detector in 2015 and 2016 at the Large Hadron Collider. The search is performed in final states in which one boson decays leptonically, and the other boson decays hadronically. The hadronically decaying W/Z boson is reconstructed as either two small-radius jets or one large-radius jet using jet substructure techniques. The electroweak production of WW/WZ/ZZ in association with two jets is measured with an observed (expected) significance of 2.7 (2.5) standard deviations, and the fiducial cross section is measured to be 45.1±8.6(stat.)-14.6+15.9(syst.) fb.

Direct dark matter detection experiments based on a liquid xenon target are leading the search for dark matter particles with masses above similar to 5 GeV/c(2), but have limited sensitivity to lighter masses because of the small momentum transfer in dark matter-nucleus elastic scattering. However, there is an irreducible contribution from inelastic processes accompanying the elastic scattering, which leads to the excitation and ionization of the recoiling atom (the Migdal effect) or the emission of a bremsstrahlung photon. In this Letter, we report on a probe of low-mass dark matter with masses down to about 85 MeV/c(2) by looking for electronic recoils induced by the Migdal effect and bremsstrahlung using data from the XENON1T experiment. Besides the approach of detecting both scintillation and ionization signals, we exploit an approach that uses ionization signals only, which allows for a lower detection threshold. This analysis significantly enhances the sensitivity of XENON1T to light dark matter previously beyond its reach.

We report constraints on light dark matter (DM) models using ionization signals in the XENON1T experiment. We mitigate backgrounds with strong event selections, rather than requiring a scintillation signal, leaving an effective exposure of (22 +/- 3) tonne day. Above similar to 0.4 keV(ee), we observe 30 MeV/c(2), and absorption of dark photons and axionlike particles for m(chi) within 0.186-1 keV/c(2).

A search for singly produced vector-like quarks Q, where Q can be either a T quark with charge +2/3 or a Y quark with charge −4/3, is performed in proton– proton collision data at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 36.1 fb−1 , recorded with the ATLAS detector at the LHC in 2015 and 2016. The analysis targets Q → W b decays where the W boson decays leptonically. No significant deviation from the expected Standard Model background is observed. Upper limits are set on the QW b coupling strength and the mixing between the Standard Model sector and a singlet T quark or a Y quark from a (B, Y ) doublet or a (T, B, Y ) triplet, taking into account the interference effects with the Standard Model background. The upper limits set on the mixing angle are as small as |sin θL| = 0.18 for a singlet T quark of mass 800 GeV, |sin θR| = 0.17 for a Y quark of mass 800 GeV in a (B, Y ) doublet model and |sin θL| = 0.16 for a Y quark of mass 800 GeV in a (T, B, Y ) triplet model. Within a (B, Y ) doublet model, the limits set on the mixing parameter |sin θR| are comparable with the exclusion limits from electroweak precision observables in the mass range between about 900 GeV and 1250 GeV.

Limits are set on the pair production of scalar leptoquarks, where all possible decays of the leptoquark into a quark (t, b) and a lepton (τ , ν) of the third generation are considered. The limits are presented as a function of the leptoquark mass and the branching ratio into charged leptons for up-type (LQu 3 → tν/bτ ) and down-type (LQd 3 → bν/tτ ) leptoquarks. Many results are reinterpretations of previously published ATLAS searches. In all cases, LHC proton-proton collision data at a centre-of-mass energy of √ s = 13 TeV recorded by the ATLAS detector in 2015 and 2016 are used, corresponding to an integrated luminosity of 36.1 fb−1 . Masses below 800 GeV are excluded for both LQu 3 and LQd 3 independently of the branching ratio, with masses below about 1 TeV being excluded for the limiting cases of branching ratios equal to zero or unity.

One correction is made to the figure 4e of the paper. The reported cross-sections, both the experimental upper limits and the theoretical prediction, corresponded only to a fiducial region of the phase space with at least two prompt same-sign electrons or muons before detector simulation (pT > 10 GeV, |η| < 2.8). The updated figure 4e instead presents the results without this requirement, i.e. in the full phase space as it should, given the y-axis label of the plot. The reported sensitivity of the analysis, i.e. the ability to exclude top squark masses below 700 GeV in this scenario, is however unchanged since the same requirements were applied to the analysis and to the theory predictions.

The differential cross section for isolated-photon production in pp collisions is measured at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC using an integrated luminosity of 36.1 fb−1. The differential cross section is presented as a function of the photon transverse energy in different regions of photon pseudorapidity. The differential cross section as a function of the absolute value of the photon pseudorapidity is also presented in different regions of photon transverse energy. Next-to-leading-order QCD calculations from Jetphox and Sherpa as well as next-to-next-to-leading-order QCD calculations from Nnlojet are compared with the measurement, using several parameterisations of the proton parton distribution functions. The predictions provide a good description of the data within the experimental and theoretical uncertainties.

This paper presents measurements of the W +→ μ +ν and W -→ μ -ν cross-sections and the associated charge asymmetry as a function of the absolute pseudorapidity of the decay muon. The data were collected in proton–proton collisions at a centre-of-mass energy of 8 TeV with the ATLAS experiment at the LHC and correspond to a total integrated luminosity of 20.2fb-1. The precision of the cross-section measurements varies between 0.8 and 1.5% as a function of the pseudorapidity, excluding the 1.9% uncertainty on the integrated luminosity. The charge asymmetry is measured with an uncertainty between 0.002 and 0.003. The results are compared with predictions based on next-to-next-to-leading-order calculations with various parton distribution functions and have the sensitivity to discriminate between them.

This paper presents a measurement of ZZ production with the ATLAS detector at the Large Hadron Collider. The measurement is carried out in the final state with two charged leptons and two neutrinos, using data collected during 2015 and 2016 in pp collisions at s = 13 TeV, corresponding to an integrated luminosity of 36.1 fb −1. The integrated cross-sections in the total and fiducial phase spaces are measured with an uncertainty of 7% and compared with Standard Model predictions, and differential measurements in the fiducial phase space are reported. No significant deviations from the Standard Model predictions are observed, and stringent constraints are placed on anomalous couplings corresponding to neutral triple gauge-boson interactions. 

Cross-sections of associated production of a Higgs boson decaying into bottomquark pairs and an electroweak gauge boson, W or Z, decaying into leptons are measured as a function of the gauge boson transverse momentum. The measurements are performed in kinematic fiducial volumes defined in the ‘simplified template cross-section’ framework. The results are obtained using 79.8 fb−1 of proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13 TeV. All measurements are found to be in agreement with the Standard Model predictions, and limits are set on the parameters of an effective Lagrangian sensitive to modifications of the Higgs boson couplings to the electroweak gauge bosons.[Figure not available: see fulltext.].

It has been found that the theoretical predictions for W and Z boson cross sections, and for the W boson charge asymmetry, which are labelled as NNPDF3.1 [1] have in fact been calculated using the NNPDF3.0 PDF set [2] instead. The reported experimental results are not affected.The corrected versions of Figs. 11, 12 and 13 of the paper are presented below.The corrected text discussing the comparison of predicted cross sections to data in Sect. 9.2 of the paper should read:A comparison of the differential cross sections shows that the predictions obtained with the NNPDF3.1 PDF set are in good agreement with the measured values, mainly because the NNPDF3.1 global fit includes high precision LHC measurements of W / Z boson production [3, 4]. On the other hand, the predictions obtained with other recent PDF sets systematically deviate from the measured values.

A measurement of fiducial and differential cross-sections for W+W− production in proton–proton collisions at s√=13 TeV with the ATLAS experiment at the Large Hadron Collider using data corresponding to an integrated luminosity of 36.1 fb−1 is presented. Events with one electron and one muon are selected, corresponding to the decay of the diboson system as WW→e±νμ∓ν. To suppress top-quark background, events containing jets with a transverse momentum exceeding 35 GeV are not included in the measurement phase space. The fiducial cross-section, six differential distributions and the cross-section as a function of the jet-veto transverse momentum threshold are measured and compared with several theoretical predictions. Constraints on anomalous electroweak gauge boson self-interactions are also presented in the framework of a dimension-six effective field theory.

The production cross-sections for W± and Z bosons are measured using ATLAS data corresponding to an integrated luminosity of 4.0 pb−1 collected at a centre-of-mass energy s√=2.76 TeV. The decay channels W→ℓν and Z→ℓℓ are used, where ℓ can be an electron or a muon. The cross-sections are presented for a fiducial region defined by the detector acceptance and are also extrapolated to the full phase space for the total inclusive production cross-section. The combined (average) total inclusive cross-sections for the electron and muon channels are:σtotW+→ℓν=σtotW−→ℓν=σtotZ→ℓℓ=2312±26 (stat.)±27 (syst.)±72 (lumi.)±30 (extr.) pb,1399±21 (stat.) ±17 (syst.)±43 (lumi.)±21 (extr.) pb,323.4±9.8 (stat.)±5.0 (syst.)±10.0 (lumi.)±5.5(extr.) pb.Measured ratios and asymmetries constructed using these cross-sections are also presented. These observables benefit from full or partial cancellation of many systematic uncertainties that are correlated between the different measurements.

A measurement of W± boson production in Pb+Pb collisions at sNN−−−√=5.02 TeV is reported using data recorded by the ATLAS experiment at the LHC in 2015, corresponding to a total integrated luminosity of 0.49nb−1. The W± bosons are reconstructed in the electron or muon leptonic decay channels. Production yields of leptonically decaying W± bosons, normalised by the total number of minimum-bias events and the nuclear thickness function, are measured within a fiducial region defined by the detector acceptance and the main kinematic requirements. These normalised yields are measured separately for W+ and W− bosons, and are presented as a function of the absolute value of pseudorapidity of the charged lepton and of the collision centrality. The lepton charge asymmetry is also measured as a function of the absolute value of lepton pseudorapidity. In addition, nuclear modification factors are calculated using the W± boson production cross-sections measured in pp collisions. The results are compared with predictions based on next-to-leading-order calculations with CT14 parton distribution functions as well as with predictions obtained with the EPPS16 and nCTEQ15 nuclear parton distribution functions. No dependence of normalised production yields on centrality and a good agreement with predictions are observed for mid-central and central collisions. For peripheral collisions, the data agree with predictions within 1.7 (0.9) standard deviations for W− (W+) bosons.

The algorithms used by the ATLAS Collaboration during Run 2 of the Large Hadron Collider to identify jets containing b-hadrons are presented. The performance of the algorithms is evaluated in the simulation and the efficiency with which these algorithms identify jets containing b-hadrons is measured in collision data. The measurement uses a likelihood-based method in a sample highly enriched in tt¯ events. The topology of the t→Wb decays is exploited to simultaneously measure both the jet flavour composition of the sample and the efficiency in a transverse momentum range from 20 to 600 GeV. The efficiency measurement is subsequently compared with that predicted by the simulation. The data used in this measurement, corresponding to a total integrated luminosity of 80.5 fb−1, were collected in proton–proton collisions during the years 2015–2017 at a centre-of-mass energy s√= 13 TeV. By simultaneously extracting both the efficiency and jet flavour composition, this measurement significantly improves the precision compared to previous results, with uncertainties ranging from 1 to 8% depending on the jet transverse momentum.

To assess the properties of the quark–gluon plasma formed in ultrarelativistic ion collisions, the ATLAS experiment at the LHC measures a correlation between the mean transverse momentum and the flow harmonics. The analysis uses data samples of lead–lead and proton–lead collisions obtained at the centre-of-mass energy per nucleon pair of 5.02 TeV, corresponding to total integrated luminosities of 22 μb−1 and 28 nb−1, respectively. The measurement is performed using a modified Pearson correlation coefficient with the charged-particle tracks on an event-by-event basis. The modified Pearson correlation coefficients for the 2nd-, 3rd-, and 4th-order flow harmonics are measured in the lead–lead collisions as a function of event centrality quantified as the number of charged particles or the number of nucleons participating in the collision. The measurements are performed for several intervals of the charged-particle transverse momentum. The correlation coefficients for all studied harmonics exhibit a strong centrality evolution, which only weakly depends on the charged-particle momentum range. In the proton–lead collisions, the modified Pearson correlation coefficient measured for the 2nd-order flow harmonics shows only weak centrality dependence. The lead-lead data is qualitatively described by the predictions based on the hydrodynamical model.

Measurements of K0S and Λ0 production in tt¯ final states have been performed. They are based on a data sample with integrated luminosity of 4.6 fb−1 from proton–proton collisions at a centre-of-mass energy of 7 TeV, collected in 2011 with the ATLAS detector at the Large Hadron Collider. Neutral strange particles are separated into three classes, depending on whether they are contained in a jet, with or without a b-tag, or not associated with a selected jet. The aim is to look for differences in their main kinematic distributions. A comparison of data with several Monte Carlo simulations using different hadronisation and fragmentation schemes, colour reconnection models and different tunes for the underlying event has been made. The production of neutral strange particles in tt¯ dileptonic events is found to be well described by current Monte Carlo models for K0S and Λ0 production within jets, but not for those produced outside jets.

A measurement of the associated production of a top-quark pair (t¯t) with a vector boson (W, Z) in proton-proton collisions at a center-of-mass energy of 13 TeV is presented, using 36.1 fb−1 of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. Events are selected in channels with two same- or opposite-sign leptons (electrons or muons), three leptons or four leptons, and each channel is further divided into multiple regions to maximize the sensitivity of the measurement. The t¯tZ and t¯tW production cross sections are simultaneously measured using a combined fit to all regions. The best-fit values of the production cross sections are σt¯tZ ¼ 0.95 0.08stat 0.10syst pb and σt¯tW ¼ 0.87 0.13stat 0.14syst pb in agreement with the Standard Model predictions. The measurement of the t¯tZ cross section is used to set constraints on effective field theory operators which modify the t¯tZ vertex.

A search for heavy charged long-lived particles is performed using a data sample of 36.1  fb−1 of proton-proton collisions at √s=13TeV collected by the ATLAS experiment at the Large Hadron Collider. The search is based on observables related to ionization energy loss and time of flight, which are sensitive to the velocity of heavy charged particles traveling significantly slower than the speed of light. Multiple search strategies for a wide range of lifetimes, corresponding to path lengths of a few meters, are defined as model independently as possible, by referencing several representative physics cases that yield long-lived particles within supersymmetric models, such as gluinos/squarks (R-hadrons), charginos and staus. No significant deviations from the expected Standard Model background are observed. Upper limits at 95% confidence level are provided on the production cross sections of long-lived R-hadrons as well as directly pair-produced staus and charginos. These results translate into lower limits on the masses of long lived gluino, sbottom and stop R-hadrons, as well as staus and charginos of 2000, 1250, 1340, 430, and 1090 GeV, respectively.

A search for Higgs boson pair production in the b¯bWW∗ decay mode is performed in the b¯b`νqq final state using 36.1 fb−1 of proton-proton collision data at a centreof-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. No evidence of events beyond the background expectation is found. Upper limits on the non-resonant pp → HH production cross section of 10 pb and on the resonant production cross section as a function of the HH invariant mass are obtained. Resonant production limits are set for scalar and spin-2 graviton hypotheses in the mass range 500 to 3000 GeV.

This paper describes a search for events with one top-quark and large missing transverse momentum in the final state. Data collected during 2015 and 2016 by the ATLAS experiment from 13 TeV proton–proton collisions at the LHC corresponding to an integrated luminosity of 36.1 fb−1 are used. Two channels are considered, depending on the leptonic or the hadronic decays of the W boson from the top quark. The obtained results are interpreted in the context of simplified models for dark-matter production and for the single production of a vector-like T quark. In the absence of significant deviations from the Standard Model background expectation, 95% confidence-level upper limits on the corresponding production cross-sections are obtained and these limits are translated into constraints on the parameter space of the models considered.

A search for flavour-changing neutral current decays of a top quark into an up-type quark (q = u, c) and the Standard Model Higgs boson, t → Hq, is presented. The search is based on a dataset of pp collisions at √ s = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 36.1 fb−1 . Two complementary analyses are performed to search for top-quark pair events in which one top quark decays into W b and the other top quark decays into Hq, and target the H → b ¯b and H → τ +τ − decay modes, respectively. The high multiplicity of b-quark jets, or the presence of hadronically decaying τ -leptons, is exploited in the two analyses respectively. Multivariate techniques are used to separate the signal from the background, which is dominated by top-quark pair production. No significant excess of events above the background expectation is found, and 95% CL upper limits on the t → Hq branching ratios are derived. The combination of these searches with ATLAS searches in diphoton and multilepton final states yields observed (expected) 95% CL upper limits on the t → Hc and t → Hu branching ratios of 1.1×10−3 (8.3×10−4 ) and 1.2×10−3 (8.3 × 10−4 ), respectively. The corresponding combined observed (expected) upper limits on the |λtcH| and |λtuH| couplings are 0.064 (0.055) and 0.066 (0.055), respectively.

A search for a pair of neutral, scalar bosons with each decaying into two W bosons is presented using 36.1 fb−1 of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. This search uses three production models: non-resonant and resonant Higgs boson pair production and resonant production of a pair of heavy scalar particles. Three final states, classified by the number of leptons, are analysed: two same-sign leptons, three leptons, and four leptons. No significant excess over the expected Standard Model backgrounds is observed. An observed (expected) 95% confidence-level upper limit of 160 (120) times the Standard Model prediction of non-resonant Higgs boson pair production cross-section is set from a combined analysis of the three final states. Upper limits are set on the production cross-section times branching ratio of a heavy scalar X decaying into a Higgs boson pair in the mass range of 260 GeV ≤ mX ≤ 500 GeV and the observed (expected) limits range from 9.3 (10) pb to 2.8 (2.6) pb. Upper limits are set on the production cross-section times branching ratio of a heavy scalar X decaying into a pair of heavy scalars S for mass ranges of 280 GeV ≤ mX ≤ 340 GeV and 135 GeV ≤ mS ≤ 165 GeV and the observed (expected) limits range from 2.5 (2.5) pb to 0.16 (0.17) pb.

Constraints on selected mediator-based dark matter models and a scalar dark energy model using up to 37 fb−1 √s = 13 TeV pp collision data collected by the ATLAS detector at the LHC during 2015–2016 are summarised in this paper. The results of experimental searches in a variety of final states are interpreted in terms of a set of spin1 and spin-0 single-mediator dark matter simplified models and a second set of models involving an extended Higgs sector plus an additional vector or pseudo-scalar mediator. The searches considered in this paper constrain spin-1 leptophobic and leptophilic mediators, spin-0 colour-neutral and colour-charged mediators and vector or pseudo-scalar mediators embedded in extended Higgs sector models. In this case, also √s = 8 TeV pp collision data are used for the interpretation of the results. The results are also interpreted for the first time in terms of light scalar particles that could contribute to the accelerating expansion of the universe (dark energy).

This paper presents measurements of W ±Z production cross sections in pp collisions at a centre-of-mass energy of 13 TeV. The data were collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider, and correspond to an integrated luminosity of 36.1fb-1. The W ±Z candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The measured inclusive cross section in the detector fiducial region for a single leptonic decay mode is σW±Z→ℓ′νℓℓfid.=63.7±1.0(stat.)±2.3(syst.)±1.4(lumi.) fb, reproduced by the next-to-next-to-leading-order Standard Model prediction of 61.5-1.3+1.4 fb. Cross sections for W +Z and W -Z production and their ratio are presented as well as differential cross sections for several kinematic observables. An analysis of angular distributions of leptons from decays of W and Z bosons is performed for the first time in pair-produced events in hadronic collisions, and integrated helicity fractions in the detector fiducial region are measured for the W and Z bosons separately. Of particular interest, the longitudinal helicity fraction of pair-produced vector bosons is also measured.

We report on the response of liquid xenon to low energy electronic recoils below 15 keV from beta decays of tritium at drift fields of 92 V/cm, 154 V/cm and 366 V/cm using the XENON100 detector. A data-to-simulation fitting method based on Markov Chain Monte Carlo is used to extract the photon yields and recombination fluctuations from the experimental data. The photon yields measured at the two lower fields are in agreement with those from literature; additional measurements at a higher field of 366 V/cm are presented. The electronic and nuclear recoil discrimination as well as its dependence on the drift field and photon detection efficiency are investigated at these low energies. The results provide new measurements in the energy region of interest for dark matter searches using liquid xenon.

A search for an invisibly decaying Higgs boson or dark matter candidates produced in association with a leptonically decaying Z boson in proton–proton collisions at s=13 TeV is presented. This search uses 36.1 fb−1 of data collected by the ATLAS experiment at the Large Hadron Collider. No significant deviation from the expectation of the Standard Model backgrounds is observed. Assuming the Standard Model ZH production cross-section, an observed (expected) upper limit of 67% (39%) at the 95% confidence level is set on the branching ratio of invisible decays of the Higgs boson with mass mH=125 GeV. The corresponding limits on the production cross-section of the ZH process with the invisible Higgs boson decays are also presented. Furthermore, exclusion limits on the dark matter candidate and mediator masses are reported in the framework of simplified dark matter models.

Searches for new heavy resonances decaying into different pairings of W, Z, or Higgs bosons, as well as dirffiffiffiectly into leptons, are presented using a data sample corresponding to 36.1 fb(-1) of pp collisions at root s = 13 TeV collected during 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Analyses selecting bosonic decay modes in the qqqq, vvqq, evqq, eeqq, evev, eevv, evee, eeee, qqbb, vvbb, evbb, and eebb final states are combined, searching for a narrow-width resonance. Likewise, analyses selecting the leptonic ev and ee final states are also combined. These two sets of analyses are then further combined. No significant deviation from the Standard Model predictions is observed. Three benchmark models are tested: a model predicting the existence of a new heavy scalar singlet, a simplified model predicting a heavy vector-boson triplet, and a bulk Randall-Sundrum model with a heavy spin-2 Kaluza-Klein excitation of the graviton. Cross section limits are set at the 95% confidence level using an asymptotic approximation and are compared with predictions for the benchmark models. These limits are also expressed in terms of constraints on couplings of the heavy vector-boson triplet to quarks, leptons, and the Higgs boson. The data exclude a heavy vector-boson triplet with mass below 5.5 TeV in a weakly coupled scenario and 4.5 TeV in a strongly coupled scenario, as well as a Kaluza-Klein graviton with mass below 2.3 TeV.

EBEX was a long-duration balloon-borne experiment to measure the polarization of the cosmic microwave background. The experiment had three frequency bands centered at 150, 250, and 410 GHz and was the first to use a kilopixel array of transition edge sensor bolometers aboard a balloon platform. We describe the design and characterization of the array and the readout system. From the lowest to highest frequency, the median measured detectors' average thermal conductances were 39, 53, and 63 pW/K, the medians of transition temperatures were 0.45, 0.48, and 0.47 K, and the medians of normal resistances were 1.9, 1.5, and 1.4 Omega; we also give the measured distributions. With the exception of the thermal conductance at 150 GHz, all measured values are within 30% of their design. We measure median low-loop-gain time constants tau(0) = 88, 46, and 57 ms. Two measurements of bolometer absorption efficiency gave results consistent within 10% and showing high (similar to 0.9) efficiency at 150 GHz and medium (similar to 0.35 and similar to 0.25) efficiency at the two higher bands. We measure a median total optical power absorbed of 3.6, 5.3, and 5.0 pW. EBEX pioneered the use of the digital version of the frequency domain multiplexing system. We multiplexed the bias and readout of 16 bolometers onto two wires. The median per-detector noise-equivalent temperatures are 400, 920, and 14,500 mu K-cmb root s. We compare these values to our preflight predictions and to a previous balloon payload. We discuss the sources of excess noise and the path for a future payload to make full use of the balloon environment.

