Publications

Searches for new resonances are performed using an unsupervised anomaly-detection technique. Events with at least one electron or muon are selected from 140fb⁻¹ of pp collisions at √s=13TeV recorded by ATLAS at the Large Hadron Collider. The approach involves training an autoencoder on data, and subsequently defining anomalous regions based on the reconstruction loss of the decoder. Studies focus on nine invariant mass spectra that contain pairs of objects consisting of one light jet or b jet and either one lepton (e,μ), photon, or second light jet or b jet in the anomalous regions. No significant deviations from the background hypotheses are observed. Limits on contributions from generic Gaussian signals with various widths of the resonance mass are obtained for nine invariant masses in the anomalous regions.

The multi-staged XENON program at INFN Laboratori Nazionali del Gran Sasso aims to detect dark matter with two-phase liquid xenon time projection chambers of increasing size and sensitivity. The XENONnT experiment is the latest detector in the program, planned to be an upgrade of its predecessor XENON1T. It features an active target of 5.9 tonnes of cryogenic liquid xenon (8.5 tonnes total mass in cryostat). The experiment is expected to extend the sensitivity to WIMP dark matter by more than an order of magnitude compared to XENON1T, thanks to the larger active mass and the significantly reduced background, improved by novel systems such as a radon removal plant and a neutron veto. This article describes the XENONnT experiment and its sub-systems in detail and reports on the detector performance during the first science run.

The Forward Search Experiment (FASER) at CERN's Large Hadron Collider (LHC) has recently directly detected the first collider neutrinos. Neutrinos play an important role in all FASER analyses, either as signal or background, and it is therefore essential to understand the neutrino event rates. In this study, we update previous simulations and present prescriptions for theoretical predictions of neutrino fluxes and cross sections, together with their associated uncertainties. With these results, we discuss the potential for possible measurements that could be carried out in the coming years with the FASER neutrino data to be collected in LHC Run 3 and Run 4.

The FASER experiment at the LHC is designed to search for light, weakly-interacting particles produced in proton-proton collisions at the ATLAS interaction point that travel in the far-forward direction. The first results from a search for dark photons decaying to an electron-positron pair, using a dataset corresponding to an integrated luminosity of 27.0 fb⁻¹ collected at centre-of-mass energy s=13.6 TeV in 2022 in LHC Run 3, are presented. No events are seen in an almost background-free analysis, yielding world-leading constraints on dark photons with couplings ϵ∼2×10⁻⁵−1×10⁻⁴ and masses ∼17 MeV−70 MeV. The analysis is also used to probe the parameter space of a massive gauge boson from a U(1)_B−L model, with couplings g_B−L∼5×10⁻⁶−2×10⁻⁵ and masses ∼15 MeV−40 MeV excluded for the first time.

FASER, the ForwArd Search ExpeRiment, is an experiment dedicated to searching for light, extremely weakly-interacting particles at CERN's Large Hadron Collider (LHC). Such particles may be produced in the very forward direction of the LHC's high-energy collisions and then decay to visible particles inside the FASER detector, which is placed 480 m downstream of the ATLAS interaction point, aligned with the beam collisions axis. FASER also includes a sub-detector, FASERν, designed to detect neutrinos produced in the LHC collisions and to study their properties. In this paper, each component of the FASER detector is described in detail, as well as the installation of the experiment system and its commissioning using cosmic-rays collected in September 2021 and during the LHC pilot beam test carried out in October 2021. FASER has successfully started taking LHC collision data in 2022, and will run throughout LHC Run 3.

The precision in reconstructing events detected in a dual-phase time projection chamber depends on an homogeneous and well understood electric field within the liquid target. In the XENONnT TPC the field homogeneity is achieved through a double-array field cage, consisting of two nested arrays of field shaping rings connected by an easily accessible resistor chain. Rather than being connected to the gate electrode, the topmost field shaping ring is independently biased, adding a degree of freedom to tune the electric field during operation. Two-dimensional finite element simulations were used to optimize the field cage, as well as its operation. Simulation results were compared to ^83mKrcalibration data. This comparison indicates an accumulation of charge on the panels of the TPC which is constant over time, as no evolution of the reconstructed position distribution of events is observed. The simulated electric field was then used to correct the charge signal for the field dependence of the charge yield. This correction resolves the inconsistent measurement of the drift electron lifetime when using different calibrations sources and different field cage tuning voltages.

In this work, we expand on the XENON1T nuclear recoil searches to study the individual signals of dark matter interactions from operators up to dimension eight in a chiral effective field theory (ChEFT) and a model of inelastic dark matter (iDM). We analyze data from two science runs of the XENON1T detector totaling 1 t×yr exposure. For these analyses, we extended the region of interest from [4.9,40.9] keVNR to [4.9,54.4] keVNR to enhance our sensitivity for signals that peak at nonzero energies. We show that the data are consistent with the background-only hypothesis, with a small background overfluctuation observed peaking between 20 and 50 keVNR, resulting in a maximum local discovery significance of 1.7σ for the Vector - Vectorstrange ChEFT channel for a dark matter particle of 70 GeV/c2 and 1.8σ for an iDM particle of 50 GeV/c2 with a mass splitting of 100 keV/c2. For each model, we report 90% confidence level upper limits. We also report upper limits on three benchmark models of dark matter interaction using ChEFT where we investigate the effect of isospin-breaking interactions. We observe rate-driven cancellations in regions of the isospin-breaking couplings, leading to up to 6 orders of magnitude weaker upper limits with respect to the isospin-conserving case.