The E and B Experiment (EBEX) was a long-duration balloon-borne cosmic microwave background (CMB) polarimeter that flew over Antarctica in 2012. We describe the experiment's optical system, receiver, and polarimetric approach and report on their in-flight performance. EBEX had three frequency bands centered on 150, 250, and 410. GHz. To make efficient use of limited mass and space, we designed a 115 cm(2) sr high-throughput optical system that had two ambient temperature mirrors and four antireflection-coated polyethylene lenses per focal plane. All frequency bands shared the same optical train. Polarimetry was achieved with a continuously rotating achromatic half-wave plate (AHWP) that was levitated with a superconducting magnetic bearing (SMB). This is the first use of an SMB in astrophysics. Rotation stability was 0.45% over a period of 10 hr, and angular position accuracy was 0 degrees.01. The measured modulation efficiency was above 90% for all bands. To our knowledge the 109% fractional bandwidth of the AHWP was the broadest implemented to date. The receiver, composed of one lens and the AHWP at a temperature of 4 K, the polarizing grid and other lenses at 1 K, and the two focal planes at 0.25 K, performed according to specifications, giving focal plane temperature stability with a fluctuation power spectrum that had a 1/f knee at 2 mHz. EBEX was the first balloon-borne instrument to implement technologies characteristic of modern CMB polarimeters, including high-throughput optical systems, and large arrays of transition edge sensor bolometric detectors with multiplexed readouts.

This Letter presents a measurement of γγ→μ+μ- production in Pb+Pb collisions recorded by the ATLAS detector at the Large Hadron Collider at sNN=5.02 TeV with an integrated luminosity of 0.49 nb-1. The azimuthal angle and transverse momentum correlations between the muons are measured as a function of collision centrality. The muon pairs are produced from γγ through the interaction of the large electromagnetic fields of the nuclei. The contribution from background sources of muon pairs is removed using a template fit method. In peripheral collisions, the muons exhibit a strong back-to-back correlation consistent with previous measurements of muon pair production in ultraperipheral collisions. The angular correlations are observed to broaden significantly in central collisions. The modifications are qualitatively consistent with rescattering of the muons while passing through the hot matter produced in the collision.

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton-proton collision data corresponding to an integrated luminosity of 36.1 fb−1 at a centre-of-mass energy of 13 TeV collected in 2015 and 2016 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons (e or μ). Several signal regions are considered with increasing requirements on the missing transverse momentum above 250 GeV. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits in models with pair-produced weakly interacting dark-matter candidates, large extra spatial dimensions, and supersymmetric particles in several compressed scenarios.[Figure not available: see fulltext.].

We report on a search for weakly interacting massive particles (WIMPs) using 278.8 days of data collected with the XENON1T experiment at LNGS. XENON1T utilizes a liquid xenon time projection chamber with a fiducial mass of (1.30 +/- 0.01) ton, resulting in a 1.0 ton yr exposure. The energy region of interest, [1.4; 10.6] keV(ee) ([4.9; 40.9] keV(nr)), exhibits an ultralow electron recoil background rate of [82(-3)(+5) (syst) +/- 3 stat)] events/ton yr keV(ee)). No significant excess over background is found, and a profile likelihood analysis parametrized in spatial and energy dimensions excludes new parameter space for the WIMP-nucleon spin-independent elastic scatter cross section for WIMP masses above 6 GeV/c(2), with a minimum of 4.1 x 10(-47) cm(2) at 30 GeV/c(2) and a 90% confidence level.

A search for a heavy neutral Higgs boson, A, decaying into a Z boson and another heavy Higgs boson, H, is performed using a data sample corresponding to an integrated luminosity of 36.1 fb−1 from proton–proton collisions at s=13 TeV recorded in 2015 and 2016 by the ATLAS detector at the Large Hadron Collider. The search considers the Z boson decaying to electrons or muons and the H boson into a pair of b-quarks. No evidence for the production of an A boson is found. Considering each production process separately, the 95% confidence-level upper limits on the pp→A→ZH production cross-section times the branching ratio H→bb are in the range of 14–830 fb for the gluon–gluon fusion process and 26–570 fb for the b-associated process for the mass ranges 130–700 GeV of the H boson and 230–800 GeV of the A boson. The results are interpreted in the context of two-Higgs-doublet models.

A search for the narrow structure, X(5568), reported by the D0 Collaboration in the decay sequence X→Bs0π±, Bs0→J/ψφ, is presented. The analysis is based on a data sample recorded with the ATLAS detector at the LHC corresponding to 4.9 fb-1 of pp collisions at 7 TeV and 19.5 fb-1 at 8 TeV. No significant signal was found. Upper limits on the number of signal events, with properties corresponding to those reported by D0, and on the X production rate relative to Bs0 mesons, ρX, were determined at 95% confidence level. The results are N(X)

A search for heavy neutral Higgs bosons and Z′ bosons is performed using a data sample corresponding to an integrated luminosity of 36.1 fb−1 from proton-proton collisions at s=13 TeV recorded by the ATLAS detector at the LHC during 2015 and 2016. The heavy resonance is assumed to decay to τ+τ− with at least one tau lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2-2.25 TeV for Higgs bosons and 0.2-4.0 TeV for Z′ bosons. The data are in good agreement with the background predicted by the Standard Model. The results are interpreted in benchmark scenarios. In the context of the hMSSM scenario, the data exclude tan β > 1.0 for mA = 0.25 TeV and tan β > 42 for mA = 1.5 TeV at the 95% confidence level. For the Sequential Standard Model, ZSSM′ with mZ′ < 2.42 TeV is excluded at 95% confidence level, while ZNU′ with mZ ′ < 2.25 TeV is excluded for the non-universal G(221) model that exhibits enhanced couplings to third-generation fermions.

A search is presented for the direct pair production of the stop, the supersymmetric partner of the top quark, that decays through an R-parity-violating coupling to a final state with two leptons and two jets, at least one of which is identified as a b-jet. The data set corresponds to an integrated luminosity of 36.1 fb-1 of proton-proton collisions at a center-of-mass energy of √s= 13 TeV, collected in 2015 and 2016 by the ATLAS detector at the LHC. No significant excess is observed over the Standard Model background, and exclusion limits are set on stop pair production at a 95% confidence level. Lower limits on the stop mass are set between 600 GeV and 1.5 TeV for branching ratios above 10% for decays to an electron or muon and a b-quark.

A search for charged Higgs bosons heavier than the top quark and decaying via H± → tb is presented. The data analysed corresponds to 36.1 fb−1 of pp collisions at s=13 TeV and was recorded with the ATLAS detector at the LHC in 2015 and 2016. The production of a charged Higgs boson in association with a top quark and a bottom quark, pp → tbH±, is explored in the mass range from mH± = 200 to 2000 GeV using multi-jet final states with one or two electrons or muons. Events are categorised according to the multiplicity of jets and how likely these are to have originated from hadronisation of a bottom quark. Multivariate techniques are used to discriminate between signal and background events. No significant excess above the background-only hypothesis is observed and exclusion limits are derived for the production cross-section times branching ratio of a charged Higgs boson as a function of its mass, which range from 2.9 pb at mH± = 200 GeV to 0.070 pb at mH± = 2000 GeV. The results are interpreted in two benchmark scenarios of the Minimal Supersymmetric Standard Model.

Charged Higgs bosons produced either in top-quark decays or in association with a top-quark, subsequently decaying via H± → τ±ντ, are searched for in 36.1 fb−1 of proton-proton collision data at s=13 TeV recorded with the ATLAS detector. Depending on whether the top-quark produced together with H± decays hadronically or leptonically, the search targets τ+jets and τ+lepton final states, in both cases with a hadronically decaying τ-lepton. No evidence of a charged Higgs boson is found. For the mass range of mH± = 90–2000 GeV, upper limits at the 95% confidence level are set on the production cross-section of the charged Higgs boson times the branching fraction ℬ (H±→ τ±ντ) in the range 4.2–0.0025 pb. In the mass range 90–160 GeV, assuming the Standard Model cross-section for tt¯ production, this corresponds to upper limits between 0.25% and 0.031% for the branching fraction ℬ(t→bH±)×ℬ(H±→τ±ντ).[Figure not available: see fulltext.]

Direct searches for lepton flavor violation in decays of the Z boson with the ATLAS detector at the LHC are presented. Decays of the Z boson into an electron or muon and a hadronically decaying τ lepton are considered. The searches are based on a data sample of proton-proton collisions collected by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb-1 at a center-of-mass energy of s=13 TeV. No statistically significant excess of events above the expected background is observed, and upper limits on the branching ratios of lepton-flavor-violating decays are set at the 95% confidence level: B(Z→eτ)

A search for dark matter (DM) particles produced in association with a hadronically decaying vector boson is performed using pp collision data at a centre-of-mass energy of s=13 TeV corresponding to an integrated luminosity of 36.1 fb−1, recorded by the ATLAS detector at the Large Hadron Collider. This analysis improves on previous searches for processes with hadronic decays of W and Z bosons in association with large missing transverse momentum (mono-W/Z searches) due to the larger dataset and further optimization of the event selection and signal region definitions. In addition to the mono-W/Z search, the as yet unexplored hypothesis of a new vector boson Z′ produced in association with dark matter is considered (mono-Z′ search). No significant excess over the Standard Model prediction is observed. The results of the mono-W/Z search are interpreted in terms of limits on invisible Higgs boson decays into dark matter particles, constraints on the parameter space of the simplified vector-mediator model and generic upper limits on the visible cross sections for W/Z+DM production. The results of the mono-Z′ search are shown in the framework of several simplified-model scenarios involving DM production in association with the Z′ boson.

A search for weakly interacting massive dark-matter particles produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and missing transverse momentum are considered. The analysis uses 36.1fb-1 of proton–proton collision data recorded by the ATLAS experiment at s=13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are interpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour-neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross-section of 300 times the predicted rate for mediators with masses between 10 and 50GeV and assuming a dark-matter mass of 1GeV and unitary coupling. Constraints on colour-charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35GeV, mediator particles with mass below 1.1TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements.

A search for doubly charged Higgs bosons with pairs of prompt, isolated, highly energetic leptons with the same electric charge is presented. The search uses a proton–proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to 36.1 fb - 1 of integrated luminosity recorded in 2015 and 2016 by the ATLAS detector at the LHC. This analysis focuses on the decays H± ±→ e±e±, H± ±→ e±μ± and H± ±→ μ±μ±, fitting the dilepton mass spectra in several exclusive signal regions. No significant evidence of a signal is observed and corresponding limits on the production cross-section and consequently a lower limit on m(H± ±) are derived at 95% confidence level. With ℓ±ℓ±= e±e±/ μ±μ±/ e±μ±, the observed lower limit on the mass of a doubly charged Higgs boson only coupling to left-handed leptons varies from 770 to 870 GeV (850 GeV expected) for B(H± ±→ ℓ±ℓ±) = 100 % and both the expected and observed mass limits are above 450 GeV for B(H± ±→ ℓ±ℓ±) = 10 % and any combination of partial branching ratios.

It has been found that Figure 30 shows the 68% and 99% confidence-level contours for the W boson and top quark mass measurements, instead of the 68% and 95% confidence-level contours, as stated in the legend.

A search for electroweak production of supersymmetric particles in scenarios with compressed mass spectra in final states with two low-momentum leptons and missing transverse momentum is presented. This search uses proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015-2016, corresponding to 36.1 fb-1 of integrated luminosity at s=13 TeV. Events with same-flavor pairs of electrons or muons with opposite electric charge are selected. The data are found to be consistent with the Standard Model prediction. Results are interpreted using simplified models of R-parity-conserving supersymmetry in which there is a small mass difference between the masses of the produced supersymmetric particles and the lightest neutralino. Exclusion limits at 95% confidence level are set on next-to-lightest neutralino masses of up to 145 GeV for Higgsino production and 175 GeV for wino production, and slepton masses of up to 190 GeV for pair production of sleptons. In the compressed mass regime, the exclusion limits extend down to mass splittings of 2.5 GeV for Higgsino production, 2 GeV for wino production, and 1 GeV for slepton production. The results are also interpreted in the context of a radiatively-driven natural supersymmetry model with nonuniversal Higgs boson masses.

A measurement of J/ ψ and ψ(2 S) production is presented. It is based on a data sample from Pb+Pb collisions at sNN=5.02TeV and pp collisions at s=5.02TeV recorded by the ATLAS detector at the LHC in 2015, corresponding to an integrated luminosity of 0.42nb-1 and 25pb-1 in Pb+Pb and pp, respectively. The measurements of per-event yields, nuclear modification factors, and non-prompt fractions are performed in the dimuon decay channel for 9

Measurements of ZZ production in the +-′+′- channel in proton-proton collisions at 13 TeV center-of-mass energy at the Large Hadron Collider are presented. The data correspond to 36.1 fb-1 of collisions collected by the ATLAS experiment in 2015 and 2016. Here and ′ stand for electrons or muons. Integrated and differential ZZ→+-′+′- cross sections with Z→+- candidate masses in the range of 66 GeV to 116 GeV are measured in a fiducial phase space corresponding to the detector acceptance and corrected for detector effects. The differential cross sections are presented in bins of twenty observables, including several that describe the jet activity. The integrated cross section is also extrapolated to a total phase space and to all standard model decays of Z bosons with mass between 66 GeV and 116 GeV, resulting in a value of 17.3±0.9[±0.6(stat)±0.5(syst)±0.6(lumi)] pb. The measurements are found to be in good agreement with the standard model. A search for neutral triple gauge couplings is performed using the transverse momentum distribution of the leading Z boson candidate. No evidence for such couplings is found and exclusion limits are set on their parameters.

We present the first search for dark matter-induced delayed coincidence signals in a dual-phase xenon time projection chamber, using the 224.6 live days of the XENON100 science run II. This very distinct signature is predicted in the framework of magnetic inelastic dark matter which has been proposed to reconcile the modulation signal reported by the DAMA/LIBRA collaboration with the null results from other direct detection experiments. No candidate event has been found in the region of interest and upper limits on the WIMP's magnetic dipole moment are derived. The scenarios proposed to explain the DAMA/LIBRA modulation signal by magnetic inelastic dark matter interactions of WIMPs with masses of 58.0 GeV/c(2) and 122.7 GeV/c(2) are excluded at 3.3 sigma and 9.3 sigma, respectively.

We report on weakly interacting massive particles (WIMPs) search results in the XENON100 detector using a nonrelativistic effective field theory approach. The data from science run II (34 kg x 224.6 live days) were reanalyzed, with an increased recoil energy interval compared to previous analyses, ranging from (6.6-240) keV(nr). The data are found to be compatible with the background-only hypothesis. We present 90% confidence level exclusion limits on the coupling constants of WIMP-nucleon effective operators using a binned profile likelihood method. We also consider the case of inelastic WIMP scattering, where incident WIMPs may up-scatter to a higher mass state, and set exclusion limits on this model as well.

We present the first constraints on the spin-dependent, inelastic scattering cross section of weakly interacting massive particles (WIMPs) on nucleons from XENON100 data with an exposure of 7.64 x 10(3) kg . days. XENON100 is a dual-phase xenon time projection chamber with 62 kg of active mass, operated at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy and designed to search for nuclear recoils from WIMP-nucleus interactions. Here we explore inelastic scattering, where a transition to a low-lying excited nuclear state of Xe-129 is induced. The experimental signature is a nuclear recoil observed together with the prompt deexcitation photon. We see no evidence for such inelastic WIMP-Xe-129 interactions. A profile likelihood analysis allows us to set a 90% C.L. upper limit on the inelastic, spin-dependent WIMP-nucleon cross section of 3.3 x 10(-38) cm(2) at 100 GeV/c(2). This is the most constraining result to date, and sets the pathway for an analysis of this interaction channel in upcoming, larger dual-phase xenon detectors.

Two-neutrino double electron capture is a rare nuclear decay where two electrons are simultaneously captured from the atomic shell. For Xe-124 this process has not yet been observed and its detection would provide a new reference for nuclear matrix element calculations. We have conducted a search for two-neutrino double electron capture from the K shell of 124Xe using 7636 kg d of data from the XENON100 dark matter detector. Using a Bayesian analysis we observed no significant excess above background, leading to a lower 90% credibility limit on the half-life T-1/2 > 6.5 x 10(20) yr. We have also evaluated the sensitivity of the XENON1T experiment, which is currently being commissioned, and found a sensitivity of T-1/2 > 6.1 x 10(22) yr after an exposure of 2 t yr.

We report on a search for electronic recoil event rate modulation signatures in the XENON100 data accumulated over a period of 4 yr, from January 2010 to January 2014. A profile likelihood method, which incorporates the stability of the XENON100 detector and the known electronic recoil background model, is used to quantify the significance of periodicity in the time distribution of events. There is a weak modulation signature at a period of 431(-14)(+16) day in the low energy region of (2.0-5.8) keV in the single scatter event sample, with a global significance of 1.9 sigma; however, no other more significant modulation is observed. The significance of an annual modulation signature drops from 2.8 sigma, from a previous analysis of a subset of this data, to 1.8 sigma with all data combined. Single scatter events in the low energy region are thus used to exclude the DAMA/LIBRA annual modulation as being due to dark matter electron interactions via axial vector coupling at 5.7 sigma.

A Rn-220 source is deployed on the XENON100 dark matter detector in order to address the challenges in calibration of tonne-scale liquid noble element detectors. We show that the Pb-212 beta emission can be used for low-energy electronic recoil calibration in searches for dark matter. The isotope spreads throughout the entire active region of the detector, and its activity naturally decays below background level within a week after the source is closed. We find no increase in the activity of the troublesome Rn-222 background after calibration. Alpha emitters are also distributed throughout the detector and facilitate calibration of its response to Rn-222. Using the delayed coincidence of Rn-220-Po-216, we map for the first time the convective motion of particles in the XENON100 detector. Additionally, we make a competitive measurement of the half-life of Po-212, t(1/2) = (293.9 +/- (1.0)(stat) +/- (0.6)(sys)) ns.

The XENON1T experiment aims for the direct detection of dark matter in a detector filled with 3.3 tons of liquid xenon. In order to achieve the desired sensitivity, the background induced by radioactive decays inside the detector has to be sufficiently low. One major contributor is the beta-emitter Kr-85 which is present in the xenon. For XENON1T a concentration of natural krypton in xenon Kr-nat/Xe

A search is conducted for new resonant and non-resonant high-mass phenomena in dielectron and dimuon fi nal states. The search uses 36 : 1 fb(-1) of proton-proton collision data, collected at root s = 13TeV by the ATLAS experiment at the LHC in 2015 and 2016. No signi fi cant deviation from the Standard Model prediction is observed. Upper limits at 95% credibility level are set on the cross-section times branching ratio for resonances decaying into dileptons, which are converted to lower limits on the resonance mass, up to 4.1 TeV for the E-6 -motivated Z(X)'. Lower limits on the qqll contact interaction scale are set between 2.4 TeV and 40 TeV, depending on the model.

The XENON1T experiment at the Laboratori Nazionali del Gran Sasso (LNGS) is the first WIMP dark matter detector operating with a liquid xenon target mass above the ton-scale. Out of its 3.2 t liquid xenon inventory, 2.0 t constitute the active target of the dual-phase time projection chamber. The scintillation and ionization signals from particle interactions are detected with low-background photomultipliers. This article describes the XENON1T instrument and its subsystems as well as strategies to achieve an unprecedented low background level. First results on the detector response and the performance of the subsystems are also presented.

We report the first dark matter search results from XENON1T, a similar to 2000-kg-target-mass dual-phase (liquid-gas) xenon time projection chamber in operation at the Laboratori Nazionali del Gran Sasso in Italy and the first ton-scale detector of this kind. The blinded search used 34.2 live days of data acquired between November 2016 and January 2017. Inside the (1042 +/- 12)-kg fiducial mass and in the [5, 40] keV(nr) energy range of interest for weakly interacting massive particle (WIMP) dark matter searches, the electronic recoil background was (1.93 +/- 0.25) x 10(-4) events/(kg x day x keV(ee)), the lowest ever achieved in such a dark matter detector. A profile likelihood analysis shows that the data are consistent with the background-only hypothesis. We derive the most stringent exclusion limits on the spin-independent WIMP-nucleon interaction cross section for WIMP masses above 10 GeV/c(2), with a minimum of 7.7 x 10(-47) cm(2) for 35-GeV/c(2) WIMPs at 90% C.L.

From the ATLAS Phase-I upgrade and onward, new or upgraded detectors and trigger systems will be interfaced to the data acquisition, detector control and timing (TTC) systems by the Front-End Link eXchange (FELIX). FELIX is the core of the new ATLAS Trigger/DAQ architecture. Functioning as a router between custom serial links and a commodity network, FELIX is implemented by server PCs with commodity network interfaces and PCIe cards with large FPGAs and many high speed serial fiber transceivers. By separating data transport from data manipulation, the latter can be done by software in commodity servers attached to the network. Replacing traditional point-to-point links between Front-end components and the DAQ system by a switched network, FELIX provides scaling, flexibility uniformity and upgradability and reduces the diversity of custom hardware solutions in favour of software.

A search for physics beyond the Standard Model, in final states with at least one high transverse momentum charged lepton (electron or muon) and two additional high transverse momentum leptons or jets, is performed using 3.2 fb (1) of proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015 at root s = 13 TeV. The upper end of the distribution of the scalar sum of the transverse momenta of leptons and jets is sensitive to the production of high-mass objects. No excess of events beyond Standard Model predictions is observed. Exclusion limits are set for models of microscopic black holes with two to six extra dimensions. 

We perform a low-mass dark matter search using an exposure of 30 kg x yr with the XENON100 detector. By dropping the requirement of a scintillation signal and using only the ionization signal to determine the interaction energy, we lowered the energy threshold for detection to 0.7 keV for nuclear recoils. No dark matter detection can be claimed because a complete background model cannot be constructed without a primary scintillation signal. Instead, we compute an upper limit on the WIMP-nucleon scattering cross section under the assumption that every event passing our selection criteria could be a signal event. Using an energy interval from 0.7 keV to 9.1 keV, we derive a limit on the spin-independent WIMP-nucleon cross section that excludes WIMPs with a mass of 6 GeV/c(2) above 1.4 x 10(-41) cm(2) at 90% confidence level.

In the upgrade process of the ATLAS detector, the innermost stations of the endcaps (Small Wheels) will be replaced. The New Small Wheel will have two chamber technologies, small-strip Thin Gap Chambers and Micromegas, each providing triggering and precision track measurement. Custom front-end Application Specific Integrated Circuits will be used to read and filter information from both types of detectors. In the context of the New Small Wheel data path, the Read Out Controller ASIC is used for handling, preprocessing and formatting the data generated by the VMM upstream chips. The Read Out Controller will concentrate the data streams from 8 VMMs, filter data based on the ATLAS Level-1 trigger which identifies bunch crossings of interest and transmit the data to FELIX via the L1DDC. The Read Out Controller is composed of 8 VMM Capture modules, a cross-bar and 4 sROC modules. The output data is sent via up to 4 serial links with a configurable speed of 80, 160 or 320 Mbps per link.

For new detector and trigger systems to be installed in the ATLAS experiment after LHC Run 2, a new approach will be followed for Front-End electronics interfacing. The FELIX (Front-End LInk eXchange) system will function as gateway connecting: on one side to detector and trigger electronics links, as well as providing timing and trigger information; and on the other side a commodity switched network built using standard technology (either Ethernet or Infiniband). The new approach is described in this paper, and results achieved so far are presented.

The XENON1T experiment is currently in the commissioning phase at the Laboratori Nazionali del Gran Sasso, Italy. In this article we study the experiment's expected sensitivity to the spin-independent WIMP-nucleon interaction cross section, based on Monte Carlo predictions of the electronic and nuclear recoil backgrounds.The total electronic recoil background in 1 tonne fiducial volume and (1, 12) keV electronic recoil equivalent energy region, before applying any selection to discriminate between electronic and nuclear recoils, is (1.80 ± 0.15) centerdot 10−4 (kgcenterdotdaycenterdotkeV)−1, mainly due to the decay of 222Rn daughters inside the xenon target. The nuclear recoil background in the corresponding nuclear recoil equivalent energy region (4, 50) keV, is composed of (0.6 ± 0.1) (tcenterdoty)−1 from radiogenic neutrons, (1.8 ± 0.3) centerdot 10−2 (tcenterdoty)−1 from coherent scattering of neutrinos, and less than 0.01 (tcenterdoty)−1 from muon-induced neutrons. The sensitivity of XENON1T is calculated with the Profile Likelihood Ratio method, after converting the deposited energy of electronic and nuclear recoils into the scintillation and ionization signals seen in the detector. We take into account the systematic uncertainties on the photon and electron emission model, and on the estimation of the backgrounds, treated as nuisance parameters. The main contribution comes from the relative scintillation efficiency Script Leff, which affects both the signal from WIMPs and the nuclear recoil backgrounds. After a 2 y measurement in 1 t fiducial volume, the sensitivity reaches a minimum cross section of 1.6 centerdot 10−47 cm2 at mχ = 50 GeV/c2.

The ATLAS Phase-I upgrade (2019) requires a Trigger and Data Acquisition (TDAQ) system able to trigger and record data from up to three times the nominal LHC instantaneous luminosity. The Front-End LInk eXchange (FELIX) system provides an infrastructure to achieve this in a scalable, detector agnostic and easily upgradeableway. It is a PC-based gateway, interfacing custom radiation tolerant optical links from front-end electronics, via PCIe Gen3 cards, to a commodity switched Ethernet or InfiniBand network. FELIX enables reducing custom electronics in favour of software running on commercial servers. The FELIX system, the design of the PCIe prototype card and the integration test results are presented in this paper.

The ATLAS experiment has performed extensive searches for the electroweak production of charginos, neutralinos, and staus. This article summarizes and extends the search for electroweak supersymmetry with new analyses targeting scenarios not covered by previously published searches. New searches use vector-boson fusion production, initial-state radiation jets, and low-momentum lepton final states, as well as multivariate analysis techniques to improve the sensitivity to scenarios with small mass splittings and low-production cross sections. Results are based on 20 fb(-1) of proton-proton collision data at root s = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. The new and existing searches are combined and interpreted in terms of 95% confidence-level exclusion limits in simplified models, where a single production process and decay mode is assumed, as well as within phenomenological supersymmetric models.

This article presents the sensitivity of the ATLAS experiment to the lepton-flavour-violating decays of tau -> 3 mu. A method utilising the production of t leptons via W -> tau nu decays is used. This method is applied to the sample of 20.3 fb(-1) of pp collision data at a centre-of-mass energy of 8 TeV collected by the ATLAS experiment at the LHC in 2012. No event is observed passing the selection criteria, and the observed (expected) upper limit on the tau lepton branching fraction into three muons, Br(tau -> 3 mu), is 3.76 x 10(-7) (3.94 x 10(-7)) at 90 % confidence level.

We have searched for periodic variations of the electronic recoil event rate in the (2–6) keV energy range recorded between February 2011 and March 2012 with the XENON100 detector, adding up to 224.6 live days in total. Following a detailed study to establish the stability of the detector and its background contributions during this run, we performed an unbinned profile likelihood analysis to identify any periodicity up to 500 days. We find a global significance of less than 1σ for all periods, suggesting no statistically significant modulation in the data. While the local significance for an annual modulation is 2.8σ, the analysis of a multiple-scatter control sample and the phase of the modulation disfavor a dark matter interpretation. The DAMA/LIBRA annual modulation interpreted as a dark matter signature with axial-vector coupling of weakly interacting massive particles to electrons is excluded at 4.8σ.

The low-background, VUV-sensitive 3-inch diameter photomultiplier tube R11410 has been developed by Hamamatsu for dark matter direct detection experiments using liquid xenon as the target material. We present the results from the joint effort between the XENON collaboration and the Hamamatsu company to produce a highly radio-pure photosensor (version R11410-21) for the XENON1T dark matter experiment. After introducing the photosensor and its components, we show the methods and results of the radioactive contamination measurements of the individual materials employed in the photomultiplier production. We then discuss the adopted strategies to reduce the radioactivity of the various PMT versions. Finally, we detail the results from screening 286 tubes with ultra-low background germanium detectors, as well as their implications for the expected electronic and nuclear recoil background of the XENON1T experiment.

EBEX is a balloon-borne telescope designed to measure the polarization of the cosmic microwave background radiation. During its eleven day science flight in the Austral Summer of 2012, it operated 955 spider-web transition edge sensor (TES) bolometers separated into bands at 150, 250 and 410 GHz. This is the first time that an array of TES bolometers has been used on a balloon platform to conduct science observations. Polarization sensitivity was provided by a wire grid and continuously rotating half-wave plate. The balloon implementation of the bolometer array and readout electronics presented unique development requirements. Here we present an outline of the readout system, the remote tuning of the bolometers and Superconducting QUantum Interference Device (SQUID) amplifiers, and preliminary current noise of the bolometer array and readout system.

XENON is a dark matter direct detection project, consisting of a time projection chamber (TPC) filled with liquid xenon as detection medium. The construction of the next generation detector, XENON1T, is presently taking place at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. It aims at a sensitivity to spin-independent cross sections of 2.10(47) cm(2) for WIMP masses around 50 GeV/c(2), which requires a background reduction by two orders of magnitude compared to XENON100, the current generation detector. An active system that is able to tag muons and muon-induced backgrounds is critical for this goal. A water Cherenkov detector of similar to 10m height and diameter has been therefore developed, equipped with 8 inch photomultipliers and cladded by a reflective foil. We present the design and optimization study for this detector, which has been carried out with a series of Monte Carlo simulations. The muon veto will reach very high detection efficiencies for muons (> 99.5%) and showers of secondary particles from muon interactions in the rock (> 70%). Similar efficiencies will be obtained for XENONnT, the upgrade of XENON1T, which will later improve the WIMP sensitivity by another order of magnitude. With the Cherenkov water shield studied here, the background from muon-induced neutrons in XENON1T is negligible.

Results of a search for the electroweak associated production of charginos and next-to-lightest neutralinos, pairs of charginos or pairs of tau sleptons are presented. These processes are characterised by final states with at least two hadronically decaying tau leptons, missing transverse momentum and low jet activity. The analysis is based on an integrated luminosity of 20.3 fb(-1) of proton-proton collisions at root s = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess is observed with respect to the predictions from Standard Model processes. Limits are set at 95% confidence level on the masses of the lighter chargino and next-to-lightest neutralino for various hypotheses for the lightest neutralino mass in simplified models. In the scenario of direct production of chargino pairs, with each chargino decaying into the lightest neutralino via an intermediate tau slepton, chargino masses up to 345 GeV are excluded for a massless lightest neutralino. For associated production of mass-degenerate charginos and next-to-lightest neutralinos, both decaying into the lightest neutralino via an intermediate tau slepton, masses up to 410 GeV are excluded for a massless lightest neutralino.

Radiation-induced proportional-electroluminescence UV signals, emitted from the holes of a Thick Gas Electron Multiplier (THGEM) electrode immersed in liquid xenon, were recorded with a PMT for the first time. Significant photon yields were observed with gamma photons and alpha particles using a 0.4 mm thick electrode with 0.3 mm diameter holes; at 2 kV across the THGEM the photon yield was estimated to be similar to 600 UV photons/electron over 4 pi. This may pave the way towards the realization of novel single-phase noble-liquid radiation detectors incorporating liquid hole-multipliers (LHM); their concept is presented.

This letter reports on a search for hypothetical heavy neutrinos, N, and right-handed gauge bosons, W-R, in events with high transverse momentum objects which include two reconstructed leptons and at least one hadronic jet. The results were obtained from data corresponding to an integrated luminosity of 2.1 fb(-1) collected in proton-proton collisions at root s = 7 TeV with the ATLAS detector at the CERN Large Hadron Collider. No excess above the Standard Model background expectation is observed. Excluded mass regions for Majorana and Dirac neutrinos are presented using two approaches for interactions that violate lepton and lepton-flavor numbers. One approach uses an effective operator framework, the other approach is guided by the Left-Right Symmetric Model. The results described in this letter represent the most stringent limits to date on the masses of heavy neutrinos and W-R bosons obtained in direct searches.

A search is made for charged Higgs bosons predicted by Two-Higgs-Doublet extensions of the Standard Model (2HDM) using electron-positron collision data collected by the OPAL experiment at root s = 189-209 GeV, corresponding to an integrated luminosity of approximately 600 pb(-1). Charged Higgs bosons are assumed to be pair-produced and to decay into q (q) over bar, tau nu(tau) or AW(+/-)*. No signal is observed. Model-independent limits on the charged Higgs-boson production cross section are derived by combining these results with previous searches at lower energies. Under the assumption BR(H-+/- -> tau nu(tau)) + BR(H-+/- -> q (q) over bar) = 1, motivated by general 2HDM type II models, excluded areas on the [m(H +/-), BR(H-+/- -> tau nu(tau))] plane are presented and charged Higgs bosons are excluded up to a mass of 76.3 GeV at 95 % confidence level, independent of the branching ratio BR(H +/- -> tau nu(tau)). A scan of the 2HDM type I model parameter space is performed and limits on the Higgs-boson masses m(H)+/- and m(A) are presented for different choices of tan beta.

Hadronic event shape distributions from e(+)e(-) annihilation measured by the OPAL experiment at centre-of-mass energies between 91 GeV and 209 GeV are used to determine the strong coupling alpha(S). The results are based on QCD predictions complete to the next-to-next-to-leading order (NNLO), and on NNLO calculations matched to the re-summed next-to-leading-log-approximation terms (NNLO+NLLA). The combined NNLO result from all variables and centre-of-mass energies is alpha(S)(m(Z0)) = 0.1201 +/- 0.0008 (stat.) +/- 0.0013(exp.) +/- 0.0010(had.) +/- 0.0024(theo.) while the combined NNLO + NLLA result is alpha(S)(m(Z0)) = 0.1189 +/- 0.0008(stat.) +/- 0.0016(exp.) +/- 0.0010(had.) +/- 0.0036(theo.) The completeness of the NNLO and NNLO + NLLA results with respect to missing higher order contributions, studied by varying the renormalization scale, is improved compared to previous results based on NLO or NLO + NLLA predictions only. The observed energy dependence of alpha(S) agrees with the QCD prediction of asymptotic freedom and excludes the absence of running.

The ATLAS Inner Detector is a composite tracking system consisting of silicon pixels, silicon strips and straw tubes in a 2 T magnetic field. Its installation was completed in August 2008 and the detector took part in data-taking with single LHC beams and cosmic rays. The initial detector operation, hardware commissioning and in-situ calibrations are described. Tracking performance has been measured with 7.6 million cosmic-ray events, collected using a tracking trigger and reconstructed with modular pattern-recognition and fitting software. The intrinsic hit efficiency and tracking trigger efficiencies are close to 100%. Lorentz angle measurements for both electrons and holes, specific energy-loss calibration and transition radiation turn-on measurements have been performed. Different alignment techniques have been used to reconstruct the detector geometry. After the initial alignment, a transverse impact parameter resolution of 22.1 +/- 0.9 mu m and a relative momentum resolution sigma(p)/p = (4.83 +/- 0.16) x 10(-4) GeV-1 x p(T) have been measured for high momentum tracks.

The ionization signals in the liquid argon of the ATLAS electromagnetic calorimeter are studied in detail using cosmic muons. In particular, the drift time of the ionization electrons is measured and used to assess the intrinsic uniformity of the calorimeter gaps and estimate its impact on the constant term of the energy resolution. The drift times of electrons in the cells of the second layer of the calorimeter are uniform at the level of 1.3% in the barrel and 2.8% in the endcaps. This leads to an estimated contribution to the constant term of (0.29(-0.04)(+0.05))% in the barrel and (0.54(-0.04)(+0.06))% in the endcaps. The same data are used to measure the drift velocity of ionization electrons in liquid argon, which is found to be 4.61 +/- 0.07 mm/mu s at 88.5 K and 1 kV/mm.

The ATLAS liquid argon calorimeter has been operating continuously since August 2006. At this time, only part of the calorimeter was readout, but since the beginning of 2008, all calorimeter cells have been connected to the ATLAS readout system in preparation for LHC collisions. This paper gives an overview of the liquid argon calorimeter performance measured in situ with random triggers, calibration data, cosmic muons, and LHC beam splash events. Results on the detector operation, timing performance, electronics noise, and gain stability are presented. High energy deposits from radiative cosmic muons and beam splash events allow to check the intrinsic constant term of the energy resolution. The uniformity of the electromagnetic barrel calorimeter response along eta (averaged over phi) is measured at the percent level using minimum ionizing cosmic muons. Finally, studies of electromagnetic showers from radiative muons have been used to cross-check the Monte Carlo simulation. The performance results obtained using the ATLAS readout, data acquisition, and reconstruction software indicate that the liquid argon calorimeter is well-prepared for collisions at the dawn of the LHC era.

The Tile hadronic calorimeter of the ATLAS detector has undergone extensive testing in the experimental hall since its installation in late 2005. The readout, control and calibration systems have been fully operational since 2007 and the detector has successfully collected data from the LHC single beams in 2008 and first collisions in 2009. This paper gives an overview of the Tile Calorimeter performance as measured using random triggers, calibration data, data from cosmic ray muons and single beam data. The detector operation status, noise characteristics and performance of the calibration systems are presented, as well as the validation of the timing and energy calibration carried out with minimum ionising cosmic ray muons data. The calibration systems' precision is well below the design value of 1%. The determination of the global energy scale was performed with an uncertainty of 4%.

The ATLAS detector at the Large Hadron Collider has collected several hundred million cosmic ray events during 2008 and 2009. These data were used to commission the Muon Spectrometer and to study the performance of the trigger and tracking chambers, their alignment, the detector control system, the data acquisition and the analysis programs. We present the performance in the relevant parameters that determine the quality of the muon measurement. We discuss the single element efficiency, resolution and noise rates, the calibration method of the detector response and of the alignment system, the track reconstruction efficiency and the momentum measurement. The results show that the detector is close to the design performance and that the Muon Spectrometer is ready to detect muons produced in high energy proton proton collisions.

The simulation software for the ATLAS Experiment at the Large Hadron Collider is being used for large-scale production of events on the LHC Computing Grid. This simulation requires many components, from the generators that simulate particle collisions, through packages simulating the response of the various detectors and triggers. All of these components come together under the ATLAS simulation infrastructure. In this paper, that infrastructure is discussed, including that supporting the detector description, interfacing the event generation, and combining the GEANT4 simulation of the response of the individual detectors. Also described are the tools allowing the software validation, performance testing, and the validation of the simulated output against known physics processes.

A search is performed for Higgs bosons decaying into invisible final states, produced in association with a Z(0) boson in e(+)e(-) collisions at energies between 183 and 209 GeV. The search is based on data samples collected by the OPAL detector at LEP corresponding to an integrated luminosity of about 660 pb(-1). The analysis aims to select events containing the hadronic decay products of the Z(0) boson and large missing momentum, as expected from Higgs boson decay into a pair of stable weakly interacting neutral particles, such as the lightest neutralino in the Minimal Supersymmetric Standard Model. The same analysis is applied to a search for nearly invisible Higgs boson cascade decays into stable weakly interacting neutral particles. No excess over the expected background from Standard Model processes is observed. Limits on the production of invisibly decaying Higgs bosons produced in association with a Z(0) boson are derived. Assuming a branching ratio BR(h(0) -> invisible) = 1, a lower limit of 108.2 GeV is placed on the Higgs boson mass at the 95% confidence level. Limits on the production of nearly invisibly decaying Higgs bosons are also obtained. (C) 2009 Elsevier B.V. All rights reserved.

Data collected around vs root s = 91 GeV by the OPAL experiment at the LEP e(+)e(-) collider are used to study the mechanism of baryon formation. As the signature, the fraction of Sigma(-) hyperons whose baryon number is compensated by the production of a (Sigma(-)) over bar, (Lambda) over bar or (Xi(-)) over bar antihyperon is determined. The method relies entirely on quantum number correlations of the baryons, and not rapidity correlations, making it more model independent than previous studies. Within the context of the JETSET implementation of the string hadronization model, the diquark baryon production model without the popcorn mechanism is strongly disfavored with a significance of 3.8 standard deviations including systematic uncertainties. It is shown that previous studies of the popcorn mechanism with Lambda(Lambda) over bar and p pi(p) over bar correlations are not conclusive, if parameter uncertainties are considered.

Inclusive jet production (e(+)e(-) -> e(+)e(-) + jet + X) is studied in collisions of quasi-real photons radiated by the LEP beams at e(+)e(-) centre-of-mass energies root s(ee) from 189 to 209 GeV. Jets are reconstructed using the k(perpendicular to) jet algorithm. The inclusive differential cross-section is measured as a function of the jet transverse momentum, p(T)(jet), in the range 5

Hadronic final states with a hard isolated photon are studied using data taken at centre-of-mass energies around the mass of the Z boson with the OPAL detector at LEP. The strong coupling alpha(s) is extracted by comparing data and QCD predictions for event shape observables at average reduced centre-of-mass energies ranging from 24 GeV to 78 GeV, and the energy dependence of alpha(s) is studied. Our results are consistent with the running of alpha(s) as predicted by QCD and show that within the uncertainties of our analysis event shapes in hadronic Z decays with hard and isolated photon radiation can be described by QCD at reduced centre-of-mass energies. Combining all values from different event shape observables and energies gives alpha(s)(M(Z))=0.1182 +/- 0.0015(stat.)+/- 0.0101(syst.).

The inclusive production of charged hadrons in the collisions of quasi-real photons (e(+)e(-) -> e(+)e(-) + X) has been measured using the OPAL detector at LEP. The data were taken at e+e- centre-of-mass energies from 183 to 209 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the hadrons are compared to theoretical calculations of up to next-to-leading order (NLO) in the strong coupling constant alpha(s), The data are also compared to a measurement by the L3 Collaboration, in which a large deviation from the NLO predictions is observed. (c) 2007 Elsevier B.V. All rights reserved.

We discuss MAXIPOL, a bolometric balloon-borne experiment designed to measure the E-mode polarization of the cosmic microwave background radiation (CMB). MAXIPOL is the first bolometric CMB experiment to observe the sky using rapid polarization modulation. To build MAXIPOL, the CMB temperature anisotropy experiment MAXIMA was retrofitted with a rotating half-wave plate and a stationary analyzer. We describe the instrument, the observations, the calibration, and the reduction of data collected with 12 polarimeters operating at 140 GHz and with a FWHM beam size of 10'. We present maps of the Q and U Stokes parameters of an 8 deg(2) region of the sky near the star beta UMi. The power spectra computed from these maps give weak evidence for an EE signal. The maximum likelihood amplitude of l(l +/- 1) C-l(EE)/2 pi is 55(-45)(+51) mu K-2 (68%), and the likelihood function is asymmetric and skewed positive such that with a uniform prior the probability that the amplitude is positive is 96%. This result is consistent with the expected concordance Lambda CDM amplitude of 14 mu K-2. The maximum likelihood amplitudes for l(l + 1) C-l(BB)/2 pi and l(l + 1) C-l(EB)/2 pi are -31(-19)(+31) and 18(-34)(+27) mu K-2 ( 68%), respectively, which are consistent with zero. All of the results are for one bin in the range 151

QCD coherence effects are studied based on measurements of correlations of particles with either restricted transverse momenta, p(T)

Hadronic events produced in e(+)e(-) collisions by the LEP collider and recorded by the OPAL detector were used to form distributions based on the number of reconstructed jets. The data were collected between 1995 and 2000 and correspond to energies of 91 GeV, 130-136 GeV and 161-209 GeV. The jet rates were determined using four different jet-finding algorithms (Cone, JADE, Durham and Cambridge). The differential two-jet rate and the average jet rate with the Durham and Cambridge algorithms were used to measure alpha(S) in the LEP energy range by fitting an expression in which O(alpha(2)(S)) calculations were matched to a NLLA prediction and fitted to the data. Combining the measurements at different centre-of-mass energies, the value of alpha(s)(Mz) was determined to be as(Mz) = 0.1177 +/- 0.0006 (stat.) +/- 0.0012 (expt.) +/- 0.0010 (had.) +/- 0.0032 (theo.)

Searches were performed for topologies predicted by gauge-mediated supersymmetry breaking models (GMSB). All possible lifetimes of the next-to-lightest SUSY particle (NLSP), either the lightest neutralino or slepton, decaying into the lightest SUSY particle, the gravitino, were considered. No evidence for GMSB signatures was found in the OPAL data sample collected at centre-of-mass energies up to root s = 209 GeV at LEP. Limits on the product of the production cross-sections and branching fractions are presented for all search topologies. To test the impact of the searches, a complete scan over the parameters of the minimal model of GMSB was performed. NLSP masses below 53.5 GeV/c(2) in the neutralino NLSP scenario, below 87.4 GeV/c(2) in the stau NLSP scenario and below 91.9 GeV/c(2) in the slepton co-NLSP scenario are excluded at 95% confidence level for all NLSP lifetimes. The scan determines constraints on the universal SUSY mass scale Lambda from the direct SUSY particle searches of Lambda > 40, 27, 21, 17, 15 TeV/c(2) for messenger indices N = 1, 2, 3, 4, 5 for all NLSP lifetimes.

The mass and width of the W boson are measured using e(+)e(-) -> W(+)W(-) events from the data sample collected by the OPAL experiment at LEP at centre-of-mass energies between 170 GeV and 209 GeV. The mass (m(W)) and width (Gamma(W)) are determined using direct reconstruction of the kinematics of W(+)W(-) -> q (q) over bar lfv and W(+)W(-) -> q (q) over barq (q) over bar events. When combined with previous OPAL measurements using W(+)W(-) -> lvlv events and the dependence on m(W) of the WW production cross-section at threshold, the results are determined to be m(W) = 80.415 +/- 0.042 +/- 0.030 +/- 0.009 GeV Gamma(W) = 1.996 +/- 0.096 +/- 0.102 +/- 0.003 GeV where the first error is statistical, the second systematic and the third due to uncertainties in the value of the LEP beam energy. By measuring m(W) in the q (q) over barq (q) over bar channel using several different determinations of the direction of jets with differing sensitivities to soft particles, a limit is also obtained on possible final-state interactions due to colour reconnection effects in W(+)W(-) -> q (q) over barq (q) over bar events. The consistency of the results for the W mass and width with those inferred from other electroweak parameters provides an important test of the Standard Model of electroweak interactions.

Data from e(+)e(-) annihilation into hadrons at centre-of-mass energies between 91 GeV and 209 GeV collected with the OPAL detector at LEP, are,used to study the four-jet rate as a function of the Durham algorithm resolution parameter y(cut). The four-jet rate is compared to next-to-leading order calculations that include the resummation of large logarithms. The strong coupling measured from the four-jet rate is alpha s(m(z)o) = 0.1182 +/- 0.0003(stat.) +/- 0.0015(exp.) +/- 0.0011(had.) +/- 0.0012(scale) +/- 0.0013(mass), in agreement with the world average. Next-to-leading order fits to the D-parameter and thrust minor event-shape observables are also performed for the first time. We find consistent results, but with significantly larger theoretical uncertainties.

Using the OPAL detector at LEP, the running of the effective QED coupling alpha(t) is measured for space-like momentum transfer from the angular distribution of small-angle Bhabha scattering. In an almost ideal QED framework, with very favourable experimental conditions, we obtain: Delta alpha(-6.07 GeV(2)) - Delta alpha(-1.81 GeV(2)) = (440 +/- 58 +/- 43 +/- 30) x 10(-5), where the first error is statistical, the second is the experimental systematic and the third is the theoretical uncertainty. This agrees with current evaluations of alpha(t). The null hypothesis that alpha remains constant within the above interval of -t is excluded with a significance above 5 sigma. Similarly, our results are inconsistent at the level of 3 sigma with the hypothesis that only leptonic loops contribute to the running. This is currently the most significant direct measurement where the running alpha(t) is probed differentially within the measured t range.

The effects of the final state interaction phenomenon known as colour reconnect ion are investigated at centre-of-mass energies in the range root s similar or equal to 189-209GeV using the OPAL detector at LEP. Colour reconnection is expected to affect observables based on charged particles in hadronic decays of W(+)W(-). Measurements of inclusive charged particle multiplicities, and of their angular distribution with respect to the four jet axes of the events, are used to test models of colour reconnection. The data are found to exclude extreme scenarios of the Sjostrand-Khoze Type I (SK-I) model and are compatible with other models, both with and without colour reconnection effects. In the context of the SK-I model, the best agreement with data is obtained for a reconnection probability of 37%. Assuming no colour reconnection, the charged particle multiplicity in hadronically decaying W bosons is measured to be (n(ch)(qq)) = 19.38 +/- 0.05(stat.) +/- 0.08(syst.).