This paper details the first application of a software tagging algorithm to reduce radon-induced backgrounds in liquid noble element time projection chambers, such as XENON1T and XENONnT. The convection velocity field in XENON1T was mapped out using Rn222 and Po218 events, and the rms convection speed was measured to be 0.30±0.01 cm/s. Given this velocity field, Pb214 background events can be tagged when they are followed by Bi214 and Po214 decays, or preceded by Po218 decays. This was achieved by evolving a point cloud in the direction of a measured convection velocity field, and searching for Bi214 and Po214 decays or Po218 decays within a volume defined by the point cloud. In XENON1T, this tagging system achieved a Pb214 background reduction of 6.2-0.9+0.4% with an exposure loss of 1.8±0.2%, despite the timescales of convection being smaller than the relevant decay times. We show that the performance can be improved in XENONnT, and that the performance of such a software-tagging approach can be expected to be further improved in a diffusion-limited scenario. Finally, a similar method might be useful to tag the cosmogenic Xe137 background, which is relevant to the search for neutrinoless double-beta decay.

The first results of the study of high-energy electron neutrino (νe) and muon neutrino (νμ) charged-current interactions in the FASERν emulsion-tungsten detector of the FASER experiment at the LHC are presented. A 128.8 kg subset of the FASERν volume was analyzed after exposure to 9.5 fb-1 of s=13.6 TeV pp data. Four (eight) νe (νμ) interaction candidate events are observed with a statistical significance of 5.2σ (5.7σ). This is the first direct observation of νe interactions at a particle collider and includes the highest-energy νe and νμ ever detected from an artificial source. The interaction cross section per nucleon σ/Eν is measured over an energy range of 560-1740 GeV (520-1760 GeV) for νe (νμ) to be (1.2-0.7+0.8)×10-38 cm2 GeV-1 [(0.5±0.2)×10-38 cm2 GeV-1], consistent with standard model predictions. These are the first measurements of neutrino interaction cross sections in those energy ranges.

A measurement of the top-quark mass (m_t) 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 m_ℓμ 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 m_t. 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 m_t = 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.].

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 protonproton 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 20152018 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 protonproton 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 (p_T) 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 p_T> 500 GeV and p_T> 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 C_cφ< 1.35 (0.97) and C_uφ < 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.

The increase in luminosity, and consequent higher backgrounds, of the LHC upgrades require improved rejection of fake tracks in the forward region of the ATLAS Muon Spectrometer. The New Small Wheel upgrade of the Muon Spectrometer aims to reduce the large background of fake triggers from track segments that don't originate from the interaction point. The New Small Wheel employs two detector technologies, the resistive strip Micromegas detectors and the \u201csmall\u201d Thin Gap Chambers, with a total of 2.45 million electrodes to be sensed. The two technologies require the design of a complex electronics system given that it consists of two different detector technologies and is required to provide both precision readout and a fast trigger. It will operate in a high background radiation region up to about 20 kHz/cm² at the expected HL-LHC luminosity of ℒ = 7.5 × 10³⁴ cm^-2 s^-1. The architecture of the system is strongly defined by the GBTx data aggregation ASIC, the newly-introduced FELIX data router and the software based data handler of the ATLAS detector. The electronics complex of this new detector was designed and developed in the last ten years and consists of multiple radiation tolerant Application Specific Integrated Circuits, multiple front-end boards, dense boards with FPGA's and purpose-built Trigger Processor boards within the ATCA standard. The New Small Wheel has been installed in 2021 and is undergoing integration within ATLAS for LHC Run 3. It should operate through the end of Run 4 (December 2032). In this manuscript, the overall design of the New Small Wheel electronics is presented.

We report the first direct observation of neutrino interactions at a particle collider experiment. Neutrino candidate events are identified in a 13.6 TeV center-of-mass energy pp collision dataset of 35.4 fb-1 using the active electronic components of the FASER detector at the Large Hadron Collider. The candidates are required to have a track propagating through the entire length of the FASER detector and be consistent with a muon neutrino charged-current interaction. We infer 153-13+12 neutrino interactions with a significance of 16 standard deviations above the background-only hypothesis. These events are consistent with the characteristics expected from neutrino interactions in terms of secondary particle production and spatial distribution, and they imply the observation of both neutrinos and anti-neutrinos with an incident neutrino energy of significantly above 200 GeV.

We developed a detector signal characterization model based on a Bayesian network trained on the waveform attributes generated by a dual-phase xenon time projection chamber. By performing inference on the model, we produced a quantitative metric of signal characterization and demonstrate that this metric can be used to determine whether a detector signal is sourced from a scintillation or an ionization process. We describe the method and its performance on electronic-recoil (ER) data taken during the first science run of the XENONnT dark matter experiment. We demonstrate the first use of a Bayesian network in a waveform-based analysis of detector signals. This method resulted in a 3% increase in ER event-selection efficiency with a simultaneously effective rejection of events outside of the region of interest. The findings of this analysis are consistent with the previous analysis from XENONnT, namely a background-only fit of the ER data.

We report on the first search for nuclear recoils from dark matter in the form of weakly interacting massive particles (WIMPs) with the XENONnT experiment, which is based on a two-phase time projection chamber with a sensitive liquid xenon mass of 5.9 ton. During the (1.09±0.03) ton yr exposure used for this search, the intrinsic Kr85 and Rn222 concentrations in the liquid target are reduced to unprecedentedly low levels, giving an electronic recoil background rate of (15.8±1.3) events/ton yr keV in the region of interest. A blind analysis of nuclear recoil events with energies between 3.3 and 60.5 keV finds no significant excess. This leads to a minimum upper limit on the spin-independent WIMP-nucleon cross section of 2.58×1047 cm2 for a WIMP mass of 28 GeV/c2 at 90% confidence level. Limits for spin-dependent interactions are also provided. Both the limit and the sensitivity for the full range of WIMP masses analyzed here improve on previous results obtained with the XENON1T experiment for the same exposure.

The nature of dark matter and properties of neutrinos are among the most pressing issues in contemporary particle physics. The dual-phase xenon time-projection chamber is the leading technology to cover the available parameter space for weakly interacting massive particles, while featuring extensive sensitivity to many alternative dark matter candidates. These detectors can also study neutrinos through neutrinoless double-beta decay and through a variety of astrophysical sources. A next-generation xenon-based detector will therefore be a true multi-purpose observatory to significantly advance particle physics, nuclear physics, astrophysics, solar physics, and cosmology. This review article presents the science cases for such a detector.