Upper limits on the cross-section of the pair-production process e(+)e(-) -> h(0)A(0), assuming 100 % branching fraction to hadrons, are derived from a new search for the h(0)A(0) -> hadrons final state, independently of the hadronic flavour of the decay products. This study, combined with previously published searches for the neutral Higgs bosons h(0) and A(0), is used to constrain the Type II Two Higgs Doublet Model (2HDM(II)) with no CP violation in the Higgs sector and no additional non-Standard Model particles besides the five Higgs bosons. The analysis combines LEP1 and LEP2 data collected with the OPAL detector up to the highest available centre-of-mass energies. The searches are sensitive to the h(0), A(0) -> q (q) over bar, gg, tau(+)tau(-) and h(0) -> A(0)A(0) decay modes of the Higgs bosons. A benchmark scan of the 2HDM(II) parameter space is performed. Large regions of the 2HDM( II) parameter space explored are excluded at the 95% CL in the (m(h), m(A)), (m(h), tan beta) and (m(A), tan beta) planes, using both direct neutral Higgs boson searches and indirect limits derived from Standard Model high precision measurements. The region 1 less than or similar to m(h) less than or similar to 55 GeV and 3 less than or similar to m(A) less than or similar to 63 GeV is excluded at 95 % CL, independently of m(h), m(A), tan beta and for selected values of alpha which are representative of a complete alpha-scan.

A new scalar resonance, called the radion, with couplings to fermions and bosons similar to those of the Higgs boson, is predicted in the framework of Randall-Sundrum models, proposed solutions to the hierarchy problem with one extra dimension. An important distinction between the radion and the Higgs boson is that the radion would couple directly to gluon pairs, and in particular its decay products would include a significant fraction of gluon jets. The radion has the same quantum numbers as the Standard Model (SM) Higgs boson, and therefore they can mix, with the resulting mass eigenstates having properties different from those of the SM Higgs boson. Existing searches for the Higgs bosons are sensitive to the possible production and decay of radions and Higgs bosons in these models. For the first time, searches for the SM I-Eggs boson and flavour-independent and decay-mode independent searches for a neutral Higgs boson are used in combination to explore the parameter space of the Randall-Sundrum model. In the dataset recorded by the OPAL experiment at LEP, no evidence for radion or Higgs particle production was observed in any of those searches at centre-of-mass energies up to 209 GeV. The results are used to set limits on the radion and Higgs boson masses. For all parameters of the Randall-Sundrurn model, the data exclude masses below 58 GeV for the mass eigenstate which becomes the Higgs boson in the no-mixing limit. (C) 2005 Elsevier B.V. All rights reserved.

Measurements of R-b, the ratio of the b (b) over bar cross-section to the q (q) over bar cross-section in e(+)e(-) collisions, are presented. The data were collected by the OPAL experiment at LEP at centre-of-mass energies between 182 and 209 GeV Lepton, lifetime and event-shape information is used to tag events containing b quarks with high efficiency. The data are compatible with the Standard Model expectation. The mean ratio of the eight measurements reported here to the Standard Model prediction is 1.055 +/- 0.031 +/- 0.037, where the first error is statistical and the second systematic. (c) 2005 Elsevier B.V. All rights reserved.

We have studied hadronic events from e(+)e(-) annihilation data at centre-of-mass energies from 91 to 209 GeV. We present distributions of event shape observables and their moments at each energy and compare with QCD Monte Carlo models. From the event shape distributions we extract the strong coupling alpha(s) and test its evolution with energy scale. The results are consistent with the running of alpha(s) expected from QCD. Combining all data, the value of alpha(s)(M(Z)) is determined to be alpha(s)(M(Z)) = 0.1191 +/- 0.0005 (stat.) +/- 0.0010 (expt.) +/- 0.0011 (hadr.) +/- 0.0044 (theo.). The energy evolution of the moments is also used to determine a value of alpha(s) with slightly larger errors: alpha(s)(M(Z)) = 0.1223 +/- 0.0005 (stat.) +/- 0.0014 (expt.) +/- 0.0016 (hadr.)(-0.0036)(+0.0054) (theo.).

Using the entire OPAL LEP1 on-peak Z hadronic decay sample, Z --> qqgamma decays were selected by tagging hadronic final states with isolated photon candidates in the electromagnetic calorimeter. Combining the measured rates of Z --> qqgamma decays with the total rate of hadronic Z decays permits the simultaneous determination of the widths of the Z into up- and down-type quarks. The values obtained, with total errors, were Gamma(u) = 300(-18)(+19) MeV and Gamma(d) = 381(-12)(+12) MeV. The results are in good agreement with the Standard Model expectation. (C) 2004 Elsevier B.V. All rights reserved.

Events with a final state consisting of two or more photons and large missing transverse energy have been observed in e(+)e(-) collisions at centre-of-mass energies in the range 192-209 GeV using the OPAL detector at LEP. Cross-section measurements are performed within the kinematic acceptance of the selection and compared with the expectations from the Standard Model process e(+)e(-) --> v (v) over bar gammagamma(gamma). No evidence for new physics contributions to this final state is observed. Upper limits on or (e(+)e(-) --> XX) (.) BR(2) (X --> Ygamma) are derived for the case of stable and invisible Y. In the case of massive Y the combined limits obtained from all the data range from 10 to 60 fb, while for the special case of massless Y the range is 20 to 40 fb. The limits apply to pair production of excited neutrinos (X = v*, Y = v), to neutralino production (X = (x) over bar (0)(2), Y = (x) over bar (0)(1)) and to 2 1 supersymmetric models in which X = (x) over bar (0)(1) and Y = (G) over bar is a light gravitino. (C) 2004 Published by Elsevier B.V.

A search for the Higgsstrahlung process e(+)e(-) --> hZ is described, where the neutral Higgs boson h is assumed to decay into hadronic final states. In order to be sensitive to a broad range of models, the search is performed independent of the flavour content of the Higgs boson decay. The analysis is based on e(+)e(-) collision data collected by the OPAL detector at energies between 192 and 209 GeV. The search does not reveal any significant excess over the Standard Model background prediction. Results are combined with previous searches at energies around 91 and at 189 GeV A limit is set on the product of the cross-section and the hadronic branching ratio of the Higgs boson, as a function of the Higgs boson mass. Assuming the hZ coupling predicted by the Standard Model, and a Higgs boson decaying only into hadronic final states, a lower bound of 104 GeV/c(2) is set on the mass at the 95% confidence level. (C) 2004 Elsevier B.V. All rights reserved.

Tau lepton decays with open strangeness in the final state are measured with the OPAL detector at LEP to determine the strange hadronic spectral function of the tau lepton. The decays tau(-) -->(Kpi)(-)nu(tau)(Kpipipi)(-)nu(tau) and (Kpipipi)(-)nu(tau) with final states consisting of neutral and charged kaons and pions have been studied. The invariant mass distributions of 93.4% of these final states have been experimentally determined. Monte Carlo simulations have been used for the remaining 6.6% and for the strange final states including eta mesons. The reconstructed strange final states, corrected for resolution effects and detection efficiencies, yield the strange spectral function of the tau lepton. The moments of the spectral function and the ratio of strange to non-strange moments, which are important input parameters for theoretical analyses, are determined. Furthermore, the branching fractions B(tau(-) --> K(-)pi(o)nu(tau)) = (0.471 +/- 0.059(stat) +/- 0.023(sys))% and B(tau(-) --> K(-)pi(+)pi(-)nu(tau)) = (0.415 +/- 0.053(stat) +/- 0.040(sys))% have been measured.

Gluon jets with a mean energy of 22 GeV and purity of 95% are selected from hadronic Z(0) decay events produced in e (+) e(-) annihilations. A subsample of these jets is identified which exhibits a large gap in the rapidity distribution of particles within the jet. After imposing the requirement of a rapidity gap, the gluon jet purity is 86%. These jets are observed to demonstrate a high degree of sensitivity to the presence of color reconnection, i.e. higher order QCD processes affecting the underlying color structure. We use our data to test three QCD models which include a simulation of color reconnection: one in the Ariadne Monte Carlo, one in the Herwig Monte Carlo, and the other by Rathsman in the Pythia Monte Carlo. We find the Rathsman and Ariadne color reconnection models can describe our gluon jet measurements only if very large values are used for the cutoff parameters which serve to terminate the parton showers, and that the description of inclusive Z(0) data is significantly degraded in this case. We conclude that color reconnection as implemented by these two models is disfavored. The signal from the Herwig color reconnection model is less clear and we do not obtain a definite conclusion concerning this model. In a separate study, we follow recent theoretical suggestions and search for glueball-like objects in the leading part of the gluon jets. No clear evidence is observed for these objects.

Triple gauge boson couplings are measured from W-pair events recorded by the OPAL detector at LEP at centre-of-mass energies of 183 - 209 GeV with a total integrated luminosity of 680 pb(-1). Only CP-conserving couplings are considered and SU(2) x U(1) relations are used, resulting in four independent couplings, k(gamma), g(1)(z), lambdagamma and g(5)(z). Determining each coupling in a separate fit, assuming the other couplings to take their Standard Model values, we obtain k(gamma) = 0.88(-0.08)(+0.09), g(1)(z) = 0.987 (-0.033),(+0.034) lambda(gamma) = -0.060(-0.033)(+0.034) and g(5)(z) = -0.04(-0.12)(+0.13), where the errors include both statistical and systematic uncertainties. Fits are also performed allowing some of the couplings to vary simultaneously. All results are consistent with the Standard Model predictions.

A study of Z-boson pair production in e (+) e(-) annihilation at center-of-mass energies between 190 GeV and 209 GeV is reported. Final states containing only leptons, (l(+)l(-)l(+)l(-) and l(+)l(-)v(v) over bar), quark and lepton pairs, (q(q) over bar l)+l-, (q(q) over bar v(v) over bar) and only hadrons (q(q) over bar q(q) over bar) are considered. In all states with at least one Z boson decaying hadronically, lifetime, lepton and event-shape tags are used to separate b(b) over bar pairs from q(q) over bar final states. Limits on anomalous ZZgamma and ZZZ couplings are derived from the measured cross sections and from event kinematics using an optimal observable method. Limits on low scale gravity with large extra dimensions are derived from the cross sections and their dependence on polar angle.

The branching ratio of beauty hadrons to final states containing two charm hadrons, Br(b-->D (D) over barX), has been measured using an inclusive method in hadronic Z(0) decays with the OPAL detector at LEP. The impact parameter significance of tracks opposite tagged b-jets is used to differentiate b-->D (D) over barX decays from other decays. The result is Br(b-->D (D) over barX) = (10.0 +/- 3.2 (stat.)(-2.9)(+2.4)(det.)(-9.0)(+10.4) (phys.))%, where "det." is the systematic uncertainty due to the modelling of the detector, and "phys." is the systematic uncertainty due to the modelling of the underlying physics. Using this result, the average number of charm plus anti-charm quarks produced in a beauty quark decay, n(c), is found to be 1.12(-0.10)(+0.11).

Anomalous quartic couplings between the electroweak gauge bosons may contribute to the nu(nu) over bar gammagamma and q (q) over bar gammagamma final states produced in e(+)e(-) collisions. This analysis uses the LEP2 OPAL data sample at center-of-mass energies up to 209 GeV. Event selections identify nu(nu) over bar gammagamma and q (q) over bar gammagamma events in which the two photons are reconstructed within the detector acceptance. The cross section for the process e(+)e(-)-->q (q) over bar gammagamma is measured. Averaging over all energies, the ratio of the observed e(+)e(-)-->q (q) over bar gammagamma cross section to the standard model expectation is R(data/SM)=0.92+/-0.07+/-0.04, where the errors represent the statistical and systematic uncertainties respectively. The nu(nu) over bar gammagamma and q (q) over bar gammagamma data are used to constrain possible anomalous W(+)W(-)gammagamma and ZZgammagamma couplings. Combining with previous OPAL results from the W(+)W(-)gamma final state, the 95% confidence level limits on the anomalous coupling parameters a(0)(Z), a(c)(Z), a(0)(W) and a(c)(W) are found to be -0.007 GeV-2

Flavour inclusive, udsc and b fragmentation functions in unbiased jets, and flavour inclusive, udsc, b and gluon fragmentation functions in biased jets are measured in e(+)e(-) annihilations from data collected at centre-of-mass energies of 91.2, and 183-209 GeV with the OPAL detector at LEP. The unbiased jets are defined by hemispheres of inclusive hadronic events, while the biased jet measurements are based on three-jet events selected with jet algorithms. Several methods are employed to extract the fragmentation functions over a wide range of scales. Possible biases are studied in the results obtained. The fragmentation functions are compared to results from lower energy e(+)e(-) experiments and with earlier LEP measurements and are found to be consistent. Scaling violations are observed and are found to be stronger for the fragmentation functions of gluon jets than for those of quarks. The measured fragmentation functions are compared to three recent theoretical next-to-leading order calculations and to the predictions of three Monte Carlo event generators. While the Monte Carlo models are in good agreement with the data, the theoretical predictions fail to describe the full set of results, in particular the b and gluon jet measurements.

This report summarizes the final results from the OPAL collaboration on searches for neutral Higgs bosons predicted by the Minimal Supersymmetric Standard Model (MSSM). CP-conserving and, for the first time at LEP, CP-violating scenarios are studied. New scenarios are also included, which aim to set the stage for Higgs searches at future colliders. The results are based on the data collected with the OPAL detector at e (+) e(-) centre-of-mass energies up to 209 GeV. The data are consistent with the prediction of the Standard Model with no Higgs boson produced. Model-independent limits are derived for the cross-sections of a number of event topologies motivated by predictions of the MSSM. Limits on Higgs boson masses and other MSSM parameters are obtained for a number of representative MSSM benchmark scenarios. For example, in the CP-conserving scenario m(h)-max where the MSSM parameters are adjusted to predict the largest range of values for m(h) at each tan beta, and for a top quark mass of 174.3 GeV, the domain 0.7 85.0 GeV are obtained. For the CP-violating benchmark scenario CPX which, by construction, enhances the CP-violating effects in the Higgs sector, the domain tan beta

The extremely high rate of events that will be produced in the future Large Hadron Collider requires the triggering mechanism to take precise decisions in a few nano-seconds. We present a study which used an artificial neural network triggering algorithm and compared it to the performance of a dedicated electronic muon triggering system. Relatively simple architecture was used to solve a complicated inverse problem. A comparison with a realistic example of the ATLAS first level trigger simulation was in favour of the neural network. A similar architecture trained after the simulation of the electronics first trigger stage showed a further background rejection. (C) 2004 Elsevier B.V. All rights reserved.

We present the first experimental results based on the jet boost algorithm, a technique to select unbiased samples of gluon jets in e(+)e(-) annihilations, i.e. gluon jets free of biases introduced by event selection or jet finding criteria. Our results are derived from hadronic Z(0) decays observed with the OPAL detector at the LEP e(+)e(-) collider at CERN. First, we test the boost algorithm through studies with HERWIG Monte Carlo events and find that it provides accurate measurements of the charged particle multiplicity distributions of unbiased gluon jets for jet energies larger than about 5 GeV, and of the jet particle energy spectra (fragmentation functions) for jet energies larger than about 14 GeV. Second, we apply the boost algorithm to our data to derive unbiased measurements of the gluon jet multiplicity distribution for energies between about 5 and 18 GeV, and of the gluon jet fragmentation function at 14 and 18 GeV. In conjunction with our earlier results at 40 GeV, we then test QCD calculations for the energy evolution of the distributions, specifically the mean and first two nontrivial normalized factorial moments of the multiplicity distribution, and the fragmentation function. The theoretical results are found to be in global agreement with the data, although the factorial moments are not well described for jet energies below about 14 GeV.

Elements of the spin density matrix for W bosons in e(+)e(-) --> W(+)W(-) --> q (q) over barq'lv(l) events are measured from data recorded by the OPAL detector at LEP. This information is used to calculate polarised differential cross-sections and to search for CP-violating effects. Results are presented for W bosons produced in e+e- Collisions with centre-of-mass energies between 183 GeV and 209 GeV. The average fraction of W bosons that are longitudinally polarised is found to be (23.9 +/- 2.1 +/- 1.1)% compared to a Standard Model prediction of (23.9 +/- 0.1)%. All results are consistent with CP conservation. (C) 2004 Elsevier B.V. All rights reserved.

Thin gap chambers (TGCs) are built for the muon trigger chambers in the endcap region of the LHC experiment ATLAS. More than 2500 ATLAS TGCs are being produced at the Weizmann institute in Israel, and in Shandong University in China. Detailed testing of these chambers is performed at the Technion and at the Tel-Aviv University. Two cosmic ray hodoscopes for testing the operation of these detectors were built in Israel. In these hodoscopes the response of the chambers to energetic cosmic ray muons is recorded and analyzed. The hodoseopes measure the exact time and space location of the cosmic ray hit and read out the chambers which are being tested to verify that they produce a corresponding signal within the required time interval. The cosmic ray hodoscopes built at the Technion and at the Tel Aviv University for the test of ATLASTGCs are described. The mechanical structure, readout electronics, data acquisition and operating scheme are presented. Typical TGC test results are presented and discussed.

Cross-sections and angular distributions for hadronic and lepton-pair final states in e (+) e(-) collisions at centre-of-mass energies between 189 GeV and 209 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. The measurements are used to determine the electromagnetic coupling constant alpha(em) at LEP 2 energies. In addition, the results are used together with OPAL measurements at 91-183 GeV within the S-matrix formalism to determine the gamma-Z interference term and to make an almost model-independent measurement of the Z mass. Limits on extensions to the Standard Model described by effective four-fermion contact interactions or the addition of a heavy Z' boson are also presented.

A study of W+W- events accompanied by hard photon radiation, Egamma > 2.5 GeV, produced in e(+)e(-) collisions at LEP is presented. Events consistent with being two on-shell W-bosons and an isolated photon are selected from 681 pb(-1) of data recorded at 180 GeV

A search for pair-produced sfermions, the scalar supersymmetric partners of the Standard Model fermions, under the assumption that R-parity is not conserved has been performed using data collected with the OPAL detector at LEP. The data samples analysed correspond to an integrated luminosity of about 610 pb(-1) collected at centre-of-mass energies of roots = 189-209 GeV. An important consequence of R-parity violation is that the lightest supersymmetric particle is expected to be unstable. Searches for R-parity violating decays of charged sleptons, sneutrinos and squarks have been performed under the assumptions that the lightest supersymmetric particle decays promptly and that only one of the R-parity violating couplings is dominant for each of the decay modes considered. Such processes would yield final states consisting of leptons, jets, or both, with or without missing energy. No significant signal-like excess of events has been observed with respect to the Standard Model expectations. Limits on the production cross-sections of sfermions in R-parity violating scenarios are obtained. Constraints on the supersymmetric particle masses are also presented in an R-parity violating framework analogous to the Constrained Minimal Supersymmetric Standard Model.

In total 1317 di-lepton events with significant missing transverse momentum were identified in a total data sample of 680 pb(-1) collected at e (+) e(-) centre-of-mass energies ranging from 183 GeV to 209 GeV. The number of di-lepton events, the dependence on centre-of-mass energy, and the event properties are consistent with expectations from Standard Model processes, predominantly W (+) W- production with both W bosons decaying leptonically. This topology is also an experimental signature for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. No evidence for new phenomena is apparent. Upper limits are presented on the production cross-section multiplied by the relevant branching ratio squared for sleptons, leptonically decaying charginos and charged Higgs bosons. Mass limits are also given.

Approximately 438 pb(-1) of e (+) e(-) data from the OPAL detector, taken with the LEP collider running at centre-of-mass energies of 192-209 GeV, are analyzed to search for evidence of chargino pair production, e(+)e(-)-->(chi) over tilde (+)(1)(chi) over tilde (-)(1), or neutralino associated production, e(+)e(-)-->(chi) over tilde (0)(2)(chi) over tilde (0)(1). Limits are set at the 95% confidence level on the product of the cross-section for the process e+e--->(chi) over tilde (+)(1)(chi) over tilde (-)(1) and its branching ratios to topologies containing jets and missing energy, or jets with a lepton and missing energy, and on the product of the cross-section for e+e--->(chi) over tilde (0)(2)(chi) over tilde (0)(1) and its branching ratio to jets. R-parity conservation is assumed throughout this paper. When these results are interpreted in the context of the Constrained Minimal Supersymmetric Standard Model, limits are also set on the masses of the (chi) over tilde (+/-)(1)(chi) over tilde (0)(1) and (chi) over tilde (0)(2), and regions of the parameter space of the model are ruled out. Nearly model-independent limits are also set at the 95% confidence level on sigma(e(+)e(-)-->(chi) over tilde (+)(1)(chi) over tilde (-)(1)) with the assumption that each chargino decays via a W boson, and on sigma(e(+)e(-)-->(chi) over tilde (0)(2)(chi) over tilde (0)(1)) with the ($chi) over tilde (0)(2) assumed to decay via a Z(0).

The ATLAS endcap muon trigger system uses custom integrated circuits and electronic modules. It will provide fast trigger information at the LHC bunch-crossing rate of 40 MHz to be used in making the first-level trigger decision. A prototype of the system has been constructed and mounted on Thin Gap Chambers. We have tested the system using a 180 GeV muon beam in the CERN SPS H8 beam line. The SPS provided bunched-beam with 25 ns structure, which allowed us to check the system performance under conditions very similar to the ATLAS experiment, before proceeding to the mass-production phase. We will present results on the evaluation of the system performance.

The ATLAS Muon Spectrometer (ATLAS Collaboration, ATLAS Muon Spectrometer Technical Design Report CERN/LHCC/97-22, ATLAS TDR 10, 1997.) will use dedicated detectors to trigger on muons and to identify the bunch-crossing at the appropriate rate. The Spectrometer has been designed to perform stand-alone triggering and measurement of muon transverse momentum up to 1 TeV with good resolution (from 3% up to 10% at 1 TeV). The magnetic system is composed of three large superconducting air-core toroids instrumented with trigger and high-precision tracking chambers, a central part (barrel) composed of eight coils and two end-cap magnets.The high-precision tracking system is based on Monitored Drift Tube (MDT) and Cathode Strip Chambers (CSC) in the small angle-regions. The Level-1 trigger is provided by Resistive Plate Chambers (RPC) in the barrel and Thin Gap Chambers (TGC) in the end-cap. These detectors will also measure the track coordinates in the magnetic field direction (second coordinate), to complement the precision tracking provided by the MDT which only measure the track coordinates in the bending direction of the magnetic field. The trigger system covers an area of 3650 m2 in the barrel and 2900 m 2 in the end-cap. In the barrel region three double-gap RPC stations are used, two in the middle and one in the outer MDT chamber layer. In the end-cap region one triple-gap TGC station is used, in front of the middle MDT station, and two double-gap TGC stations behind it. The mass production of both systems is under way.The systems were involved in extensive beam tests in 2002–2003, testing their compliance with LHC timing requirements using 25ns beam bunching to emulate the LHC beam structure, aging under critical environment conditions and so on.

Bose-Einstein correlations between like-sign charged-particle pairs in e(+)e(-) --> W(+) W(-) events recorded with the OPAL detector at LEP at centre-of-mass energies between 183 GeV and 209 GeV are studied. Recently proposed methods which allow direct searches for correlations in the data via distributions of test variables are used to investigate the presence of correlations between hadrons originating from different W bosons in W(+)W(-) --> q (q) over barq (q) over bar events. Within the statistics of the data sample no evidence for inter - WW Bose-Einstein correlations is obtained. The data are also compared with predictions of a recent implementation of Bose-Einstein correlation effects in the Monte Carlo model PYTHIA.

Di-jet production is studied in collisions of quasi-real photons at e(+)e(-) centre-of-mass energies roots(ee) from 189 to 209 GeV at LEP. The data were collected with the OPAL detector. Jets are reconstructed using an inclusive k(perpendicular to)-clustering algorithm for all cross-section measurements presented. A cone jet algorithm is used in addition to study the different structure of the jets resulting from either of the algorithms. The inclusive di-jet cross-section is measured as a function of the mean transverse energy (E) over bar (jet)(T) of the two leading jets, and as a function of the estimated fraction of the photon momentum carried by the parton entering the hard sub-process, x(gamma), for different regions of (E) over bar (jet)(T). Angular distributions in di-jet events are measured and used to demonstrate the dominance of quark and gluon initiated processes in different regions of phase space. Furthermore the inclusive di-jet cross-section as a function of \eta(jet)\ and \Deltaeta(jet)\ I is presented, where eta(jet) is the jet pseudo-rapidity. Different regions of the x(gamma)(+)-x(gamma)(-)-space are explored to study and control the influence of an underlying event. The results are compared to next-to-leading order perturbative QCD calculations and to the, predictions of the leading order Monte Carlo generator PYTHIA.

The process e(+)e(-) --> gammagamma(gamma) is studied using data collected by the OPAL detector at LEP between the years 1997 and 2000. The data set corresponds to an integrated luminosity of 672.3 pb(-1) at centre-of-mass energies lying between 181 GeV and 209 GeV. Total and differential cross-sections are determined and found to be in good agreement with the predictions of QED. Fits to the observed angular distributions are used to set limits on parameters from several models of physics beyond the Standard Model such as cut-off parameters, contact interactions of the type e(+)e(-) gammagamma, gravity in extra spatial dimensions and excited electrons. In events with three photons in the final state the mass spectrum of photon pairs is investigated. No narrow resonance X --> gammagamma is found and limits are placed on the product of the Xgamma production cross-section and branching ratio.

We observe Bose-Einstein correlations in pi(0) pairs using back-to-back two jet hadronic events from Z(0) decays in the data sample collected by the OPAL detector at LEP I from 101 to 1995. Using a static Gaussian picture for the pion emitter source, we obtain the chaoticity parameter lambda = 0.55 +/- 0.10 +/- 0.10 and the source radius R = (0.59 +/- 0.08 +/- 0.05) fm. According to the JETSET and HERWIG Monte Carlo models, the Bose-Einstein correlations in our data sample largely connect pi(0)s originating from the decays of different hadrons. Prompt pions formed at string break-ups or cluster decays only form a small fraction of the sample. (C) 2003 Elsevier Science B.V. All rights reserved.

A search for the single production of doubly-charged Higgs bosons is performed using e(+)e(-) collision data collected by the OPAL experiment at centre-of-mass energies between 189 GeV and 209 GeV. No evidence for the existence of H(+/-+/-) is observed. Upper limits are derived on h(ee), the Yukawa coupling of the H(+/-+/-) to like-signed electron pairs. A 95% confidence level upper limit of hee

A search for pair-produced leptoquarks is performed using e(+)e(-) collision events collected by the OPAL detector at LEP at centre-of-mass energies between 189 and 209 GeV. The data sample corresponds to a total integrated luminosity of 596 pb(-1). The leptoquarks are assumed to be produced via couplings to the photon and the Z(0). For a given search channel only leptoquark decays involving a single lepton generation are considered. No evidence for leptoquark pair production is observed. Lower limits on masses for scalar and vector leptoquarks are calculated. The results improve most of the LEP limits derived from previous searches for the pair production process by 10-25 GeV, depending on the leptoquark quantum numbers.

A search was performed for charginos with masses close to the mass of the lightest neutralino in e(+)e(-) collisions at centre-of-mass energies of 189-209 GeV recorded by the OPAL detector at LEP. Events were selected if they had an observed high-energy photon from initial state radiation, reducing the dominant background from two-photon scattering to a negligible level. No significant excess over Standard Model expectations has been observed in the analysed data set corresponding to an integrated luminosity of 570 pb(-1). Upper limits were derived on the chargino pair-production cross-section, and lower limits on the chargino mass were derived in the context of the Minimal Supersymmetric Extension of the Standard Model for the gravity- and anomaly-mediated Supersymmetry breaking scenarios.

We have analysed the data collected by OPAL at centre-of-mass energies between 189 and 209 GeV searching for Higgs boson candidates from the process e(+)e(-) --> h(0)Z(0) followed by the decay of h(0) --> A(0)A(0) where A(0) is the CP-odd Higgs boson. The search is done in the region where the A(0) mass, m(A), is below the production threshold for b (b) over bar, and the CP-even Higgs boson mass m(h) is within the range 45-86 GeV/c(2). In this kinematic range, the decay of h(0) --> A(0)A(0) may be dominant and previous Higgs boson searches have very small sensitivities. This search can be interpreted within any model that predicts the existence of at least one scalar and one pseudoscalar Higgs boson. No excess of events is observed above the expected Standard Model backgrounds. Model-independent limits on the cross-section for the process e(+)e(-) --> h(0)Z(0) are derived assuming 100% decays of the h(0) into A(0)A(0) and 100% decays of the A(0)A(0) into each of the following final states: c (c) over barc (c) over bar, gggg, tau(+)tau(-)tau(+)tau(-), c (c) over bar gg, ggtau(+)tau(-) and c (c) over bar tau(+)tau(-). The results are also interpreted in the CP-conserving no-mixing MSSM scenario, where the region 45 less than or equal to m(h) less than or equal to 85 GeV/c(2), and 2 less than or equal to m(A) less than or equal to 9.5 GeV/c(2) is excluded.

A measurement of the forward-backward asymmetries of e(+)e(-) --> b (b) over bar and e(+)e(-) --> c (c) over bar events using electrons and muons produced in semileptonic decays of bottom and charm hadrons is presented. The outputs of two neural networks designed to identify b --> l(-) and c --> l(+) decays are used in a maximum likelihood fit to a sample of events containing one or two identified leptons. The b and c quark forward-backward asymmetries at three centre-of-mass energies roots and the average B mixing parameter are determined simultaneously in the fit. Using all data collected by OPAL near the Z resonance, the asymmetries are measured to be: A(FB)(b (b) over bar) = (4.7 +/- 1.8 +/- 0.1)%, A(FB)(c (c) over bar) = (-6.8 +/- 2.5 +/- 0.9)% at = 89.51 GeV, A(FB)(b (b) over bar) = (9.72 +/- 0.42 +/- 0.15)%, A(FB)(c (c) over bar) = (5.68 +/- 0.54 +/- 0.39)% at = 91.25 GeV, A(FB)(b (b) over bar) = (10.3 +/- 1.5 +/- 0.2)%, A(FB)(c (c) over bar) = (14.6 +/- 2.0 +/- 0.8)% at = 92.95 GeV, For the average B mixing parameter, a value of: (chi) over bar = (13.12 +/- 0.49 +/- 0.42)% is obtained. In each case the first uncertainty is statistical and the second systematic. These results are combined with other OPAL measurements of the b and c forward-backward asymmetries, and used to derive a value for the effective electroweak mixing angle for leptons sin(2)theta(eff)(l) of 0.23238 +/- 0.00052. (C) 2003 Published by Elsevier B.V.

This paper summarises the search for the Standard Model Higgs boson in e(+)e(-) collisions at centre-of-mass energies up to 209 GeV performed by the OPAL Collaboration at LEP. The consistency of the data with the background hypothesis and various Higgs boson mass hypotheses is examined. No indication of a signal is found in the data and a lower bound of 112.7 GeV/c(2) is obtained on the mass of the Standard Model Higgs boson at the 95% CL.

The decay chain b --> (B) over bar --> D(**0) l(-) (nu) over bar X , D(**0) --> D(*+)pi(-), D(*+) --> D(0)pi(+), D(0) --> (Kpi or K3pi) is identified in a sample of 3.9 million hadronic Z decays collected with the OPAL detector at LEP. The branching ratio BR (b --> (B) over bar) x BR ((B) over bar --> D(1)(0) l(-) (nu) over bar X) x BR (D(1)(0) --> D(*+)pi(-)) is measured to be (2.64 +/- 0.79 (stat) +/- 0.39 (syst)) x 10(-3) for the J(P)=1(+) (D(1)(0)) state. For decays into the J(P)=2(+) (D(2)(*0)) state, an upper limit of 1.4 x 10(-3) is placed on the branching ratio at the 95% confidence level.

A study of b quark hadronisation is presented using inclusively reconstructed B hadrons in about four million hadronic Z decays recorded in 1992-2000 with the OPAL detector at LEP. The data are compared to different theoretical models, and fragmentation function parameters of these models are fitted. The average scaled energy of weakly decaying B hadrons is determined to be [xE] = 0.7193 +/- 0.0016(stat)(-0.0033)(+0.0038)(syst).

A novel method of determining the mass of the W boson in the W(+)W(-) --> lvl'v' channel is presented and applied to 667 pb(-) (1) of data recorded at center-of-mass energies in the range 183-207 GeV with the OPAL detector at LEP. The measured energies of charged leptons and the results of a new procedure based on an approximate kinematic reconstruction of the events are combined to give: M(w) = 80.41 +/- 0.41 +/- 0.13 GeV, where the first error is statistical and the second is systematic. The systematic error is dominated by the uncertainty on the lepton energy, which is calibrated using data, and the parameterization of the variables used in the fitting, which is obtained using Monte Carlo events. Both of these are limited by statistics.

We discuss MAXIPOL, a bolometric balloon-borne experiment designed to measure the E-mode polarization anisotropy of the cosmic microwave background radiation (CMB) on angular scales of 10' to 2degrees. MAXIPOL is the first CMB experiment to collect data with a polarimeter that utilizes a rotating half-wave plate and fixed wire-grid polarizer. We present the instrument design, elaborate on the polarimeter strategy and show the instrument performance during flight with some time domain data. Our primary dataset was collected during a 26 h turnaround flight that was launched from the National Scientific Ballooning Facility in Ft. Sumner, New Mexico in May 2003. During this flight five regions of the sky were mapped. Data analysis is in progress. (C) 2003 Elsevier B.V. All rights reserved.

This paper describes topological searches for neutral scalar bosons S(0) produced in association with a Z(0) boson via the Bjorken process e(+)e(-) --> S(0)Z(0) at centre-of-mass energies of 91 GeV and 183209 GeV. These searches are based on studies of the recoil mass spectrum of Z(0) --> e(+)e(-) and mu(+)mu(-) events and on a search for S(0)Z(0) with Z(0) --> v (v) over bar and So --> e(+)e(-) or photons. They cover the decays of the S(0) into an arbitrary combination of hadrons, leptons, photons and invisible particles as well as the possibility that it might be stable. No indication for a signal is found in the data and upper limits on the cross section of the Bjorken process are calculated. Cross-section limits are given in terms of a scale factor k with respect to the Standard Model cross section for the Higgs-strahlung process e(+)e(-) --> H(SM)(0)Z(0). These results can be interpreted in general scenarios independently of the decay modes of the S(0). The examples considered here are the production of a single new scalar particle with a decay width smaller than the detector mass resolution, and for the first time, two scenarios with continuous mass distributions, due to a single very broad state or several states close in mass.

For the first time at LEP the production of prompt photons is studied in the collisions of quasi-real photons using the OPAL data taken at e(+)e(-) centre-of-mass energies between 183 GeV and 209 GeV. The total inclusive production cross-section for isolated prompt photons in the kinematic range of transverse momentum p(T)(gamma)>3.0 GeV and pseudorapidity \eta(gamma)\

Charged particle momentum distributions are studied in the reaction e(+)e(-) --> hadrons, using data collected with the OPAL detector at centre-of-mass energies from 192 GeV to 209 GeV. The data correspond to an average centre-of-mass energy of 201.7 GeV and a total integrated luminosity of 433 pb(-1). The measured distributions and derived quantities, in combination with corresponding results obtained at lower centre-of-mass energies, are compared to QCD. predictions in various theoretical approaches to study the energy dependence of the strong interaction and to test QCD as the theory describing it. In general, a good agreement is found between the measurements and the corresponding QCD predictions.

Non-commutative QED would lead to deviations from the Standard Model depending on a new energy scale A(NC) and a unique direction in space defined by two angles eta and xi. In this analysis, eta is defined as the angle between the unique direction and the rotation axis of the earth. The predictions of a tree level calculation for the process e(+)e(-) --> gammagamma are evaluated for the specific orientation of the OPAL detector and compared to the measurements. Distributions of the polar and azimuthal photon angles are used to extract limits on the energy scale A(NC) depending on the model parameter eta. It is shown that the time dependence of the total cross-section could be used to determine the model parameter xi if there were a detectable signal. This is the first experimental study of non-commutative QED at an e(+)e(-) collider. (C) 2003 Published by Elsevier B.V.

The exclusive production of proton-antiproton pairs in the collisions of two quasi-real photons has been studied using data taken at rootS(ee) = 183 GeV and 189 GeV with the OPAL detector at LEP. Results are presented for p (p) over bar invariant masses, W, in the range 2.15

The tau(-) --> mu(-)(nu) over bar (mu)nu(tau) branching ratio has been measured using data collected from 1990 to 1995 by the OPAL detector at the LEP collider. The resulting value of B (tau(-) --> mu-(nu) over bar (mu)nu(tau))= 0. 1734 +/- 0.0009(stat) +/- 0.0006(syst) has been used in conjunction with other OPAL measurements to test lepton universality, yielding the coupling constant ratios g(mu)/g(e) = 1.0005 +/- 0.0044 and g(tau)/g(e) = 1.0031 +/- 0.0048, in good agreement with the Standard Model prediction of unity. A value for the Michel parameter eta = 0.004 +/- 0.037 has also been determined and used to find a limit for the mass of the charged Higgs boson, m(H)+/- > 1.28 tan beta, in the Minimal Supersymmetric Standard Model. (C) 2002 Elsevier Science B.V. All rights reserved.

Searches for a scalar top quark and a scalar bottom quark have been performed using a data sample of 438 pb(-1) at centre-of-mass energies of roots- = 192-209 GeV collected with the OPAL detector at LEP. No evidence for a signal was found. The 95% confidence level lower limit on the scalar top quark mass is 97.6 GeV if the mixing angle between the supersymmetric partners of the left- and right-handed states of the top quark is zero. When the scalar top quark decouples from the Z(0) boson, the lower limit is 95.7 GeV. These limits were obtained assuming that the scalar top quark decays into a charm quark and the lightest neutralino, and that the mass difference between the scalar top quark and the lightest neutralino is larger than 10 GeV. The complementary decay mode of the scalar top quark decaying into a bottom quark, a charged lepton and a scalar neutrino has also been studied. The lower limit on the scalar top quark mass is 96.0 GeV for this decay mode, if the mass difference between the scalar top quark and the scalar neutrino is greater than 10 GeV and if the mixing angle of the scalar top quark is zero. From a search for the scalar bottom quark, a mass limit of 96.9 GeV was obtained if the mass difference between the scalar bottom quark and the lightest neutralino is larger than 10 GeV. (C) 2002 Elsevier Science B.V. All rights reserved.

A search is performed for production of short-lived particles in e(+)e(-) --> XY, with X --> gammagamma and Y --> f (f) over bar, for scalar X and scalar or vector Y. Model-independent limits in the range of 25-60 femtobarns are presented on a(e(+)e(-) --> XY) x B(X --> gammagamma) x B(Y --> f (f) over bar) for centre-of-mass energies in the range 205-207 GeV The data from all LEP centre-of-mass energies 88209 GeV are also interpreted in the context of fermiophobic Higgs boson models, for which a lower mass limit of 105.5 GeV is obtained for a "benchmark" fermiophobic Higgs boson. (C) 2002 Elsevier Science B.V. All rights reserved.

A search for charged excited leptons decaying into a lepton and a photon has been performed using approximately 680 pb(-1) of e(+)e(-) collision data collected by the OPAL detector at LEP at centre-of-mass energies between 183 and 209 GeV No evidence for their existence was found. Upper limits on the product of the cross-section and the branching fraction are inferred. Using results from the search for singly produced excited leptons, upper limits on the ratio of the excited lepton coupling constant to the compositeness scale are calculated. From pair production searches, 95% confidence level lower limits on the masses of excited electrons, muons and taus are determined to be 103.2 GeV (C) 2002 Elsevier Science B.V. All rights reserved.

The b quark forward-backward asymmetry has been measured using hadronic Z(0) decays collected by the OPAL experiment at LEP. Z(0) --> b (b) over bar decays were selected using a combination of secondary vertex and lepton tags, and the sign of the b quark charge was determined using an inclusive tag based on jet, vertex and kaon charges. The results, corrected to the quark level, are: A(FB)(b)=0.0582 +/- 0.0153 +/- 0.0012 at, roots = 89.50GeV, A(FB)(b)=0.0977 +/- 0.0036 +/- 0.0018 at roots=91.26GeV, A(FB)(b)=0.1221 +/- 0.0123 +/- 0.0025 at roots=92.91GeV, where the first error is statistical and the second systematic in each case. Within the framework of the Standard Model, the result is interpreted as a measurement of the effective weak mixing angle for electrons of sin(2) theta(W)(eff.e) = 0.32305 +/- 0.00068 (C) 2002 Elsevier Science B.V. All rights reserved.

The interaction of virtual photons is investigated using the reaction e(+)e(-) --> e(+)e(-) hadrons based on data taken by the OPAL experiment at e(+)e(-) centre-of-mass energies roots(ec) = 189-209 GeV, for W > 5 GeV and at an average Q(2) of 17.9 GeV(2). The measured cross-sections are compared to predictions of the Quark Parton Model (QPM), to the Leading Order QCD Monte Carlo model PHOJET, to the NLO prediction for the reaction e(+)e(-) --> e(+)e(-) q (q) over bar, and to BFKL calculations. PHOJET, NLO e(+)e(-) --> e(+)e(-) q (q) over bar, and QPM describe the data reasonably well, whereas the cross-section predicted by a Leading Order BFKL calculation is too large.

The process e(+)e(-) --> e(+)e(-)Z/gamma* is studied with the OPAL detector at LEP at a centre of mass energy of roots = 189 GeV. The cross-section times the branching ratio of the Z/gamma* decaying into hadrons is measured within Lorentz invariant kinematic limits to be (1.2 +/- 0.3 +/- 0.1) pb for invariant masses of the hadronic system between 5 GeV and 60 GeV and (0.7 +/- 0.2 +/- 0.1) pb for hadronic masses above 60 GeV. The differential cross-sections of the Mandelstam variables (s) over cap, (t) over cap, and (u) over cap are measured and compared with the predictions from the Monte Carlo generators grc4f and PYTHIA. From this, based on a factorisation ansatz, the total and differential cross-sections for the subprocess egamma --> eZ/gamma* are derived.

A search for first generation scalar and vector leptoquarks (LQ) as well as for squarks (El)in R-parity violating SUSY models with the direct decay of the q into Standard Model particles has been performed using e(+)e(-) collisions collected with the OPAL detector at LEP at an e+e- centre-of-mass energy roots(ee) of 189 GeV. The data correspond to an integrated luminosity of about 160 pb(-1). The dominant process for this search is eq --> LQ/q --> eq, vq, where a photon, which has been radiated by one of the beam electrons, serves as a source of quarks. The numbers of selected events found in the two decay channels are in agreement with the expectations from Standard Model processes. This result allows to set lower limits at the 95% confidence level on the mass of first generation scalar and vector leptoquarks, and of squarks in R-parity violating SUSY models. For Yukawa couplings lambda to fermions larger than root4pialpha(em), the mass limits range from 121 GeV/c(2) to 175 GeV/c(2) (149 GeV/c(2) to 188 GeV/c(2)) depending on the branching ratio 3 of the scalar (vector) leptoquark state. Furthermore, limits are set on the Yukawa couplings A for leptoquarks and lambda(1)(jk)(') for squarks, and on beta as a function of the scalar leptoquark/squark mass.

A search for pair-produced doubly charged Higgs bosons has been performed using data samples corresponding to an integrated luminosity of about 614 pb(-1) collected with the OPAL detector at LEP at centre-of-mass energies between 189 GeV and 209 GeV. No evidence for a signal has been observed. A mass limit of 98.5 GeV/c(2) at the 95% confidence level has been set for the doubly charged Higgs particle in left-right symmetric models. This is the first search for doubly charged Higgs bosons at centre-of-mass energies larger than 91 GeV. (C) 2002 Elsevier Science B.V. All rights reserved.

Searches for first generation scalar and vector leptoquarks, and for squarks in R-parity violating SUSY models with the direct decay of the squark into Standard Model particles, have been performed using e(+)e(-) collisions collected with the OPAL detector at LEP at e(+)e(-) centre-of-mass energies between 189 and 209 GeV. No excess of events is found over the expectation from Standard Model background processes. Limits are computed on the leptoquark couplings for different values of the branching ratio to electron-quark final states. (C) 2002 Elsevier Science B.V. All rights reserved.

The prototype of the Read Out Driver for the ATLAS Muon Endcap trigger system is described. The hardware is based on a single large Xilinx Virtex FPGA that accepts data from four gigabit optical links and sends processed output to the ATLAS central DAQ via an S-link and sampled data to a VME processor via the VMEbus. A C-like procedural language was used for a large part of the design. In addition to using pipelined logic and other standard FPGA design techniques, the design uses some architectural elements that are more common in microprocessor architectures. The ROD's design features, implementation details, DAQ software, experience with the procedural language and performance are described.

The hadronic structure function of the photon F(2)(gamma) (x, Q(2)) is measured as a function of Bjorken x and of the photon virtuality Q(2) using deep-inelastic scattering data taken by the OPAL detector at LEP at e(+)e(-) centre-of-mass energies from 183 to 209 GeV Previous OPAL measurements of the x dependence of F(2)(gamma) are extended to an average Q(2) of = 780 GeV(2) using data in the kinematic range 0.15

This final analysis of hadronic and leptonic cross-sections and of leptonic forward-backward asymmetries in e(+)e(-) collisions With the OPAL detector makes use of the full LEP 1 data sample comprising 161pb(-1) of integrated luminosity and 4.5 x 10(6) selected Z decays. An interpretation of the data in terms of contributions from pure Z exchange and from gamma /Z interference allows the parameters of the Z resonance to be determined in a model-independent way. Our results are in good agreement with lepton universality anti consistent with the vector and axial-vector couplings predicted in the Standard Model. A fit to the complete dataset yields the fundamental Z resonance parameters: m(z) = (91.1852 +/- 0.0030) GeV. Gamma (Z) = 12.4948 +/- 0.0041) GeV. sigma (0)(h) = (41.501 +/- 0.055) nb. R(l) = 20.823 +/- 0.044, and A(FB)(0,l) = 0.0145 +/- 0.0017. Transforming these parameters gives a measurement of the ratio between the decay width into invisible particles and the width to a single species of charged lepton. Gamma (inv)/Gamma (ll) = 5.942 +/- 0.027. Attributing the entire invisible width to neutrino, decays and assuming the Standard Model couplings for neutrinos. this translates into a measurement of the effective number of light neutrino species. N(nu) = 2.984 +/- 0.013. Interpreting the data within the context of the Standard Model allows the mass of the top quark, m(t) = (162(-16)(+29)) GeV, to be determined through its influence oil radiative corrections. Alternatively, utilising the direct external measurement of m(t) as an additional constraint leads to a measurement of the strong coupling constant and the mass of the Higgs boson: alpha (s)(m(Z)) = 0.127 +/- 0.005 and m(H) = (390(-280)(+750)) GeV.

Measurements are presented of the polarisation of W+W- boson pairs produced in e(+)e(-) collisions. and of CP-violating WWZ and WW gamma trilinear gauge couplings. The data were recorded by the OPAL experiment at LEP during 1998. where a total integrated luminosity of 183 pb(-1) was obtained at a centre-of-mass energy of 189 GeV. The measurements are performed through a spin density matrix analysis of the W boson decay products. The fraction of W bosons produced with longitudinal polarisation was found to be sigma (L)/sigma (total) = (21.0 +/- 3.3 +/- 1.6)% where the first error is statistical and the second systematic. The joint W boson pair production fractions were found to be sigma (TT)/sigma (total) = (78.1 +/- 9.0 +/- 3.2)%, sigma (LL)/sigma (total) = (20.1 +/- 7.2 +/- 1.8)% and sigma (LL)/sigma (total) = (1.8 +/- 14.7 +/- 3.8)%. In the CP-violating trilinear gauge coupling sector we find )over tilde> = -0.02 (+0.10)(-0.07), g(4)(z) = -0.02(-0.33)(+0.32) and )over tilde>(z) = -0.18(-0.16)(+0.24), where errors include both statistical and systematic uncertainties. In each case the coupling is determined with all other couplings set to their Standard model values except those related to the measured coupling via SU(2)(L) x U(1)(Y) symmetry. These results are consistent with Standard Model expectations.