Multiple viable theoretical models predict heavy dark matter particles with a mass close to the Planck mass, a range relatively unexplored by current experimental measurements. We use 219.4 days of data collected with the XENON1T experiment to conduct a blind search for signals from multiply interacting massive particles (MIMPs). Their unique track signature allows a targeted analysis with only 0.05 expected background events from muons. Following unblinding, we observe no signal candidate events. This Letter places strong constraints on spin-independent interactions of dark matter particles with a mass between 1×1012 and 2×1017 GeV/c2. In addition, we present the first exclusion limits on spin-dependent MIMP-neutron and MIMP-proton cross sections for dark matter particles with masses close to the Planck scale.

The XENONnT detector uses the latest and largest liquid xenon-based time projection chamber (TPC) operated by the XENON Collaboration, aimed at detecting Weakly Interacting Massive Particles and conducting other rare event searches. The XENONnT data acquisition (DAQ) system constitutes an upgraded and expanded version of the XENON1T DAQ system. For its operation, it relies predominantly on commercially available hardware accompanied by open-source and custom-developed software. The three constituent subsystems of the XENONnT detector, the TPC (main detector), muon veto, and the newly introduced neutron veto, are integrated into a single DAQ, and can be operated both independently and as a unified system. In total, the DAQ digitizes the signals of 698 photomultiplier tubes (PMTs), of which 253 from the top PMT array of the TPC are digitized twice, at ×10 and ×0.5 gain. The DAQ for the most part is a triggerless system, reading out and storing every signal that exceeds the digitization thresholds. Custom-developed software is used to process the acquired data, making it available within ∼30 s for live data quality monitoring and online analyses. The entire system with all the three subsystems was successfully commissioned and has been operating continuously, comfortably withstanding readout rates that exceed ∼500 MB/s during calibration. Livetime during normal operation exceeds 99% and is ∼90% during most high-rate calibrations. The combined DAQ system has collected more than 2 PB of both calibration and science data during the commissioning of XENONnT and the first science run.

We perform a blind search for particle signals in the XENON1T dark matter detector that occur close in time to gravitational-wave signals in the LIGO and Virgo observatories. No particle signal is observed in the nuclear recoil and electronic recoil channels within ±500 seconds of observations of the gravitational-wave signals GW170104, GW170729, GW170817, GW170818, and GW170823. We use this null result to constrain monoenergetic neutrinos and axion-like particles emitted in the closest coalescence GW170817, a binary neutron star merger. We set new upper limits on the fluence (time-integrated flux) of coincident neutrinos down to 17 keV at the 90% confidence level. Furthermore, we constrain the product of the coincident fluence and cross section of axion-like particles to be less than 10-29 cm2/cm2 in the [5.5-210] keV energy range at the 90% confidence level.

The Large Hadron Collider (LHC) at CERN is expected to be upgraded to the High-Luminosity LHC (HL-LHC) by 2029 and achieve instantaneous luminosity around 5- 7.5 x 1034 cm-2 s-1. This represents a more than 3-4 fold increase in the instantaneous luminosity compared to what has been achieved in Run 2. The New Small Wheel (NSW) upgrade is designed to be able to operate efficiently in this high background rate environment. In this article, we summarize multiple performance studies of the small-strip Thin Gap Chamber (sTGC) at high rate using nearly final front-end electronics. We demonstrate that the efficiency versus rate distribution can be well described by an exponential decay with electronics dead-time being the primary cause of loss of efficiency at high rate. We then demonstrate several methods that can decrease the electronics dead-time and therefore minimize efficiency loss. One such method is to install either a pi-network input filter or pull-up resistor to minimize the charge input into the amplifier. We optimized the pi-network capacitance and pull-up resistor resistance using the results from our measurements. The results shown here were not only critical to finalizing the components on the front-end board, but also are critical for setting the optimal operating parameters of the sTGC detector and electronics in the ATLAS cavern.

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 ²²²Rn 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 ²²²Rn 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 protonproton 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 m_X in the range 251 GeV≤m_X≤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 T_1/2^2νECEC=(1.1±0.2_stat±0.1_sys)×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 T_1/2^0νββ>1.2×10²⁴yr 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 T_1/2^0νββ>2.1×10²⁵yr at 90% CL, corresponding to an effective Majorana mass range of ⟨m_ββ⟩

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.

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 simulationthe calorimeter shower simulationwith 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.

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 novel online distillation technique was developed for the XENON1T dark matter experiment to reduce intrinsic background components more volatile than xenon, such as krypton or argon, while the detector was operating. The method is based on a continuous purification of the gaseous volume of the detector system using the XENON1T cryogenic distillation column. A krypton-in-xenon concentration of (360 ± 60) ppq was achieved. It is the lowest concentration measured in the fiducial volume of an operating dark matter detector to date. A model was developed and fitted to the data to describe the krypton evolution in the liquid and gas volumes of the detector system for several operation modes over the time span of 550 days, including the commissioning and science runs of XENON1T. The online distillation was also successfully applied to remove 37Ar after its injection for a low-energy calibration in XENON1T. This makes the usage of 37Ar as a regular calibration source possible in the future. The online distillation can be applied to next-generation liquid xenon time projection chamber experiments to remove krypton prior to, or during, any science run. The model developed here allows further optimization of the distillation strategy for future large-scale detectors.

FASER is a new experiment designed to search for new light weakly-interacting long-lived particles (LLPs) and study high-energy neutrino interactions in the very forward region of the LHC collisions at CERN. The experimental apparatus is situated 480 m downstream of the ATLAS interaction-point aligned with the beam collision axis. The FASER detector includes four identical tracker stations constructed from silicon microstrip detectors. Three of the tracker stations form a tracking spectrometer, and enable FASER to detect the decay products of LLPs decaying inside the apparatus, whereas the fourth station is used for the neutrino analysis. The spectrometer has been installed in the LHC complex since March 2021, while the fourth station is not yet installed. FASER will start physics data taking when the LHC resumes operation in early 2022. This paper describes the design, construction and testing of the tracking spectrometer, including the associated components such as the mechanics, readout electronics, power supplies and cooling system.