Searches for final states expected in models with light gravitinos have been performed, including experimental topologies with multi-leptons with missing energy, leptons and photons with missing energy, and jets and photons with missing energy. No excess over the expectations from the Standard Model has been observed. Limits are placed on production cross-sections in the different experimental topologies. Additionally, combining with searches for the anomalous production of lepton and photon pairs with missing energy, results are interpreted in the context of minimal models of gauge mediated SUSY breaking, Exclusion limits at the 95% confidence level on the supersymmetric particle masses of m((l) over tilde) > 83 GeV and m()over tilde>1)(0) > 85 GeV for tan beta = 2, and m()over tilde>), 69 GeV, m((e) over tilde,)over tilde>) > 88 GeV and m()over tilde>1)(0) > 76 GeV for tan beta = 20, are established, (C) 2001 Elsevier Science B.V, All rights reserved.

From an analysis of the ionisation energy loss of charged particles selected from 110326 e(+)e(-) --> tau (+)tau (-) candidates recorded by the OPAL detector at e(+)e(-) centre-of-mass energies near the Z(0) resonance, we determine the one-prong tan decay branching ratios: Br(tau (-) --> nu (tau)K(-) greater than or equal to 0h(0)) = (1.528 +/- 0.039 +/- 0.040)% Br(tau (-) --> nu (tau)K(-)) = (0.658 +/- 0.027 +/-0.029)% where the h(0) notation refers to a pi (0), an eta, a K(S)(0), or a K(L)(0), and where the first uncertainty is statistical and the second is systematic.

The rates are measured per hadronic Z(0) decay for gluon splitting to b (b) over bar quark pairs, g(b (b) over bar), and of events containing two b (b) over bar quark pairs, g(4b), using a sample of four-jet events selected from data collected with the OPAL detector. Events with an enhanced signal of gluon splitting to b (b) over bar quarks are selected if two of the jets are close in phase-space and contain detached secondary vertices. For the event sample containing two b (b) over bar quark pairs, three of the four jets are required to have a significantly detached secondary vertex. Information from the event topology is combined in a likelihood fit to extract the values of g(b (b) over bar) and g(4b), namely g(b (b) over bar) = (3.07 +/- 0.53(stat) +/- 0.97(syst)) x 10(-3), g(4b) = (0.36 +/- 0.17(stat) +/- 0.27(syst)) x 10(-3).

A measurement of triple gauge boson couplings is presented, based on W-pair data recorded by the OPAL detector at LEP during 1998 at a centre-of-mass energy of 189 GeV with an integrated luminosity of 183 pb(-1). After combining with our previous measurements at centre-of-mass energies of 161-183 GeV we obtain kappa = 0.97(-0.16)(+0.20), g(1)(z) = 0.991(-0.057)(+0.060) and lambda = -0.110(-0.055)(+0.058), where the errors include both statistical and systematic uncertainties and each coupling is determined by setting the other two couplings to their Standard Model values. These results are consistent with the Standard Model expectations.

The mass and width of the W boson are determined in e(+)e(-) collisions at LEP using 183 pb(-1) of data recorded at a centre-of-mass energy roots = 189 GeV with the OPAL detector, The invariant mass distributions from 970 W+W- --> qqqq and 1118 W+W- --> qq(l)v(l) candidate events are used to measure the mass of the W boson, M-W = 80.451 +/- 0.076 (stat.) +/- 0.049 (syst.) GeV, A direct measurement of the width of the W boson gives Gamma (W) = 2.09 +/- 0.18 (stat,) +/- 0.09 (syst,) GeV, The results are combined with previous OPAL results from 78 pb(-1) of data recorded with roots from 161 to 183 GeV, to obtain: M-W = 80.432 +/- 0.066 (stat,) +/- 0.035 (syst,) GeV, Gamma (W) = 2.04 +/- 0,16 (stat,) +/- 0.09 (syst.) GeV, The consistency of the direct measurement of Mw with that inferred from other measurements of electroweak parameters provides an important test of the Standard Model of electroweak interactions. (C) 2001 Elsevier Science B,V, All rights reserved.

Bose-Einstein correlations in pairs of charged kaons produced in a sample of 3.9 million hadronic Z(0) decays have been measured with the OPAL experiment at LEP. Charged kaons were identified in the central tracking detector using their specific energy loss in the drift chamber gas. The correlation function was studied in two-jet events using a double ratio, formed by the number of like-sign pairs normalised by a reference sample in the data, divided by the same ratio in a Monte Carlo simulation. The enhancement at small values of the four-momentum difference of the pair was parametrised using a Gaussian function. The parameters of the Bose-Einstein correlations were measured to be a lambda = 0.82 +/-0.22(-0.12)(+0.17) for the strength and R-0 = 0.56 +/-0.08(-0.06)(+0.08) fm for the kaon source radius, where the first errors are statistical and the second systematic. Corrections for final-state interactions are discussed.

Data on muon pair production obtained by the OPAL Collaboration at centre of mass energies near the Z peak are analysed. Small angular mismatches between the directions of the two muons are used to assess the effects of initial state photon radiation and initial-final-state radiation interference on the forward-backward asymmetry of muon pairs. The dependence of the asymmetry on the invariant mass of the pair is measured in a model-independent way. Effective vector and axial-vector couplings of the Z boson are determined and compared to the Standard Model expectations. (C) 2001 Elsevier Science B.V. All rights reserved.

The magnitude of the CKM matrix element /V-ub/ is determined by measuring the inclusive charmless semileptonic branching fraction of beauty hadrons at OPAL based on b --> X(u)l nu event topology and kinematics. This analysis uses OPAL data collected between 1991 and 1995, which correspond to about four million hadronic Z decays. We measure Br(b --> X(u)l nu) to be (1.63 +/-0.53 (+0.15)(-0.62)) X 10(-3). The first uncertainty is the statistical error and the second is the systematic error. From this analysis,. /V-ub/ is determined to be: /Vub/ = (4.00 +/-0.65(stat)(-0.76)(+0.67) (sys) +/- 0.19(HQE)) X 10(-3). The last error represents the theoretical uncertainties related to the extraction of /V-ub/ from Br(b --> X(u)l nu) using the Heavy Quark Expansion.

A search for the Standard Model Higgs boson has been performed with the OPAL detector at LEP based on the full data sample collected at roots approximate to 192-209 GeV in 1999 and 2000, corresponding to an integrated luminosity of approximately 426 pb(-1). The data are examined for their consistency with the background-only hypothesis and various Higgs boson mass hypotheses. A lower bound of 109.7 GeV is obtained on the Higgs boson mass at the 95% confidence level. At higher masses, the data are consistent with both the background and the signal-plus-background hypotheses. (C) 2001 Elsevier Science B.V. All rights reserved.

A sample of 3.73 million hadronic Z decays, recorded with the OPAL detector at LEP in the years 1991.95. has been used to search for a narrow resonance corresponding to the decay of the D*(/+/-)(2629) meson into D*(+/-)pi (+)pi (-). The D*(+) mesons are reconstructed in the decay channel D*(+) --> D(0)pi (+) with D-0 --> K(-)pi (+). No evidence for a narrow D*(/+/-)(2629) resonance is found. A limit on the production of narrow D*(/+/-)(2629) in hadronic Z decays is derived: f(Z --> D*(/+/-)(2629)) x Br(D*(/+) --> D*(+)pi (+)pi (-))

From data collected around the Z resonance by the OPAL detector at LEP, a sample of B-s(0) decays was obtained using D(s)(-)l(+) combinations. where the D-s(-) was fully reconstructed in the phi pi (-), K*K-0(-) and (KsK-)-K-0 decay channels or partially reconstructed in the phil(-)(nu)over bar>(X) decay channel. These events mere used to study B-s(0) oscillation. The flavor (b or (b) over bar) at decay was determined from the lepton charge while the flavor at production was determined from a combination of techniques. The expected sensitivity of the experiment is 4.1 ps(-1) The experiment was not able to resolve the oscillatory behavior. and we deduced that the B-s(0) oscillation frequency Deltam(s) > 1.0 ps(-1) at the 95% confidence level.

About 4.4 million hadronic decays of Z bosons, recorded by the OPAL detector at LEP at a centre-of-mass energy of around roots = 91.2 GeV, are used to determine the mean charged particle multiplicities for the three light quark flavours. Events from primary u, d, and s quarks are tagged by selecting characteristic particles which carry a large fraction of the beam energy. The charged particle multiplicities are measured in the hemispheres opposite to these particles. An unfolding procedure is applied to obtain these multiplicities for each primary light quark flavour. This yields [n(u)] = 17.77 +/- 0.51(-1.20)(+0.86), [n(d)] = 21.44 +/- 0.63(-1.17)(+1.46), [n(s)] = 20.02 +/- 0.13(-0.37)(+0.39), where statistical and systematic errors are given. The results for [n(u)] and [n(d)] are almost fully statistically anti-correlated. Within the errors the result is consistent with the flavour independence of the strong interaction for the particle multiplicities in events from the light up, down, and strange quarks.

Using data from e(+)e(-) annihilation into hadrons, taken with the OPAL detector at LEP at the Z pole between 1991 and 1995, we performed a simultaneous measurement of the colour factors of the underlying gauge group of the strong interaction, C(F) and C(A), and the strong coupling, alpha (s). The measurement was carried out by fitting next-to-leading order perturbative predictions to measured angular correlations of 4-jet events together with multi-jet related variables. Our results, C(A) = 3.02 +/- 0.25(stat.) +/- 0.49(syst.), CF = 1.34 +/- 0.13(stat.) +/- 0.22(syst.), alpha (s) (M(Z)) = 0.120 +/- 0.011(stat.) +/- 0.020(syst,), provide a test of perturbative QCD in which the only assumptions are non-abelian gauge symmetry and standard hadronization models. The measurements are in agreement with SU(3) expectations for C(F) and C(A) and the world average of alpha (s)(M(Z)).

Measurements of the tau lepton polarization and forward-backward polarization asymmetry near the Z(0) resonance using the OPAL detector are described. The measurements are based on analyses of tau -->e nu (e)nu (tau), tau --> mu nu (mu)nu (tau), tau --> pi nu (tau), tau --> rho nu (tau) and tau -->a(1)nu (tau) decays from a sample of 144,810 e(+) e(-) --> tau (+) tau (-) candidates corresponding to an integrated luminosity of 151 pb(-1). Assuming that the tau lepton decays according to V-A theory, we measure the average tau polarization near roots = M(z) to be = (-14.10 +/- 0.73 +/-0.55)% and the tau polarization forward-backward asymmetry to be A(pol)(FB) = (-10.55 +/-0.76 +/-0.25)%, where the first error is statistical and the second systematic. Taking into account the small effects of the photon propagator, photon-Z(0) interference and photonic radiative corrections, these results can be expressed in terms of the lepton neutral current asymmetry parameters: A(tau) = 0.1456 +/- 0.0076 +/- 0.0057 A(e) = 0.1454 +/- 0.0108 +/- 0.0036 These measurements are consistent with the hypothesis of lepton universality and combine to give A(t) = 0.1455 +/-0.0073. Within the context of the Standard Model this combined result corresponds to = 0.23172 +/-0.00092. Combing these results with those from the other OPAL neutral current measurements yields a value of = 0.23211 +/-0.00068.

In hadronic decays of Z bosons recorded with the OPAL detector at LEP, events containing b quarks were selected using the long lifetime of b flavoured hadrons. Comparing the 3-jet rate in b events with that in d,u,s and c quark events, a significant difference was observed. Using O(alpha (2)(S)) calculations for massive quarks, this difference was used to determine the b quark mass in the YI-S renormalisation scheme at the scale of the Z boson mass. By combining the results from seven different jet finders the running b quark mass was determined to be (m) over bar (b) (m(Z)) = (2.67 +/-0.03(stat.)(-0.37)(+0.29)(syst.)+/-0.19(theo.)) GeV. Evolving this value to the b quark mass scale itself yields (m) over bar (b)((m) over bar (b)) = (3.95(-0.62)(+0.52))GeV, consistent with results obtained at the b quark production threshold. This determination confirms the QCD expectation of a scale dependent quark mass. A constant mass is ruled out by 3.9 standard deviations.

Correlations among hadrons with the same electric charge produced in Z(0) decays are studied using the high statistics data collected from 1991 through 1995 with the OPAL detector at LEP. Normalized factorial cumulants up to fourth order are used to measure genuine particle correlations as a function of the size of phase space domains in rapidity, azimuthal angle and transverse momentum. Both all-charge and like-sign particle combinations show strong positive genuine correlations. One-dimensional cumulants initially increase rapidly with decreasing size of the phase space cells but saturate quickly. In contrast, cumulants in two- and three-dimensional domains continue to increase. The strong rise of the cumulants for all-charge multiplets is increasingly driven by that of like-sign multiplets. This points to the likely influence of Bose-Einstein correlations. Some of the recently proposed algorithms to simulate Bose-Einstein effects, implemented in the Monte Carlo model PYTHIA, are found to reproduce reasonably well the measured second- and higher-order correlations between particles with the same charge as well as those in all-charge particle multiplets. (C) 2001 Elsevier Science B.V. All rights reserved.

A search for single top quark production via flavour changing neutral currents (FCNC) was performed with data collected by the OPAL detector at the e(+)e(-) collider LEP. Approximately 600 pb(-1) of data collected at roots = 189-209 GeV were used to search for the FCNC process e(+)e(-) --> tc(u) --> bWc(u). This analysis is sensitive to the leptonic and the hadronic decay modes of the W boson. No evidence for a FCNC process is observed. Upper limits at the 95% confidence level on the single top production cross-section as a function of the centre-of-mass energy are derived. Limits on the anomalous coupling parameters K(gamma) and K(Z) are determined from these results. (C) 2001 Elsevier Science B.V. All rights reserved.

We search for lepton flavour violating events (e mu, e tau and mu tau) that could be directly produced in e(+) e(-) annihilations, using the full available data sample collected with the OPAL detector at centre-of-mass energies between 189 GeV and 209 GeV. In general, the Standard Model expectations describe the data well for all the channels and at each roots. A single ett event is observed where according to our Monte Carlo simulations only 0.019 events are expected from Standard Model processes. We obtain the first limits on the cross-sections sigma (e(+)e(-) --> e mu, e tau and mu tau) as a function of roots at LEP2 energies. (C) 2001 Elsevier Science B.V. All rights reserved.

Searches for the neutral Higgs bosons h(0) and A(0) are used to obtain limits on the Type II Two Higgs Doublet Model (2HDM(II)) with no CP-violation in the Higgs sector and no additional particles besides the five Higgs bosons. The analysis combines approximately 170 pb(-1) of data collected with the OPAL detector at roots approximate to 189 GeV with previous runs at roots approximate to m(Z) and roots approximate to 183 GeV. The searches are sensitive to the h(0), A(0) --> q (q) over bar, gg. tau (+)tau (-) and h(0) --> A(0)A(0) decay modes of the Higgs bosons. For the first time, the 2HDM(II) parameter space is explored in a detailed scan, and new flavour independent analyses are applied to examine regions in which the neutral Higgs bosons decay predominantly into light quarks or gluons. Model-independent limits are also given.

The inclusive branching ratio for the process b --> tau (-)(v) over bar X-tau has been measured using hadronic Z decays collected by the OPAL experiment at LEP in the years 1992-2000. The result is: BR(b --> tau (-)(v) over bar X-tau) = (2.78 +/- 0.18 +/- 0.51)%. This measurement is consistent with the Standard Model expectation and puts a constraint of tan beta /M-H(+/-)

Using about 3.9 million hadronic Z decays from e(+)e(-) collisions recorded by the OPAL detector at LEP at centre-of-mass energies,roots approximate to M-Z, the branching ratio for the decay D-s(-)--> tau (-)(v) over bar (tau) has been measured to be BR(D-s(-) --> tau (-)(v)over bar>) = (7.0 +/- 2.1(stat) +/- 2.0(syst))%. This result can be used to derive the decay constant of the D-s(-) meson: f(Ds) = (286 +/- 44(stat) +/- 41 (syst)) MeV. (C) 2001 Elsevier Science B.V. All rights reserved.

The inclusive production of D*(+/-) mesons in photon-photon collisions has been measured using rile OPAL detector at LEP at e(+)e(-) centre-of-mass energies root S-ee of 183 and 189 GeV. The D*(+) mesons are reconstructed in their decay to D(0)pi(+) with the D-0 observed in the two decay modes K(-)pi(+) and K(-)pi(+)pi(-)pi(+) After background subtraction, 100.4 +/- 12.6 (stat) D*(+/-) mesons have been selected in events without observed scattered beam electron ("anti-tagged") and 29.8 +/- 5.9 (stat)D*(+/-) mesons in events where one beam electron is scattered into the detector ("single-tagged"). Direct and single-resolved events are studied separately. Differential cross-sections d sigma/dp(T)(D*) and d sigma/d \eta(D*)\ as functions of the D*+/- transverse momentum p(T)(D*) and pseudorapidity eta(D*) are presented in the kinematic region 2 GeV

The branching ratio of the tau lepton to a neutral kaon meson is measured from a sample of approximately 200,000 tau decays recorded by the OPAL detector at centre-of-mass energies near the Z(0) resonance. The measurement is based on two samples which identify one-prong tau decays with K(L)(0) and K(S)(0) mesons. The combined branching ratios are measured ed to be B(tau(-) --> pi(-)(K) over bar(0)nu(tau)(9.33 +/- 0.68 +/- 0.49) x 10(-3) B(tau(-) --> pi(-)(K) over bar(0)[greater than or equal to 1 pi(0)]nu(tau)) = (3.24 +/- 0.74 +/- 0.66) x 10(-3) B(tau(-) --> K(-)K(0)[greater than or equal to 0 pi(0)]nu(tau)) = (3.30 +/- 0.55 +/- 0.39) x 10(-3) where the first error is statistical and the second systematic.

A search for the decay B(+/-) --> K(+/-)K(+/-)pi(-/+) was performed using data collected by the OPAL detector at LEP. These decays are strongly suppressed in the Standard Model but could occur with a higher branching ratio in supersymmetric models, especially in those with R-parity violating couplings. No evidence for a signal was observed and a 90% confidence level upper limit of 1.29 X 10(-4) was set for the branching ratio. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

The correlated production of Lambda and baryons has been studied using 4.3 million multihadronic Z(0) decays recorded with the OPaL detector at LEP. Lambda pairs were investigated in the full data sample and for the first time also in 2-jet and 3-jet events selected with the k(perpendicular to) algorithm. The distributions of rapidity differences, from correlated Lambda pairs exhibit short-ranger local correlations and prove to be a sensitive tool to test models: particularly for 2-jet events. The JETSET model describes the data best but some extra parameter tuning is needed to improve agreement with the experimental results in the rates and the rapidity spectra simultaneously. The recently developed modification of JETSET, the MOdified Popcorn Scenarium (MOPS), and also HERWIG do not give satisfactory results. This study of di-lambda production in 2- and 3-jet events supports the short-range compensation of quantum numbers.

The measurement of small-angle Bhabha scattering is used to determine the luminosity at the OPAL interaction point for the LEP I data recorded between 1993 and 1995. The measurement is based on the OPAL Silicon-Tungsten Luminometer which is composed of two calorimeters encircling the LEP beam pipe, on opposite sides of the interaction point. The luminometer detects Electrons from small-angle Bhabha scattering at angles betn een 25 and 58 mrad. At LEP center-of-mass energies around the Z(0), about half of all Bhabha electrons entering the detector fall within a 79 nb fiducial acceptance region. The electromagnetic showers generated in the stack of 1 radiation length tungsten absorber plates are sampled by 608 silicon detectors with 38,912 radial pads of 2.5 mm width. The fine segmentation of the detector, combined with the precise knowledge of its physical dimensions, allows the trajectories of incoming 45 GeV electrons or photons to be determined with a total systematic error of less than 7 microns. We have quantified all significant sources of systematic experimental error in the luminosity determination Lv direct physical measurement. All measured properites of the luminosity event sample are found to bk in agreement with current theoretical expectations. The total systematic measurement uncertainty is 3.4x10(-4), significantly below the theoretical error of 5.4x10(-4) currently assigned to the QED calculation of the Bhabha acceptance, and contributes negligibly to the total uncertainty in the OPAL measurement of Gamma(inv)/Gamma(l+l-), a quantity of basic physical interest which depends crucially on the luminosity measurement.

From a data sample of 183 pb(-1) recorded at a center-of-mass energy of roots=189 GeV with the OPAL detector at LEP, 3068 W-pair candidate events are selected. Assuming Standard Model W boson decay branching fractions, the W-pair production cross section is measured to be sigma (WW) = 16.30 +/- 0.34(stat.) +/- 0.18(syst.) pb. When combined with previous OPAL measurements, the W boson branching fraction to hadrons is determined to be 68.32 +/- 0.61(stat.) +/- 0.28(syst.)% assuming lepton universality. These results are consistent with Standard Model expectations. (C) 2000 Published by Elsevier Science B.V.

We measure the mass of the tau to be 1775.1 +/- 1.6(m(cn)(stat.)) +/- 1.0(m(cn)(sys.)) MeV using tau from Z(0) decays. To test CPT invariance we compare the masses of the positively and negatively charged tau. The relative mass difference is found to be smaller than 3.0 x 10(-3) at the 90%. confidence level. (C) 2000 Elsevier Science B.V. All rights reserved.

Using data recorded at centre-of-mass energies around 183 and 189 GeV with the OPAL detector at LEP, the fundamental coupling of the charm quark to the W boson has been studied. The ratio R(c)(W) = Gamma (W --> cX)/Gamma(W --> hadrons) has been measured from jet properties, lifetime information, and leptons produced in charm decays. A value compatible with the Standard Model expectation of 0.5 is obtained: R(c)(W) = 0.481 +/- 0.042 (stat.) +/- 0.032(syst.). By combining this result with measurements of the W boson total width and hadronic branching ratio, the magnitude of the CKM matrix element IV,,I is determined to be /V(cs)/ = 0.969 +/- 0.058. (C) 2000 Elsevier Science B.V. All rights reserved.

Photonic events with large missing energy have been observed in e(+)e(-) collisions at a centre-of-mass energy of 189 GeV using the OPAL detector at LEP. Results are presented for event topologies consistent with a single photon or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection, and the number of light neutrino species is measured. Cross-section results are compared with the expectations from the Standard Model process e(+)e(-) --> nu)over bar> + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on sigma (e(+)e(-) --> XY) . BR(X --> Y-gamma) and sigma (e(+)e(-) --> XX) . BR2(X --> Y gamma) are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos (X = nu*,Y = nu), to neutralino production (X = )over tilde>(0)(2),Y = )over tilde>(0)(1)) and to supersymmetric models in which X = )over tilde>(0)(1) and Y = (G) over tilde is a Light gravitino. The case of macroscopic decay lengths of particle X is considered for e(+)e(-) --> XX, X --> Y gamma, when M-Y approximate to 0. The single-photon results are also used to place upper limits on superlight gravitino pair production as well as graviton-photon production in the context of theories with additional space dimensions.

The inclusive branching fraction of phi masons from the decay of b hadrons produced in Z decays was measured to be Br(b --> phiX) = 0.0282 +/- 0.0013 (stat.) +/- 0.0019 (syst.), using data collected by the OPAL detector at LEP. (C) 2000 Elsevier Science B.V. All rights reserved.

We compared the multiplicities of pi (0), eta, K-0 and of charged particles in quark and gluon jets in 3-jet events, as measured by the OPAL experiment at LEP. The comparisons were performed for distributions unfolded to 100% pure quark and gluon jets, at an effective scale Q(jet) which took into account topological dependences of the 3-jet environment. The ratio of particle multiplicity in gluon jets to that in quark jets as a function of Q(jet) for pi (0), eta or K-0 was found to be independent of the particle species. This is consistent with the QCD prediction that the observed enhancement in the mean particle rate in gluon jets with respect to quark jets should be independent of particle species. In contrast to some theoretical predictions and previous observations, we observed no evidence for an enhancement of eta meson production in gluon jets with respect to quark jets, beyond that observed for charged particles. We measured the ratio of the slope of the average charged particle multiplicity in gluon jets to that in quark jets, C, and we compared it to a next-to-next-to-next-to leading order calculation. Our result, C = 2.27 +/- 0.20(stat. + syst.), is about one standard deviation higher than the perturbative prediction.