The accurate simulation of additional interactions at the ATLAS experiment for the analysis of protonproton collisions delivered by the Large Hadron Collider presents a significant challenge to the computing resources. During the LHC Run 2 (20152018), 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 protonproton 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 |κ_c|

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. 

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.]

We report on a search for nuclear recoil signals from solar B8 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 B8 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.

We report the results of a search for the inelastic scattering of weakly interacting massive particles (WIMPs) in the XENON1T dark matter experiment. Scattering off Xe129 is the most sensitive probe of inelastic WIMP interactions, with a signature of a 39.6 keV deexcitation photon detected simultaneously with the nuclear recoil. Using an exposure of 0.83 tonne-years, we find no evidence of inelastic WIMP scattering with a significance of more than 2σ. A profile-likelihood ratio analysis is used to set upper limits on the cross section of WIMP-nucleus interactions. We exclude new parameter space for WIMPs heavier than 100 GeV/c2, with the strongest upper limit of 3.3×10-39 cm2 for 130 GeV/c2 WIMPs at 90% confidence level.

The selection of low-radioactive construction materials is of utmost importance for the success of low-energy rare event search experiments. Besides radioactive contaminants in the bulk, the emanation of radioactive radon atoms from material surfaces attains increasing relevance in the effort to further reduce the background of such experiments. In this work, we present the ²²²Rn emanation measurements performed for the XENON1T dark matter experiment. Together with the bulk impurity screening campaign, the results enabled us to select the radio-purest construction materials, targeting a ²²²Rn activity concentration of 10μBq/kg in 3.2t of xenon. The knowledge of the distribution of the ²²²Rn sources allowed us to selectively eliminate problematic components in the course of the experiment. The predictions from the emanation measurements were compared to data of the ²²²Rn activity concentration in XENON1T. The final ²²²Rn activity concentration of (4.5±0.1)μBq/kg in the target of XENON1T is the lowest ever achieved in a xenon dark matter experiment.

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 202123, 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

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.

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 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).

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).

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.

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.

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 the electroweak production of charginos, neutralinos and sleptons decaying into final states involving two or three electrons or muons is presented. The analysis is based on 36.1 fb⁻¹ of √s = 13 TeV proton proton collisions recorded by the ATLAS detector at the Large Hadron Collider. Several scenarios based on simplified models are considered. These include the associated production of the next-to-lightest neutralino and the lightest chargino, followed by their decays into final states with leptons and the lightest neutralino via either sleptons or Standard Model gauge bosons; direct production of chargino pairs, which in turn decay into leptons and the lightest neutralino via intermediate sleptons; and slepton pair production, where each slepton decays directly into the lightest neutralino and a lepton. No significant deviations from the Standard Model expectation are observed and stringent limits at 95% confidence level are placed on the masses of relevant supersymmetric particles in each of these scenarios. For a massless lightest neutralino, masses up to 580 GeV are excluded for the associated production of the next-to-lightest neutralino and the lightest chargino, assuming gauge-boson mediated decays, whereas for slepton-pair production masses up to 500 GeV are excluded assuming three generations of mass-degenerate sleptons.

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 σ and 9.3 σ, 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×224.6 live days) were reanalyzed, with an increased recoil energy interval compared to previous analyses, ranging from (6.6-240) keVnr. 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 7.64×103 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 Xe129 is induced. The experimental signature is a nuclear recoil observed together with the prompt deexcitation photon. We see no evidence for such inelastic WIMP-Xe129 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 3.3×10-38 cm2 at 100 GeV/c2. 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.

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 Rn220 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 Pb212 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 Rn222 background after calibration. Alpha emitters are also distributed throughout the detector and facilitate calibration of its response to Rn222. Using the delayed coincidence of Rn220-Po216, 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 Po212, t1/2=(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 β-emitter ⁸⁵Kr which is present in the xenon. For XENON1T a concentration of natural krypton in xenon natKr/Xe

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 ∼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] keVnr energy range of interest for weakly interacting massive particle (WIMP) dark matter searches, the electronic recoil background was (1.93±0.25)×10-4 events/(kg×day×keVee), 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/c2, with a minimum of 7.7×10-47 cm2 for 35-GeV/c2 WIMPs at 90% C.L.

Direct searches for lepton flavour violation in decays of the Higgs and Z bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: H→ eτ, H→μτ, and Z→μτ. The searches are based on the data sample of protonproton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3fb^-1 at a centre-of-mass energy of √s=8 TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95% confidence level: Br (H→ eτ) < 1.04%, Br (H→μτ) 

The cross section of a top-quark pair produced in association with a photon is measured in proton-proton collisions at a centre-of-mass energy of s=8 TeV with 20.2 fb⁻¹ of data collected by the ATLAS detector at the Large Hadron Collider in 2012. The measurement is performed by selecting events that contain a photon with transverse momentum p_T > 15 GeV, an isolated lepton with large transverse momentum, large missing transverse momentum, and at least four jets, where at least one is identified as originating from a b-quark. The production cross section is measured in a fiducial region close to the selection requirements. It is found to be 139 ± 7 (stat.) ± 17 (syst.) fb, in good agreement with the theoretical prediction at next-to-leading order of 151 ± 24 fb. In addition, differential cross sections in the fiducial region are measured as a function of the transverse momentum and pseudorapidity of the photon.[Figure not available: see fulltext.].

Two-neutrino double electron capture is a rare nuclear decay where two electrons are simultaneously captured from the atomic shell. For Xe124 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 Xe124 using 7636 kgd 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 T1/2>6.5×1020 yr. We have also evaluated the sensitivity of the XENON1T experiment, which is currently being commissioned, and found a sensitivity of T1/2>6.1×1022 yr after an exposure of 2 tyr.

We present results of searches for vector and pseudoscalar bosonic super-weakly interacting massive particles (WIMPs), which are dark matter candidates with masses at the keV-scale, with the XENON100 experiment. XENON100 is a dual-phase xenon time projection chamber operated at the Laboratori Nazionali del Gran Sasso. A profile likelihood analysis of data with an exposure of 224.6 live days ×34 kg showed no evidence for a signal above the expected background. We thus obtain new and stringent upper limits in the (8-125) keV/c2 mass range, excluding couplings to electrons with coupling constants of gae>3×10-13 for pseudo-scalar and α/α>2×10-28 for vector super-WIMPs, respectively. These limits are derived under the assumption that super-WIMPs constitute all of the dark matter in our galaxy.

The XENON1T dark matter experiment aims to detect weakly interacting massive particles (WIMPs) through low-energy interactions with xenon atoms. To detect such a rare event necessitates the use of radiopure materials to minimize the number of background events within the expected WIMP signal region. In this paper we report the results of an extensive material radioassay campaign for the XENON1T experiment. Using gamma-ray spectroscopy and mass spectrometry techniques, systematic measurements of trace radioactive impurities in over one hundred samples within a wide range of materials were performed. The measured activities allowed for stringent selection and placement of materials during the detector construction phase and provided the input for XENON1T detection sensitivity estimates through Monte Carlo simulations.

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.

We perform a low-mass dark matter search using an exposure of 30 kg×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/c2 above 1.4×10-41 cm2 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 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 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 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 √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.

We report on WIMP search results of the XENON100 experiment, combining three runs summing up to 477 live days from January 2010 to January 2014. Data from the first two runs were already published. A blind analysis was applied to the last run recorded between April 2013 and January 2014 prior to combining the results. The ultralow electromagnetic background of the experiment, ∼5×10-3 events/(keVee×kg×day)) before electronic recoil rejection, together with the increased exposure of 48 kg×yr, improves the sensitivity. A profile likelihood analysis using an energy range of (6.6-43.3) keVnr sets a limit on the elastic, spin-independent WIMP-nucleon scattering cross section for WIMP masses above 8 GeV/c2, with a minimum of 1.1×10-45 cm2 at 50 GeV/c2 and 90% confidence level. We also report updated constraints on the elastic, spin-dependent WIMP-nucleon cross sections obtained with the same data. We set upper limits on the WIMP-neutron (proton) cross section with a minimum of 2.0×10-40 cm2 (52×10-40 cm2) at a WIMP mass of 50 GeV/c2, at 90% confidence level.

We have searched for periodic variations of the electronic recoil event rate in the (26) 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.

Laboratory experiments searching for galactic dark matter particles scattering off nuclei have so far not been able to establish a discovery. We use data from the XENON100 experiment to search for dark matter interacting with electrons. With no evidence for a signal above the low background of our experiment, we exclude a variety of representative dark matter models that would induce electronic recoils. For axial-vector couplings to electrons, we exclude cross sections above 6 × 10^-35 cm² for particle masses of m_χ = 2 GeV/c². Independent of the dark matter halo, we exclude leptophilic models as an explanation for the long-standing DAMA/LIBRA signal, such as couplings to electrons through axial-vector interactions at a 4.4σ confidence level, mirror dark matter at 3.6σ, and luminous dark matter at 4.6σ.

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 c ² for WIMP masses around 50 GeV², 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 ∼ 10 m 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√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 ∼ 600 UV photons/electron over 4π. 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.

A search for long-lived particles is performed using a data sample of 4.7 fb^-1 from proton-proton collisions at a centre-of-mass energy. √s = 7 TeV collected by the ATLAS detector at the LHC. No excess is observed above the estimated background and lower limits, at 95% confidence level, are set on the mass of the long-lived particles in different scenarios, based on their possible interactions in the inner detector, the calorimeters and the muon spectrometer. Long-lived staus in gauge-mediated SUSY-breaking models are excluded up to a mass of 300 GeV for tan β = 5-20. Directly produced long-lived sleptons are excluded up to a mass of 278 GeV. R-hadrons, composites of gluino (stop, sbottom) and light quarks, are excluded up to a mass of 985 GeV (683 GeV, 612 GeV) when using a generic interaction model. Additionally two sets of limits on R-hadrons are obtained that are less sensitive to the interaction model for R-hadrons. One set of limits is obtained using only the inner detector and calorimeter observables, and a second set of limits is obtained based on the inner detector alone. (c) 2013 CERN. Published by Elsevier B.V. All rights reserved.

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 √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 qq̄ τν_τ 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^± → τν_τ) + BR (H^± → qq̄) = 1, motivated by general 2HDM type II models, excluded areas on the [m_H± BR(H^± → τν_τ)] 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^±→τν_τ). 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 tanβ.

A search for long-lived charged particles reaching the muon spectrometer is performed using a data sample of 37 pb^-1 from pp collisions at √s = 7 TeV collected by the ATLAS detector at the LHC in 2010. No excess is observed above the estimated background. Stable τ over bar sleptons are excluded at 95% CL up to a mass of 136 GeV, in GMSB models with N-5 = 3 ,m_messenger= 250 TeV, sign(μ) = 1 and tan β = 5. Electroweak production of sleptons is excluded up to a mass of 110 GeV. Gluino R-hadrons in a generic interaction model are excluded up to masses of 530 GeV to 544 GeV depending on the fraction of R-hadrons produced as (g) over bar -balls. 

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 α_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 resummed next-to-leading-log-approximation terms (NNLO + NLLA). The combined NNLO result from all variables and centre-of-mass energies is [Equation not available: see fulltext.] while the combined NNLO + NLLA result is [Equation not available: see fulltext.] 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 α_S agrees with the QCD prediction of asymptotic freedom and excludes the absence of running.

A search is performed for Higgs bosons decaying into invisible final states, produced in association with a Z⁰ 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 Z⁰ 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 Z⁰ boson are derived. Assuming a branching ratio BR (h⁰ → 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.