The cross-section ratio R-b = sigma(e(+)e(-) --> b (b) over bar)/sigma(e(+)e(-) --> q (q) over bar) and the bottom and charm forward-backward asymmetries A(FB)(b) and A(FB)(c) are measured using event samples collected by the OPAL detector at centre-of-mass energies between 130 and 189 GeV. Events with bottom quark production are selected with a secondary vertex tag, and a hemisphere charge algorithm is used to extract A(FB)(b). In addition, the bottom and charm asymmetries are measured using leptons from semileptonic decays of heavy hadrons and pions from D*(+) --> D(0)pi(+) decays. The results are in agreement with the Standard Model predictions.

The total hadronic cross-section sigma gamma gamma(W) for the interaction of real photons, gamma gamma --> hadrons, is measured for gamma gamma centre-of-mass energies 10 less than or equal to W less than or equal to 110 GeV. The cross-section is extracted from a measurement of the process e(+)e(-) --> e(+)e(-) gamma*gamma* --> e(+)e(-) + hadrons, using a luminosity function for the photon flux together with form factors for extrapolating to real photons (Q(2) = 0 GeV(2)). The data, were taken with the OPAL detector at LEP at e(+)e(-) centre-of-mass energies root s(ee) = 161, 172 and 183 GeV. The cross-section sigma gamma gamma (W) is compared with Regge factorisation and with the energy dependence observed in yp and pp interactions. The data are also compared to models which predict a faster rise of sigma gamma gamma(W) compared to gamma p and pp interactions due to additional hard gamma gamma interactions not present in hadronic collisions.

Bose-Einstein correlations in pairs of identical charged pions produced in a sample of 4.3 million Z(0) hadronic decays are studied as a function of the three components of the momentum difference, transverse ("out" and "side") and longitudinal with respect to the thrust direction of the event. A significant difference between the transverse. r(tside), and longitudinal, r(iota), dimensions is observed, indicating that the emitting source of identical pions, as observed in the Longitudinally CoMoving System, hs an elongated shape. This is observed with a variety of selection techniques. Specifically, the values of che parameters obtained by fitting the extended Goldhaber parametrisation to the correlation function C' = C-DATA/C-MC for two-jet events, selected with the Durham algorithm and resolution parameter y(cut) = 0.04, are r(tside) = (0.809 +/- 0.000 (stat)(-0.032)(+0.019) (syst)) fm, r(iota) = (0.989 +/- 0.011 (stat)(-0.015)(+0.030) (syst)) fm and r(iota)/r(tside) = 1.222 +/- 0.027 (stat)(-0.012)(+0.075) (syst). The results are discussed in the context of a recent model of Bose-Einstein correlations based on string fragmentation. The results of a unidimensional analysis are also presented.

The helicity density matrix elements rho(00) of rho(770)(+/-) and omega(782) mesons produced in Z(0) decays have been measured using the OPAL detector at LEP. Over the measured meson energy range, the values are compatible with 1/3, corresponding to a statistical mix of helicity -1, 0 and +1 states. For the highest accessible scaled energy range 0.3

A study of Z boson pair production in e(+)e(-) annihilation at center-of-mass energies near 183 GeV and 189 GeV is reported. Final states containing only leptons, (l(+)l(-)l(+)l(-) and l(+)l(-) v (v) over bar), quark and lepton pairs, (q (q) over bar l(+)l(-), q (q) over bar v (v) over bar) and the all-hadronic final state (q (q) over bar q (q) over bar) are considered. In all states with at least one Z boson decaying hadronically, q (q) over bar and b (b) over bar final states are considered separately using lifetime and event-shape tags, thereby improving the cross-section measurement. At root s = 189 GeV the Z-pair cross section was measured to be 0.80(-0.13)(+0.14)(stat.)(-0.05)(+0.06)(syst.) pb, consistent with the Standard Model prediction. At root s = 183 GeV the 95% C.L. upper limit is 0.55 pb. Limits on anomalous ZZ gamma and ZZZ couplings are derived. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

A measurement of inclusive semileptonic branching fractions of b hadrons produced in Z(0) decays is presented. An enriched Z(0) --> b (b) over bar sample is obtained with a lifetime flavour-tagging technique. The leptonic events are then selected from, this sample, and classified according to their origin, which is determined by comparing the distribution of several kinematic variables using artificial neural network techniques. Using 3.6 million multihadronic events collected with the OPAL detector at energies near the Z(0) resonance, the values BR(b --> lX) = (10.84 +/- 0.09 (stat.) +/- 0.21 (syst.) (+0.21)(-0.13) (model)) BR(b --> c --> lX) = (8.39 +/- 0.15 (stat.) +/- 0.22 (syst.) -(+0.33)(0.29) (model)). are measured, where b denotes all weakly decaying b hadrons and l represents either e or mu The second error includes all experimental systematic uncertainties whereas the last error is due to uncertainties in modelling of the lepton momentum spectrum in semileptonic decays and b quark fragmentation. The average fraction of the beam energy carrried by the weakly decaying b hadron, is measured to be = 0.709 +/- 0.003 (stat.) +/- 0.003 (syst.) +/- 0.013 (model) where the modelling error is dominated by the choice of b fragmentation model. The agreement between data and various semileptonic decay models and fragmentation functions is also investigated.

A selection of di-lepton events with significant missing transverse momentum has been performed using a total data sample of 77.0 pb(-1) at e(+)e(-) centre-of-mass energies of 161 GeV, 172 GeV and 183 GeV. The observed numbers of events: four at 161 GeV, nine at 172 GeV, and 78 at 183 GeV, are consistent with the numbers expected from Standard Model processes, which arise predominantly from W+W- production with each W decaying leptonically. This topology is an experimental signature also for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Further event selection criteria are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is apparent and limits on the production cross-section times branching ratio squared for various new physics processes are presented in a manner intended to minimise the number of model assumptions. Assuming a 100% branching ratio for the decay rho(R)(+/-) --> rho(+/-) (0)(1), we exclude aat 95% CL: right-handed smuons with masses below 65 GeV for m() - m(1)(0) > 2 GeV and right-handed status with masses below 64 GeV for m(-) - m(1)(0) > 10 GeV. Right-handed selectrons are excluded at 95% CL for masses below 77 GeV for m((o) over tilde-) - m(1)(0) > 5 GeV within the framework of the Minimal Supersymmetric Standard Model assuming mu

A readout system for the ATLAS end cap muon trigger chamber has been developed. The system has a star architecture and a source synchronous serial communication is employed for the data path. FPGA modules are developed for prototyping and trial fabrication of ASIC is on going. (C) 2000 Elsevier Science B.V. All rights reserved.

A search for pair-produced leptoquarks has been performed using a sample of e(+)e(-) collision events collected by the OPAL detector at LEP at e(+)e(-) centre-of-mass energies of about 183 GeV. The data sample corresponds to an integrated luminosity of 55.9 pb(-1). The leptoquarks were assumed to be produced via couplings to the photon and the Z(0) and then to decay within a single fermion generation. No evidence for contributions from leptoquark pair production processes was observed. Lower limits on scalar and vector leptoquark masses are obtained. The existing limits are improved in the region of large decay branching ratio to quark-neutrino.

Searches fur the neutral Higgs bosons predicted by the Standard Model (SM) and the Minimal Supersymmetric extension of the Standard Model (MSSM) have been performed with the OPAL detector at LEP. Approximately 170 pb(-1) of e(+)e(-) collision data collected at root s approximate to 189 GeV were used to search for Higgs boson production in the SM processes e+e- --> H(0)Z(0) and the MSSM processes e(+)e(-) --> h(0)Z(0) and e(+)e(-) --> A(0)h(0). The searches are sensitive to the and tau(+)tau(-) decay modes of the Higgs bosons, and also to the MSSM decay mode h(0) --> A(0)A(0). OPAL search results at lower centre-of-mass energies have been incorporated in the limits, which are valid at the 95% confidence level. For the SM Higgs boson, a lower mass bound of 91.0 GeV is obtained. In the MSSM, the limits are m(H) > 74.8 GeV and m(A) > 76.5 GeV, assuming tan beta > 1, that the mixing of the scalar top quarks is either zero or maximal, and that the soft SUSY-breaking masses are 1 TeV. For the case of zero scalar top mixing, the values of tan beta between 0.72 and 2.19 are excluded.

A selection of di-lepton events with significant missing transverse momentum has been performed using a total data sample of 237.4 pb(-1) at e(+)e(-) centre-of-mass energies of approximately 183 GeV and 189 GeV. The observed numbers of events - 78 at 183 GeV and 301 at 189 GeV are consistent with the numbers expected from Standard Model processes, which arise predominantly from W+W- production with both W bosons decaying leptonically. This topology is also an experimental signature for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Discrimination techniques are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is apparent. Upper limits on the production cross-section times branching ratio squared for sleptons and for leptonically decaying charginos and charged Higgs are presented in a manner intended to minimise the number of model assumptions. Assuming a 100% branching ratio for the decay (l)over tilde(R)(+/-) --> l(+/-)(chi)over tilde(1)(0), where (chi)over tilde>(0)(1) is the lightest neutralino, we exclude at 95% CL: right-handed smuons with masses below 82.3 GeV for m((mu)over tilde) - m((chi)over tilde)(1)(0) > 3 GeV and right-handed staus with masses below 81.0 GeV for m((tau)over tilde>) - m((chi)over tilde)(1)(0) > 8 GeV. Right-handed selectruns are excluded at 95% CL for masses below 87.1 GeV for m(e)(over tilde)(-) - m(chi)(over tilde>)(0)(1) > 5 GeV, within the framework of the Minimal Supersymmetric Standard Model assuming mu tau(+/-)nu(tau).

From an analysis of the ionisation energy loss of charged particles selected from a sample of 147926 e(+)e(-)-->tau(+)tau(-) candidates recordec. in the OPAL detector at e(+)e(-) centre-of-mass energies near the Z(0) resonance, we determine the branching ratios: Br(tau(-) --> nu(tau)K(-)pi(-)pi(+)(pi(0))) = 0.343 +/- 0.073 +/- 0.031 % Br(tau(-)--> nu(tau) K(-) pi(-) K(+)(pi(0))) = 0.159 +/- 0.053 +/- 0.020 %, where the (pi(0)) notation refers to decay modes with or without an accompanying pi(0). The tau(-) --> nu(tau)K(-)pi(-)pi(+)(pi(upsilon)) final states occurring through tau(-) --> nu(tau)K-K(S)(0)(pi(0)) are treated as background in this analysis. We also examine the resonant structure of tau(-) --> nu(tau) K(-)pi(-)pi(+) candidates. Under the assumption that the resonant structure is dominated by the K(1) resonances, we determine: R=Br(tau(-)-->nu(tau) K(1)(1270))/Br(tau(-) --> nu(tau) K(1)(1400)) + Br(tau(-) K(1)(1270)) = 0.71 +/- 0.16 +/- 0.11. In all results, the first uncertainties are statistical and the second are systematic.

The lifetimes of the B+ and B-0 mesons, and their ratio, have been measured in the OPAL experiment using 2.4 million hadronic Z(0) decays recorded at LEP. Z(0) --> b (b) over bar decays were tagged using displaced secondary vertices and high momentum electrons and muons. The lifetimes were then measured using well-reconstructed charged and neutral secondary vertices selected in this tagged data sample. The results are tau(B+) = 1.643 +/- 0.037 +/- 0.025 ps tau(Bo) = 1.523 +/- 0.057 +/- 0.053 ps tau(B+)/tau(Bo) = 1.079 +/- 0.064 +/- 0.041, where in each case the first error is statistical and the second systematic. A larger data sample of 3.1 million hadronic Z(o) decays has been used to search for CP and CPT violating effects by comparison of inclusive b and (b) over bar hadron decays, No evidence fur such effects is seen. The CP violation parameter Re(epsilon(B)) is measured to be Re(epsilon(B)) = 0.001 +/- 0.014 +/- 0.003 and the fractional difference between b and (b) over bar hadron lifetimes is measured to (Delta tau/tau)(b) = tau(b hadron) - tau((b) over bar hadron)/tau(average) = -0.001 +/- 0.012 +/- 0.008.

A search for charginos and neutralinos, predicted by supersymmetric theories, is performed using a data sample of 182.1 pb(-1) taken at a centre-of-mass energy of 189 GeV with the OPAL detector at LEP. No evidence for chargino or neutralino production is found. Upper limits on chargino and neutralino pair production ((+)(1)(-)(1), (0)(1)(0)(2)) cross-sections are obtained as a function of the chargino mass (m(chi 1)(+/-)), the lightest neutralino mass (m(chi 1)(0)) and the second lightest neutralino mass (m(chi 2)(0)). Within the Constrained Minimal Supersymmetric Standard Model framework, and for m(chi 1)(+/-) - m(chi 1)(0) greater than or equal to 5 GeV, the 95% confidence level lower limits on m(chi 1)(-/+) are 93.6 GeV for tan beta = 1.5 and 94.1 GeV for tan beta = 35. These limits are obtained assuming a universal scalar mass m(0) greater than or equal to 500 GeV. The corresponding limits for all m(0) are 78.0 and 71.7 GeV. The 95% confidence level lower limits on the lightest neutralino mass, valid for any value of tan beta are 32.8 GeV for m(0) greater than or equal to 500 GeV and 31.6 GeV for all m(0).

The data recorded at a centre-of-mass energy of 189 GeV by the OPAL detector at LEP are used to search for trilinear couplings of the neutral gauge bosons in the process e(+)e(-) --> Z gamma. The cross-sections for multihadronic events with an energetic isolated photon, and for events with a high energy photon accompanied by missing energy are measured. These cross-sections and the photon energy, polar angle and isolation angle distributions are compared to the Standard Model predictions and to the theoretical expectations of a model allowing for Z gammaZ and Z gamma gamma vertices. Since no significant deviations with respect to the Standard Model expectations are found, constraints are derived on the strength of neutral trilinear gauge couplings.

The photon structure function F-2(gamma)(x, Q(2)) has been measured using data taken by the OPAL detector at e(+)e(-) centre-of-mass energies of 91 GeV, 183 GeV and 189 GeV, in Q(2) ranges of 1.5-30.0 GeV2 (LEP1), and 7.0-30.0 GeV2 (LEP2), probing lower values of x than ever before. Since previous OPAL analyses, new Monte Carlo models and new methods, such as multi-variable unfolding, have been introduced, reducing significantly the model dependent systematic errors in the measurement. The results do not conclusively prove, but are completely consistent with, the presence of a rise in F-2(gamma) at low-x as expected from QCD.

The energy distribution and type of the particle with the highest momentum in quark jets are determined for each of the live quark flavours making only minimal model assumptions. The analysis is based on a large statistics sample of hadronic Z(0) decays collected with the OPAL detector at the LEP e(+)e(-) collider. These results provide a basis for future studies of light flavour production at other cerltre-of-mass energies. We use our results to study the hadronisation mechanism in light flavour jets and compare the data to the QCD models JETSET and HERWIG. Within the JETSET model we also directly determine the suppression of strange quarks to be gamma(s) = 0.-122 +/- 0.039(stat.) +/- 0.059(syst.) by comparing the production of charged and neutral kaons in strange and non-strange light quark events. Finally we study the features of baryon production.

We have studied hadronic events fi om e(+)e(-) annihilation data at centre-of-mass energies of root s = 172. 183 and 189 GeV. The total integrated luminosity of the three samples, measured with the OPAL detector, corresponds to 250 pb(-1). We present distributions of event shape variables, charged particle multiplicity and momentum, measured separately in the three data samples. From these we extract measurements of the strong coupling a,, the mean charged particle multiplicity (n(ch)) and the peak position xi(0) in the xi(p) = ln(1/x(p)) distribution. In general the data are described well by analytic QCD calculations and Monte Carlo models. Our measured values of alpha(s), (n(ch)) and xi(0) are consistent with previous determinations at root s = M(Z)o.

The rate of secondary charm-quark-pair production has been measured in 4.4 million hadronic Z(0) decays collected by OPAL. By selecting events with three jets and tagging charmed hadrons in tie gluon jet candidate using leptons and D*+/- mesons, the average number of secondary charm-quark pairs per hadronic event is found to be (3.20+/-0.21+/-0.38) x 10(-2).

Cross-sections and angular distributions for hadronic and lepton pair final states in e(+) e(-) collisions at a centre-of-mass energy near 189 GcV. measured with the OPAL detector at LEP. are presented and compared with the predictions of the Standard Model. The results are used to measure the energy dependence of the electromagnetic coupling constant alpha(em), and to place limits: on new physics as described by four-fermion contact interactions or bq the exchange of a new heavy particle such as a sneutrino in super symmetric theories with R-parity violation. A search for the indirect effects of the gravitational interaction in extra dimensions on the mu(+)mu(-) and tau(+)tau(-) final states ia also presented.

The lifetime and oscillation frequency of the B-0 meson has been measured using (B) over bar (0) --> D(*+)l(-))over bar> decays recorded on the Z(0) peak with the OPAL detector at LEP. The D*+ --> D(0)pi (+) decays were reconstructed using an inclusive technique and the production flavour of the B-0 mesons was determined using a combination of tags from the rest of the event. The results tau (B0) = 1.541 +/- 0.028 +/- 0.023 ps, Deltam(d) = 0.497 +/- 0.024 +/- 0.025 ps(-1) were obtained, where in each case the first error is statistical and the second is systematic. (C) 2000 Elsevier Science B.V. All rights reserved.

The magnitude of the Cabibbo-Kobayashi-Maskawa matrix element V(cb) has been measured using (B) over bar(0) --> D*(+)l(-)(v) over bar decays recorded on the Z(0) peak using the OPAL detector at LEP. The D*(+) --> D(0)pi(+) decays were reconstructed both in the particular decay modes D*(+) --> D(0)pi(+) and D*(+) --> D(0)pi(+)pi(0) and via an inclusive technique. The product of \V(cb)\ and the decay form factor of the (B) over bar(0) --> D*(+)l(-)(v) over bar transition at zero recoil F(1) was measured to be F(1)\V(cb)\ = (37.1 +/- 1.0 +/- 2.0) x 10(-3), where the uncertainties are statistical and systematic respectively. By using Heavy Quark Effective Theory calculations for F(1), a value of \V(cb)\ = (40.7 +/- 1.1 +/- 2.2 +/- 1.6) x 10(-3) was obtained. where the third error is due to theoretical uncertainties in the value of F(1). The branching ratio Br((B) over bar(0) --> D*(+)l(-)(v) over bar) was also measured to be (5.26 +/- 0.20 +/- 0.46)%. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Cross-sections for hadronic. bl; and lepton pair final states in e(+)e(-) collisions at. root s=183 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. Forward-backward asymmetries for the leptonic final state:! have also been measured. Cross-sections and asymmetries are also presented for data recorded in 1997 at root s=130 and 136 GeV. The results are used to measure the energy dependence of the electromagnetic coupling constant alpha(em), and to place limits on new physics as described by four-fermion contact interactions or by the exchange of a new heavy particle such as a leptoquark, or of a squark or sneutrino in supersymmetric theories with R-parity violation.

The production of charged hadrons and K(S)(0) mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at e(+)e(-) centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the charged hadrons and K(S)(0) mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading order (NLO) QCD calculations. The distributions have been measured in the range 10

The structure of both quasi-real and highly virtual photons is investigated using the reaction e(+)e(-) --> e(+)e(-) mu(+)mu(-), proceeding via the exchange of two photons. The results are based on the complete OPAL dataset taken at e(+)e(-) centre-of-mass energies close to the mass of the Z boson. The QED structure function F(2)(gamma) and the differential cross-section d sigma/dx for quasi-real photons are obtained as functions of the fractional momentum x from the muon momentum which is carried by the struck muon in the quasi-real photon for Values of Q(2) ranging from 1.5 to 400 GeV(2). The differential cross-section d sigma/dx for highly virtual photons is measured for 1.5

Gluon jets are identified in hadronic Z(0) decays as all the particles in a hemisphere opposite to a hemisphere containing two tagged quark jets. Gluon jets defined in this manner are equivalent to gluon jets produced from a color singlet point source and thus correspond to the definition employed for most theoretical calculations. In a separate stage of the analysis, we select quark jets in a manner to correspond to calculations, as the particles in hemispheres of flavor tagged light quark (uds) events. We present the distributions of rapidity, scaled energy, the logarithm of the momentum, and transverse momentum with respect to the jet axes, for charged particles in these gluon and quark jets. We also examine the charged particle multiplicity distributions of the jets in restricted intervals of rapidity. For soft particles at large pr, we observe the charged particle multiplicity ratio of gluon to quark jets to be 2.291+/-0.09 (stat.)+/-0.15 (syst.), in agreement with the prediction that this ratio should approximately equal the ratio of QCD color factors, C(A)/C(F) = 2.25. The intervals used to define soft particles and large p(T) for this result, p

Searches for a scalar top quark and a scalar bottom quack have been performed using a data sample of 182 pb-l at a centre-of-mass energy of root s = 189 GeV collected with the OPAL detector at LEP. No evidence for a signal was found. The 95% confidence level (C.L.) lower limit on the scalar top quark mass is 90.3 GeV if the mixing angle between the supersymmetric partners of the left- and right-handed st;states of the top quark is zero. In the worst case, when the scalar top quark decouples from the Z(0) boson, the lower limit is 87.2 GeV. These limits were obtained assuming that the scalar top quark decays into a charm quark and the lightest neutralino, and that the mass difference between the scalar top quark and the lightest neutralino is larger than 10 GeV. The complementary decay mode of the scalar top quark decaying into a bottom quark, a charged lepton and a scalar neutrino has also been studied. From a search for the scalar bottom quark, a mass limit of 88.6 GeV was obtained if the mass difference between the scalar bottom quark and the lightest neutralino is larger than 7 GeV. These limits significantly improve the previous OPAL limits. (C) 1999 Elsevier Science B.V. All rights reserved.

A search is described for the generic process e(+) e(-) --> XY, where X is a neutral heavy scalar boson decaying into a pair of photons, and Y is a neutral heavy boson (scalar or vector) decaying into a fermion pair. The search is motivated mainly by the cases where either X, or both X and Y, are Higgs bosons. In particular, we investigate the case where X is the Standard Model Higgs boson and Y the Z(0) boson. Other models with enhanced Higgs boson decay couplings to photon pairs are also considered. The present search combines the data set collected by the OPAL collaboration at 189 GeV collider energy, having an integrated luminosity of 182.6 pb(-1), with data samples collected at lower energies. The search results have been used to put 95% confidence level bounds, as functions of the mass M-X, on the product of the cross-section and the relevant branching ratios, both in a model independent manner and for the particular models considered. (C) 1999 Elsevier Science B.V. All rights reserved.

We present a test of the flavour independence of the strong coupling constant for charm and bottom quarks with respect to light (uds) quarks, based on a hadronic event sample obtained with the OPAL detector at LEP. Five observables related to global event shapes were used to measure ct, in three flavour tagged samples (uds, c and b). The event shape distributions were fitted by O(alpha(s)(2)) calculations of jet production taking into account mass effects for the c and b quarks. We find: alpha(s)(c)/alpha(s)(uds) = 0.997 +/- 0.038 (stat.) +/- 0.030 (syst.) +/- 0.012 (theory) and alpha(s)(b)/alpha(s)(uds) = 0.993 +/- 0.008 (stat.) +/- 0.006 (syst.) +/- 0.011 (theory).