Data collected around √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 ∑^- hyperons whose baryon number is compensated by the production of a ∑̄^- Λ̄ Ξ ̄^- 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 Λ Λ ̄ and p πp̄ correlations are not conclusive, if parameter uncertainties are considered.

The ATLAS detector at CERN's Large Hadron Collider (LHC) will be exposed to proton-proton collisions from beams crossing at 40 MHz. A three-level trigger system will select potentially interesting events in order to reduce the read-out rate to about 200 Hz. The first trigger level is implemented in custom-built electronics and makes an initial fast selection based on detector data of coarse granularity. It has to reduce the rate by a factor of 10 ⁴ to less than 100 kHz. The other two consecutive trigger levels are in software and run on PC farms. We present an overview of the first-level central trigger and the muon barrel trigger system and report on the current installation status. Moreover, we show analysis results of cosmic-ray data recorded in situ at the ATLAS experimental site with final or close-to-final hardware.

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 sqrt(s_ee) from 189 to 209 GeV. Jets are reconstructed using the k_⊥ jet algorithm. The inclusive differential cross-section is measured as a function of the jet transverse momentum, p_T^jet, in the range 5 < p_T^jet < 40   GeV for pseudo-rapidities, η^jet, in the range - 1.5 < η^jet < 1.5. The results are compared to predictions of perturbative QCD in next-to-leading order in the strong coupling constant.

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 α_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 α_s is studied. Our results are consistent with the running of α_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 α_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 α_s. The data are also compared to a measurement by the L3 Collaboration, in which a large deviation from the NLO predictions is observed.

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 < p_T^cut, where p_T is defined with respect to the thrust axis, or restricted absolute momenta, p ≡ | p | < p^cut, using about four million hadronic Z decays recorded at LEP with the OPAL detector. The correlations are analyzed in terms of normalized factorial and cumulant moments. The analysis is inspired by analytical QCD calculations which, in conjunction with Local Parton-Hadron Duality (LPHD), predict that, due to colour coherence, the multiplicity distribution of particles with restricted transverse momenta should become Poissonian as p_T^cut decreases. The expected correlation pattern is indeed observed down to p_T^cut ≈ 1   GeV but not at lower transverse momenta. Furthermore, for p^cut → 0   GeV a strong rise is observed in the data, in disagreement with theoretical expectation. The Monte Carlo models reproduce well the measurements at large p_T^cut and p^cut but underestimate their magnitudes at the lowest momenta. The e⁺ e^- data are also compared to the measurements in deep-inelastic e⁺ p collisions at HERA. It is shown that for soft particles, the often assumed equivalence of a single hemisphere in e⁺ e^- annihilation with the current region in the Breit frame of a deep-inelastic collision may be misleading. Our study indicates difficulties with the LPHD hypothesis when applied to many-particle inclusive observables of soft hadrons.

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 (Γ_W) are determined using direct reconstruction of the kinematics of W⁺W ^- → qq̄ℓv and W⁺W^- → qq̄qq̄ 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 Γ_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 qq̄qq̄ 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^- → qq̄qq̄ 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.

Using the OPAL detector at LEP, the running of the effective QED coupling α(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: Δα(-6.07 GeV²) - Δα(-1.81 GeV²) = (440 ± 58 ± 43 ± 30) × 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 α(t). The null hypothesis that α remains constant within the above interval of -t is excluded with a significance above 5σ. Similarly, our results are inconsistent at the level of 3 σ with the hypothesis that only leptonic loops contribute to the running. This is currently the most significant direct measurement where the running α(t) is probed differentially within the measured t range.

Thin gap chambers (TGCs) are used for the muon trigger system in the forward region of the LHC experiment ATLAS. The TGCs are expected to provide a trigger signal within 25 ns of the bunch spacing. An extensive system test of the ATLAS muon spectrometer has been performed in the H8 beam line at the CERN SPS, during the last few years. A relational database was used for storing the conditions of the tests as well as the configuration of the system. This database has provided the detector control system with the information needed for configuration of the front end electronics. The database is used to assist the online operation and maintenance. The same database is used to store the nonevent condition and configuration parameters needed later for the offline reconstruction software. A larger scale of the database has been produced to support the whole TGC system. It integrates all the production, QA tests and assembly information. A 1/12th model of the whole TGC system is currently in use for testing the performance of this database in configuring and tracking the condition of the system. A prototype of the database was first implemented during the H8 test beams. This paper describes the database structure, its interface to other systems and its operational performance.

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 α_s in the LEP energy range by fitting an expression in which script O sign(α_s²) calculations were matched to a NLLA prediction and fitted to the data. Combining the measurements at different centre-of-mass energies, the value of α_s(M_Z) was determined to be α_s(M_Z) = 0.1177 ± 0.0006 (stat.) ± 0.0012 (expt.) ± 0.0010 (had.) ± 0.0032 (theo.).

The effects of the final state interaction phenomenon known as colour reconnection are investigated at centre-of-mass energies in the range √s ≃ 189-209 GeV 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 Sjöstrand-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.).

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 →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/c² in the neutralino NLSP scenario, below 87.4 GeV/c² 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 A from the direct SUSY particle searches of A > 40, 27, 21, 17, 15 TeV/c² for messenger indices N = 1, 2, 3, 4, 5 for all NLSP lifetimes.

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 equation presented, 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.

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 Higgs 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-Sundrum model, the data exclude masses below 58 GeV for the mass eigenstate which becomes the Higgs boson in the no-mixing limit.

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.).

Thin gap chambers (TGCs) is the end-cap muon trigger detector of the ATLAS experiment, one of the major projects being built at CERN, Geneva, Switzerland. The TGC detector will be inaccessible during operation due to high radiation levels in the ATLAS cavern. The detector requires a detector-control system (DCS) to monitor important detector and environmental parameters, calibrate, set, and maintain the configuration of front-end electronics, and take appropriate corrective action to maintain the detector stability and reliable performance. The TGC DCS is a distributed system, comprising a central control and configuration master station, and about 1500 microcontroller slaves connected to it using a controller area network (CAN). The CAN nodes control hardware devices such as the thresholds for amplifier shaper discriminators (ASDs), and data-acquisition parameters. CAN nodes are distributed on the on-chamber trigger electronic boards, to service many channels close to their source. In contrast to many other control systems, the TGC DCS makes full use of the intelligence offered by the ATLAS ELMB CAN nodes, in order to distribute the control of complex tasks on the front-end nodes and reduce CAN bus traffic. These nodes implement JTAG and I2C protocols, monitor hardware parameters, generate test patterns, and histogram chamber charge for diagnostics. The hardware and software design, integration, performance, and radiation test results are described.

A heavy charged stable or semi-stable particle is predicted in many models of physics beyond the standard model. A case in point is Gauge Mediate Supersymmetry Breaking (GMSB) with high tanβ where the is the Next to Lightest Supersymmetric Particle (NLSP) and couples weakly to the gravitino. The signal we are looking for is a heavy long-lived charged particle with low beta. The momentum spectrum of those sleptons is model dependent. The ones with beta significantly lower than 1 can be identified as staus, and are the subject of this note. In ATLAS, in order to match event fragments from different sub detectors into events correctly, great emphasis is placed on beam crossing identification, BCID. This assumes that the particles that leave signals in the detector travel nearly at the speed of light (beta=1). In this note we will consider a particle with beta smaller than one. Such a particle may be completely lost during data collection, or might be marked with the wrong BCID. The concept of wrong BCID due to low particle speed was not considered in the ATLAS trigger and data acquisition design. ATLAS was definitely not designed to deal with it. Therefore, the primary interest of this document is in assuring that design issues will not cause ATLAS to miss a potential big discovery.

Measurements of R_b, the ratio of the bb̄ cross-section to the qq̄ 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.

Using the entire OPAL LEP1 on-peak Z hadronic decay sample, Z→qq̄γ decays were selected by tagging hadronic final states with isolated photon candidates in the electromagnetic calorimeter. Combining the measured rates of Z→qq̄γ 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 Γ_u=300⁺¹⁹_-18 MeVandΓ_d= 381⁺¹²_-12 MeV. The results are in good agreement with the Standard Model expectation.

Gluon jets with a mean energy of 22 GeV and purity of 95% are selected from hadronic Z⁰ 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 Z⁰ 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 axe 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)×U(1) relations are used, resulting in four independent couplings, κ_γ, g₁^z, λ_γ and g₅^z. Determining each coupling in a separate fit, assuming the other couplings to take their Standard Model values, we obtain κ_γ,=0.88_-0.08 ^+0.09, g₁^z=0.987_-0.033 ^+0.034, A7=-0.060lg:g|J and g₅^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.

The branching ratio of beauty hadrons to final states containing two charm hadrons, Br(b → DD̄X), has been measured using an inclusive method in hadronic Z⁰ decays with the OPAL detector at LEP. The impact parameter significance of tracks opposite tagged b-jets is used to differentiate b → DD̄X decays from other decays. The result is Br(b → DD̄X) = (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.

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 β, and for a top quark mass of 174.3GeV, the domain 0.7 < tan β < 1.9 is excluded at the 95% confidence level and Higgs boson mass limits of m_h > 84.5GeV and m_A > 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 β < 2.8 is excluded but no universal limit can be set on the Higgs boson masses.

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.

Elements of the spin density matrix for W bosons in e⁺e ^-→W⁺W ^-→qq̄ℓν_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.

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 hodoscopes 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 ATLAS TGCs 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 α_em at LEP2 energies. In addition, the results are used together with OPAL measurements at 91-183 GeV within the S-matrix formalism to determine the γ-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 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 √s = 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.

A prototype of the ATLAS End-cap Muon Level 1 Trigger system has been constructed and mounted on thin gap chambers, which are used for muon trigger chambers for the ATLAS endcap region. We have tested the system with the chambers using 100 GeV muon beams at CERN SPS H8 beam line. The performance of both the trigger signal generation and the chamber data readout has been evaluated in this configuration. In the beam test, the system has been integrated with the muon central trigger processor interface as well as the ATLAS standard central data acquisition system and detector control system. We present the evaluation results of the system and discuss validity as a level 1 trigger generator system for ATLAS.

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^- → νν̄γγ (γ). No evidence for new physics contributions to this final state is observed. Upper limits on σ(e⁺e^- → XX)·BR²(X → Yγ) 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 = ν*, Y = ν), to neutralino production (X = χ̃ ₂⁰, Y = χ̃₁⁰) and to supersymmetric models in which X=χ̃10 and Y=G̃ is a light gravitino.

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/c² is set on the mass at the 95% confidence level.

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 τ lepton. The decays τ^-→ (Kπ) ^-ντ, (Kππ)^-ντ and (Kπππ) ^- 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 η mesons. The reconstructed strange final states, corrected for resolution effects and detection efficiencies, yield the strange spectral function of the τ 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(τ ^- → K^-π⁰ντ) = (0.471 ±0.059_stat±0.023_sys) % and B(τ ^-→ K^-π⁺π^-ντ) = (0.415 ± 0.053_stat ± 0.040_sys) % have been measured.

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^- →₁ ⁺χ̃₁^- or neutralino associated production, e⁺e^- → χ̃₂ ⁰χ̃₁⁰ Limits are set at the 95% confidence level on the product of the cross-section for the process e ⁺e^- →₁ ⁺χ̃₁^- 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^- →₂⁰χ̃₁⁰ 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 χ̃₁±,χ̃₁⁰ and χ̃₂⁰, 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 σ(e⁺e^- →₁ ⁺χ̃₁^- assumption that each chargino decays via a W boson, and on σ(e⁺e^- →₂⁰ with the χ̃₂⁰ assumed to decay via a Z⁰.