A multidimensional study of local multiplicity fluctuations and multiparticle correlations of hadrons produced in Z(0) decays is performed. The study is based on the data sample of more than 4 x 10(6) events recorded with the OPAL detector at LEP. The fluctuations and correlations are analysed in terms of the normalized scaled factorial moments and cumulants up to the fifth order. The moments are observed to have intermittency-like behaviour, which is found to be more pronounced with increasing dimension. The large data sample allows for the first time a study of the factorial cumulants in e(+)e(-) annihilation. The analysis of the cumulants shows the existence of genuine multiparticle correlations with a strong intermittancy rise up to higher orders. These correlations are found to be stronger in higher dimensions. The decomposition of the factorial moments into lower-order correlations shows that the dynamical fluctuations have important contributions from genuine many-particle correlations. The Monte Carlo models JETSET 7.4 and HERWIG 5.9 are found to reproduce the trend of the measured moments and cumulants but they underestimate the magnitudes. The results are found to be consistent with QCD jet formation dynamics, although additional contributions from other mechanisms cannot be excluded.

A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of 57 pb(-1) at centre-of-mass energies of 181-184 GeV taken with the OPAL detector at LEP. No evidence for chargino or neutralino production has been found. Upper limits on chargino and neutralino pair production ((+)(1)(-)(1)(0)(1)(0)(2)) cross-sections are obtained as a function of the chargino mass (m(1+/-)), the lightest neutralino mass (m(10)) and the second lightest neulralino mass (m(20)). For large chargino masses the limits have been improved with respect to the previous analyses at. lower centre-of-mass energies. Exclusion regions at 95% confidence level (C.L.) of parameters of the Constrained Minimal Supersymmetric Standard Model are determined for the case of a large universal scalar mass, mo, implying heavy scalar fermions, and for the case of a small mo resulting in light scaler fermions and giving the worst-case Limits. Within this framework and for m(1+/-) - m(10) greater than or equal to 5 GeV the 95% C.L. lower limits on m(1+/-) for m(0) = 500 GeV are 90.0 and 90.2 GeV for tan beta = 1.5 and 35 respectively. These limits for all m(0) (the worst-case) are 69.1 and 65.2 GeV for tan beta = 1.5 and 35 respectively. Exclusion regions are also presented for neutralino masses, including an absolute lower limit at 95% C.L. for the mass of the lightest neutralino of 30.1 GeV for m(0) = 500 GeV (25.2 GeV for all m(0)), with implications for experimental searches for the lightest neutralino as a dark matter candidate.

Di-jet production is studied in collisions of quasi-real photons radiated by the LEP beams at e(-)e(-) centre-of-mass energies root s(ee) = 161 and 172 GeV. The jets are reconstructed using a cone jet finding algorithm. The angular distributions of direct and double-resolved processes are measured and compared to the predictions of leading order and nest-to-leading order perturbative QCD. The jet energy profiles are also studied. The inclusive di-jet cross-section is measured as a function of E-T(jet) and \eta(jet)\ and compared to next-to-leading order perturbative QCD calculations. The inclusive di-jet cross-section as a function of \eta(jet)\ is compared to the prediction of the leading order Monte Carlo generators PYTHIA and PHOJET. The Monte Carlo predictions are calculated with different parametrisations of the parton distributions of the photon. The influence of the 'underlying event' has been studied to reduce the model dependence of the predicted jet cross-sections from the Monte Carlo generators.

Using a data sample of 57 pb(-1) recorded at a centre-of-mass energy of 183 GeV with the OPAL detector at LEP, 282 W(+)W(-) --> q (q) over bar q (q) over bar and 300 W(+)W(-) --> q (q) over bar l(l) candidate events are used to obtain a measurement of the mass of the W boson, M(W) = 80.39 +/- 0.13 (stat.) +/- 0.05 (syst.) GeV, assuming the Standard Model relation between M(W) and Gamma(W). A second fit provides a direct measure of the width of the W boson and gives Gamma(W) = 1.96 +/- 0.34 (stat.) +/- 0.20 (syst.) GeV. These results are combined with previous OPAL results to obtain M(W) = 80.38 +/- 0.12 (stat.) +/- 0.05 (syst.) GeV and Gamma(W) = 1.84 +/- 0.32 (stat.) +/- 0.20 (syst.) GeV. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

The Michel parameters of the leptonic tau decays are measured using the OPAL detector at LEP. The parameters pe, Ee, (E delta)e (with l = e, mu) and eta(mu), are extracted from the energy spectra of the charged decay leptons and from their energy-energy correlations. A new method involving a global likelihood fit of Monte Carlo generated events with complete detector simulation and background treatment has been applied to the data recorded at center-of-mass energies close to root s = m(Z)o corresponding to an integrated luminosity of 155 pb(-1). If e-mu universality is assumed and inferring the tau polarization from neutral current data, the measured Michel parameters are: rho = 0.781 +/- 0.028 +/- 0.018, xi = 0.98 +/- 0.22 +/- 0.10, eta = 0.027 +/- 0.055 +/- 0.005, xi delta = 0.65 +/- 0.14 +/- 0.07. where the value of eta has been constrained using the published OPAL measurements of the leptonic branching ratios and the tau lifetime; Limits on non-standard coupling constants and on the masses of new gauge bosons are obtained. The results are in agreement with the V-A prediction of the Standard Model.

Searches for a scalar top quark and a scalar bottom quark have been performed using a total data sample of 56.8 pb(-1) at a centre-of-mass energy of root s = 183 GeV collected with the OPAL detector at LEP. No candidate events were observed. Combining this result with those obtained at lower centre-of-mass energies, the 95% C.L. lower limit on the scalar top quark mass is 85.0 GeV if the mixing angle between the supersymmetric partners of the left- and right-handed states of the top quark is zero. The lower limit is 81.3 GeV, even if the scalar top quark decouples from the Z(0) boson. These limits were obtained assuming that the scalar top quark decays into a charm quark and the lightest neutralino, and that the mass difference between the scalar top quark and the lightest neutralino is larger than 10 GeV. The complementary decay mode of the scalar top quark in which it decays into a bottom quark, a charged lepton and a scalar neutrino was also studied. From a similar analysis, a mass limit on the light scalar bottom quark was set at 82.7 GeV for a mass difference between the scalar bottom quark and the lightest neutralino larger than 7 GeV, and at 84.0 GeV for the mass difference larger 10 GeV and the lightest neutralino heavier than 30 GeV. These limits were obtained assuming that the scalar bottom quark decays into a bottom quark and the lightest neutralino, and that a mixing angle between the supersymmetric partners of the left- and right-handed states of the bottom quark is zero.

A sample of Z(0) decays containing b-flavoured hadrons is tagged using leptons, and events having precise proper time measurements are selected. These events are used to study B(s)(0) oscillations. The flavour (b or (b) over bar) at decay is determined from the lepton charge while the flavour at production is determined from jet charge or the charge of a second lepton, where available. The experiment was not able to resolve the oscillatory behaviour, and we deduce that the B(s)(0) oscillation frequency Delta m(s) > 5.2 ps(-1) at the 95% confidence level.

Photonic events with large missing energy have been observed in e(+)e(-) collisions at centre-of-mass energies of 130, 136 and 183 GeV collected in 1997 using the OPAL detector at LEP. Results are presented for event topologies with a single photon and missing transverse energy or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection. These results are compared with the expectations from the Standard Model process e(+)e(-) --> v (v) over bar + photon(s). No evidence is observed for new physics contributions to these final states. Using the data at root s = 183 GeV, upper limits on sigma(e(+)e(-) --> XY) BR(X --> Y gamma) and sigma(e(+)e(-) --> XX) BR2 (X --> Y gamma) are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos (X = nu*,Y = nu), to neutralino production (X = (0)(2), Y = (0)(1)) and to supersymmetric models in which X = (0)(1) and Y = (G) over bar is a light gravitino.

The data collected by the OPAL experiment at root s = 183 GeV used used tu search for Higgs bosons which are predicted by the Standard Model and various extensions, such as general models with two Higgs field doublets and the Minimal Supersymmetric Standard Model (MSSM). The data correspond to an integrated luminosity of approximately 54 pb(-1). None of the searches for neutral and charged Higgs bosons have revealed an excess of events beyond the expected background. This negative outcome, in combination with similar results from searches at lower energies, leads to new limits for the Higgs boson masses and other model parameters. In particular, the 95% confidence level lower limit fur the mass of the Standard Model Higgs boson is 88.3 GeV. Charged Higgs bosons can be excluded for masses up to 59.5 GeV. In the MSSM, m(ho) > 70.5 GeV and m(Ao) > 72.0 GeV are obtained for tan beta > if no and maximal scalar top mixing and soft SUSY-breaking masses of 1 TeV. The range 0.8

A study of W-pair production in e(+)e(-) annihilations at LEP is presented, based on 877 W(+)W(-) candidates corresponding to an integrated luminosity of 57 pb(-1) at root s = 183 GeV. Assuming that the angular distributions of the W-pair production and decay, as well as their branching fractions, are described by the Standard Model, the W-pair production cross-section is measured to be 15.43 0.61(stat.)+/- 0.26(syst.) pb. Assuming lepton universality and combining with our results from lower centre-of-mass energies, the W branching fraction to hadrons is determined to be 67.9 +/- 1.2(stat.) +/- 0.5(syst.)%. The number of W-pair candidates and the angular distributions for each final state (qv(l), q (q) over bar q (q) over bar,(l)l'(v) over bar(l')) are used to determine the triple gauge boson couplings. After combining these values with our results from lower centre-of-mass energies we obtain Delta kappa(gamma), = 0.11(-0.37)(+0.52), Delta g(1)(z) = 0.01(-0.12)+(0.13) and lambda = -0.10(-0.12)(+0.13), where the errors include both statistical and systematic uncertainties and each coupling is determined by setting: the other two couplings to the Standard Model value. The fraction of W bosons produced with a longitudinal polarisation is measured to be 0.242 +/- 0.091(stat.) +/- 0.023(syst.). All these measurements are consistent with the Standard Model expectations.

The process e(+)e(-) --> gamma gamma(gamma) is studied using data recorded with the OPAL detector at LEP. The data sample taken at a centre-of-mass energy of 189 GeV corresponds to a total integrated luminosity of 178 pb(-1). The measured cross-section agrees well with the expectation from QED. A fit to the angular distribution is used to obtain improved limits at 95% CL on the QED cut-off parameters: Lambda(+)> 304 GeV and Lambda(-)> 295 GeV as well as a mass limit for an excited electron, M(e) * > 306 GeV assuming equal e*e gamma and ee gamma couplings. Graviton exchange in the context of theories with higher dimensions is excluded for scales G(+)

Using data collected with the OPAL detector at LEP, we have searched for the processes e(+)e(-) --> Z(0) --> pe(-),p mu(-) and the charge conjugate final-states. These would violate the conservation of the baryon-number B, lepton-number L and the fermion-number n = (B + L). No evidence for such decays has been found, and the 95% confidence level upper limits on the partial widths Gamma(Z(0) --> pe) and Gamma(Z(0) --> p mu) are found to be 4.6 and 4.4 keV respectively. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

The production of K(S)(0) mesons and Lambda baryons in quark and gluon jets has been investigated using two complementary techniques. In the first approach, which provides high statistical accuracy, jets were selected using different jet finding algorithms and ordered according to their energy. Production rates were determined taking into account the dependences of quark and gluon compositions as a function of jet energy as predicted by Monte Carlo models. Selecting three-jet events with the k(perpendicular to) (Durham) jet finder (Y(cut) = 0.005), the ratios of K(S)(0) and Lambda production rates in gluon and quark jets relative to the mean charged particle multiplicity were found to be 1.10 +/- 0.02 +/- 0.02 and 1.41 +/- 0.04 +/- 0.04, respectively, where the first uncertainty is statistical and the second is systematic. In the second approach, a new method of identifying quark jets based on the collimation of energy flow around the jet axis is introduced and was used to anti-tag gluon jets in symmetric (Y-shaped) three-jet events. Using the cone jet finding algorithm with a cone size of 30 degrees, the ratios of relative production rates in gluon and quark jets were determined to be 0.94 +/- 0.07 +/- 0.07 for K(S)(0) and 1.18 +/- 0.10 +/- 0.17 for Lambda. The results of both analyses are compared to the predictions of Monte Carlo models.

A study of W(+)W(-) events accompanied by hard photon radiation produced in e(+)e(-) collisions at LEP is presented. Events consistent with two on-shell W-bosons and an isolated photon are selected from 183 pb(-1) of data recorded at root s = 189 GeV. From these data, 17 W(+)W(-) gamma candidates are selected with photon energy greater than 10 GeV, consistent with the Standard Model expectation. These events are used to measure the e(+)e(-) --> W(+)W(-) gamma cross-section within a set of geometric and kinematic cuts, (WW gamma) = 136 +/- 37 +/- 8 fb, where the first error is statistical and the second systematic. The photon energy spectrum is used to set the first direct, albeit weak, limits on possible anomalous contributions to the W(+)W(-) gamma gamma and W(+)W(-) gamma Z(0) vertices: 0.070GeV(-2)

The fraction of Z(0) --> b (b) over bar events in hadronic Z(0) decays has been measured by the OPAL experiment using the data collected at LEP between 1992 and 1995. The Z(0) --> b (b) over bar decays were tagged using displaced secondary vertices, and high momentum electrons and muons. Systematic uncertainties were reduced by measuring the b-tagging efficiency using a double tagging technique. Efficiency correlations between opposite hemispheres of an event are small, and are well understood through comparisons between real and simulated data samples. A value of R(b)= sigma(e(+)e(-) --> b (b) over bar)/sigma(e(+)e(-) --> hadrons) = 0.2178 +/- 0.0011 +/- 0./0013 was obtained, where the first error is statistical and the second systematic. The uncertainty on R(c), the fraction of Z(0) --> c (c) over bar events in hadronic Z(0) decays, is not included in the errors. The dependence on R(c) is where Delta R(b)/R(b) = -0.056 Delta R(c)/R(c), Delta R(c) is the deviation of R(c) from the value 0.172 predicted by the Standard Model. The result for R(b) agrees with the value of 0.2155 +/- 0.0003 predicted by the Standard Model.

Searches for pair-produced charginos and neutralinos with R-parity violating decays have been performed using a data sample corresponding to an integrated luminosity of 56 pb(-1) collected with the OPAL detector at LEP at a centre-of-mass energy of root s = 183 GeV. An important consequence of R-parity violation is that the lightest supersymmetric particle becomes unstable. The searches have been performed under the assumptions that the lightest supersymmetric particle promptly decays and that only one R-parity violating coupling is dominant for each of the decay modes considered. Such processes would yield multiple leptons, jets plus leptons, or multiple jets with or without significant missing energy in the final state. No excess of such events above Standard Model backgrounds has been observed. Limits are presented on the production cross-sections of gauginos in R-parity violating scenarios. Limits are also presented in the framework of the Minimal Supersymmetric Standard Model.

The spectral functions of the vector current and the axial-vector current have been measured in hadronic tau decays using the OPAL detector at LEP. Within the framework of the Operator Product Expansion a simultaneous determination of the strong coupling constant or,, the non-perturbative operators of dimension 6 and 8 and of the gluon condensate has been performed. Different perturbative descriptions have been compared to the data. The Contour Improved Fixed Order Perturbation Theory gives alpha(s)(m(tau)(2)) = 0.348 +/- 0.009(exp) +/- 0.019(theo) at the tau-mass scale and alpha(s)(m(Z)(2)) = 0.1219 +/- 0.0010(exp) +/- 0.0017(the)o at the Z(0)-mass scale. The values obtained for alpha(s)(m(Z)(2)) using Fixed Order Perturbation Theory or Renormalon Chain Resummation are 2.3% and 4.1% smaller, respectively. The 'running' of the strong coupling between s(0) similar or equal to 1.3 GeV(2) and s(0) = m(tau)(2) has been tested from direct fits to the integrated differential hadronic decay rate R(tau)(s(0)). A test of the saturation of QCD sum rules at the tau-mass scale has been performed.

The branching ratios of the decay of the tau lepton to five charged hadrons have been measured with the OPAL detector at LEP using data collected between 1991 and 1995 at e(+)e(-) centre-of-mass energies close to the Z(0) resonance. The branching ratios are measured to be B(tau(-) --> 3h(-)2h(+)v(tau)) = (0.091 +/- 0.014 +/- 0.006)% B(tau(-) --> 3h(-)2h(+)pi(0)v(tau)) = (0.027 +/0 0.018 +/- 0.009)% where the first error is statistical and the second systematic.

The branching ratio for the decay tau(-) --> e(-) (e)nu(tau) has been measured using Z(0) decay data collected by the OPAL experiment at LEP. In total 33 073 tau(-) --> e(-) (e)nu(tau) candidates were identified from a sample of 186 197 selected tau. decays, giving a branching ratio of B(tau(-) --> e(-) (e)nu(tau)) = (17.81 +/- 0.09 (stat) +/- 0.06 (syst))%. This result is combined with other measurements to test e - mu and mu - tau universality in charged-current weak interactions. Additionally, the strong coupling constant alpha(s)(m(tau)(2)) has been extracted from B(tau(-) --> e(-) (e)nu(tau)) and evolved to the Z(0) mass scale, giving alpha(s)(m(Z)(2)) = 0.1204 +/- 0.0011 (exp) +/- 0.0019 (theory). (C) 1999 Published by Elsevier Science B.V. All rights reserved.

The inclusive charm hadron semileptonic branching fractions B(c --> e) and B(c --> mu) in Z(0) --> c (c) over bar events have been determined using 4.4 million hadronic Z(0) decays collected with the OPAL detector at LEP. A charm-enriched sample is obtained by. selecting events with reconstructed D*(+/-) mesons. Using leptons found in the hemisphere opposite that of the D*(+/-) mesons, the semileptonic branching fractions of charm hadrons are measured to be B(c --> e) = 0.103 +/- 0.009(-0.008)(+0.009) and B(c --> mu) = 0.090 +/ 0.007(-0.006)(+0.007), where the first errors are in each case statistical and the second systematic. Combining these measurements, an inclusive semileptonic branching fraction of charm hadrons of B(c --> l) = 0.095 +/- 0.006(-0.006)(+0.007) is obtained.

The product branching ratio, f(b --> Lambda(b)) . BR(Lambda(b) --> Lambda X), where Lambda(b) denotes any weakly-decaying b-baryon, has been measured using the OPAL detector at LEP. Lambda(b)'s are selected by the presence of energetic Lambda particles in bottom events tagged by the presence of displaced secondary vertices. A fit to the moments of the Lambda particles separates signal from B meson and fragmentation backgrounds. The measured product branching ratio is f(b --> Lambda(b)) . BR(Lambda(b) --> Lambda X) = (2.67) +/- 0.38(stat)(-0.60)(+0.67)(sys))%. Combined with a previous OPAL measurement, one obtains f(b --> Lambda(b)) . BR(Lambda(b) --> Lambda X) = (3.50) +/- 0.32(stat) +/- 0.35(sys))%.

Bose-Einstein correlations between like-charge pions are studied in hadronic final states produced by e(+)e(-) annihilations at center-of-mass energies of 172 and 183 GeV. Three event samples are studied, each dominated by one of the processes W(+)W(-) --> q (q) over bar l (v) over bar i, W(+)W(-) --> q (q) over bar q (q) over bar, or (Z(0)/gamma)* --> q (q) over bar. After demonstrating the existence of Bose-Einstein correlations in W decays, an attempt is made to determine Bose-Einstein correlations for pions originating front the same W boson and from different W bosons, as well as for pions from (Z(0)/gamma)* --> q (q) over bar events. The following results are obtained for the individual chaoticity parameters lambda, assuming a common source radius R: lambda(same) = 0.63+/- 0.19 +/- 0.14 lambda(diff) = 0.22 +/- 0.53 +/- 0.14 lambda(Z*) = 0.47 +/- 0.11 +/- 0.08 R = 0.92 =/- 0.09 +/- 0.09 fm In each case, the first error is statistical and the second is sq systematic. At the current level of statistical precision it is not established whether Bose-Einstein correlations, between pions from different W bosons exist or not.

The predicted effects of final state interactions such as colour reconnection are investigated by measuring properties of hadronic decays of W bosons, recorded at a centre-of-mass energy of root s similar or equal to 182.7 GeV in the OPAL detector at LEP. Dependence on the modelling of hadronic W decays is avoided by comparing W(+)W(-) --> q (q) over bar q (q) over bar events with the non-leptonic component of W(+)W(-) --> q (q) over bar'l(l) events. The scaled momentum distribution, its mean value, [x(p)], and that of the charged particle multiplicity, [n(ch)], are measured and found to be consistent in the two channels. The measured differences are: Delta[n(ch)] = [n(ch)(4q)] - 2[n(ch)(qql nu)] = + 0.7 +/- 0.8 +/- 0.6 Delta[x(p)] = [x(p)(4q)] - [x(p)(qql nu)] = (- 0.09 +/- 0.09 +/- 0.05) X 10(-2) In addition, measurements of rapidity and thrust are performed for W(+)W(-) --> q (q) over bar'q (q) over bar' events. The data are described well by standard QCD models and disfavour one model of colour reconnection within the ARIADNE program. The current implementation of the Ellis-Geiger model of colour reconnection is excluded. At the current level of statistical precision no evidence for colour reconnection effects was found in the observables studied. The predicted effect of colour reconnection on OPAL measurements of M(W) is also quantified in the context of models studied. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

Fragmentation functions for charged particles in Z(0) --> q (q) over bar events have been measured Tor bottom (b), charm ic) and light (uds) quarks as well as for all flavours together. The results are based un data recorded between 1990 and 1995 using the OPAL detector at LEP. Event samples with different flavour composition were formed using reconstructed D*(+/-) mesons and secondary vertices. The xi(p) = In(1/x(p)) distributions and the position of their maxima xi(0) are also presented separately for uds, c and b quark events. The fragmentation function for b quarks is significantly softer than for uds quarks.

In the process e(+)e(-) --> hadrons, one of the effects of gluon emission is to modify the (1 + cos(2)theta) form of the angular distribution of the thrust axis, an effect which may be quantified by the longitudinal cross-section. Using the OPAL detector at LEP, we have determined the longitudinal to total cross-section ratio to be sigma(L)/sigma(tot) = 0.0127 +/- 0.0016 +/- 0.0013 at the parton level, in good agreement with the expectation of QCD computed to O(alpha(s)(2)). Comparisons at the hadron level with Monte Carlo models are presented. The dependence of the longitudinal cross-section on the value of thrust has also been studied, and provides a new test of QCD. (C) 1998 Elsevier Science B.V. All rights reserved.

We report the first observation of Z/gamma* production in Compton scattering of quasi-real photons. This is a subprocess of the reaction e(+)e(-) --> e(+)e(-)Z/gamma*, where one of the final state electrons is undetected. Approximately 55 pb(-1) of data collected in the year 1997 at an e(+)e(-) centre-of-mass energy of 183 GeV with the OPAL detector at LEP have been analysed. The Z/gamma* from Compton scattering has been detected in the hadronic decay channel. Within well defined kinematic bounds, we measure the product of cross-section and Z/gamma* branching ratio to hadrons to be (0.9 +/- 0.3 +/- 0.1) pb for events with a hadronic mass larger than 60 GeV, dominated by (e)eZ production. In the hadronic mass region between 5 GeV and 60 GeV, dominated by (e)e gamma* production, this product is found to be (4.1 +/- 1.6 +/- 0.6) pb. Our results agree with the predictions of two Monte Carlo event generators, grc4f and PYTHIA. (C) 1998 Elsevier Science B.V. All rights reserved.

Inclusive production of the f(0)(980), f(2)(1270) and phi(1020) resonances has been studied in a sample of 4.3 million hadronic Z(0) decays from the OPAL experiment at LEP. A coupled channel analysis has been used for the f(0) in simultaneous fits to the resonances in inclusive pi(+)pi(-) and K(+)K(-) mass spectra. Fragmentation functions are reported for the three states. Total inclusive rates are measured to be 0.141 plus or minus 0.007 plus or minus 0.011 f(0), 0.155 plus or minus 0.011 plus or minus 0.018 f(2) and 0.091 plus or minus 0.002 plus or minus 0.003 phi mesons per hadronic Z(0) decay. The production properties of the f(0), including those in three-jet events, are compared with those of the f(2) and phi, and with the Lund string model of hadron production. All measurements are consistent with the hypothesis that the f(0)(980) is a conventional q(q)over-bar scalar meson.