A study of W⁺W^- events accompanied by hard photon radiation, Eγ > 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 < √s < 209 GeV. From the sample of 187 selected W⁺W^-γ candidates with photon energies greater than 2.5 GeV, the W⁺W^-γ cross-section is determined at five values of √s. The results are consistent with the Standard Model expectation. Averaging over all energies, the ratio of the observed cross-section to the Standard Model expectation is R(data/SM) = 0.99 ± 0.09 ± 0.04, where the errors represent the statistical and systematic uncertainties respectively. These data provide constraints on the related O(α) systematic uncertainties on the measurement of the W-boson mass at LEP. Finally, the data are used to derive 95% confidence level upper limits on possible anomalous contributions to the W⁺W^-γγ and W⁺W^-Z⁰γ vertices: -0.020 GeV ^-2 < a₀/∧² < 0.020 GeV-2, -0.053 GeV^-2 < a_c/∧² < 0.037 GeV ^-2, -0.16 GeV^-2 < a_n/∧² < 0.15 GeV^-2, where ∧ represents the energy scale for new physics and a₀, a_c and a_n are dimensionless coupling constants.

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 ^-νν̄), quark and lepton pairs, (qq̄ℓ ⁺ℓ^-, qq̄νν̄) and only hadrons (qq̄qq̄) are considered. In all states with at least one Z boson decaying hadronically, lifetime, lepton and event-shape tags are used to separate bb̄ pairs from qq̄ final states. Limits on anomalous ZZγ 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.

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.

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^- → qq̄qq̄ 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.

Anomalous quartic couplings between the electroweak gauge bosons may contribute to the ννγγ and qq̄γγ 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 ννγγ and qq̄γγ events in which the two photons are reconstructed within the detector acceptance. The cross section for the process e⁺e^-→qq̄γγ is measured. Averaging over all energies, the ratio of the observed e ⁺e^-→qq̄γγ 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 ννγγ and qq̄γγ data are used to constrain possible anomalous W⁺W^- γγ and ZZγγ couplings. Combining with previous OPAL results from the W ⁺ W^- γ final state, the 95% confidence level limits on the anomalous coupling parameters a₀^Z, a _c^Z, a₀^W and a_c ^W are found to be -0.007 GeV^-2

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 svell described for jet energies below about 14 GeV.

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^- 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.

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 20022003, 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.

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 hee, the Yukawa coupling of the H^±± to like-signed electron pairs. A 95% confidence level upper limit of hee < 0. 071 is inferred for M(H^±±) < 160 GeV assuming that the sum of the branching fractions of the H^±± to all lepton flavour combinations is 100%. Additionally, indirect constraints on hee from Bhabha scattering at centre-of-mass energies between 183 GeV and 209 GeV, where the H^±± would contribute via t-channel exchange, are derived for M(H^±±) < 2 TeV. These are the first results both from a single production search and on constraints from Bhabha scattering reported from LEP.

A measurement of the forward-backward asymmetries of e⁺e ^- → bb̄ and e⁺e^- → cc̄ 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 √s and the average B mixing parameter χ are determined simultaneously in the fit. Using all data collected by OPAL near the Z resonance, the asymmetries are measured to be: A_FB ^bb̄ = (4.7 ± 1.8 ± 0.1)%, A_FB ^cc̄ = (-6.8 ± 0.9)% at 〈√s〉 = 89.51 GeV, A_FB^bb̄ = (9.72 ± 0.42 ± 0.15)%, A _FB^cc̄ = (5.68 ± 0.54 ± 0.39)% at 〈√s〉 = 91.25 GeV, A_FB^bb̄ = (10.3 ± 1.5 ± 0.2)%, A_FB^cc̄ = (14.6 ± 2.0 ± 0.8)% at 〈√s〉 = 92.95 GeV. For the average B mixing parameter, a value of: χ = (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 sin²θ _eff^ℓ of 0.23238 ± 0.00052.

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.

Di-jet production is studied in collisions of quasi-real photons at e ⁺e^- centre-of-mass energies √s_ee from 189 to 209 GeV at LEP. The data were collected with the OPAL detector. Jets are reconstructed using an inclusive k_⊥-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 Ē_T^jet 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_γ, for different regions of Ē_T^jet. 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 |η^jet| and |Δη^jet| is presented, where η^jet is the jet pseudo-rapidity. Different regions of the x_γ⁺-x _γ^--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.

Non-commutative QED would lead to deviations from the Standard Model depending on a new energy scale Λ_NC and a unique direction in space defined by two angles η and ξ. In this analysis, η 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 ^- → γγ 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 Λ_NC depending on the model parameter η. It is shown that the time dependence of the total cross-section could be used to determine the model parameter ξ if there were a detectable signal. This is the first experimental study of non-commutative QED at an e⁺e^- collider.

The exclusive production of proton-antiproton pairs in the collisions of two quasi-real photons has been studied using data taken at √s_ee = 183 GeV and 189 GeV with the OPAL detector at LEP. Results are presented for pp invariant masses, W, in the range 2.15 < W < 3.95 GeV. The cross-section measurements are compared with previous data and with recent analytic calculations based on the quark-diquark model.

The τ^- → μ^-v̄_μv_τ 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(τ^- → μ^-v̄_μv_τ)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_μ/g_e = 1.0005±0.0044 and g_τ/g_e = 1.0031±0.0048, in good agreement with the Standard Model prediction of unity. A value for the Michel parameter η = 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 β, in the Minimal Supersymmetric Standard Model.

The process e⁺e^- → γγ(γ) 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. Pits 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^-γγ, 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 → γγ is found and limits are placed on the product of the X_γ production cross-section and branching ratio.

We observe Bose-Einstein correlations in π⁰ pairs using back-to-back two jet hadronic events from Z⁰ decays in the data sample collected by the OPAL detector at LEP 1 from 1991 to 1995. Using a static Gaussian picture for the pion emitter source, we obtain the chaoticity parameter λ = 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 π⁰s 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.

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^γ > 3.0 GeV and pseudorapidity |η^γ| < 1 is determined to be σ_tot = 0.32 ± 0.04 (stat) ± 0.04 (sys) pb. Differential cross-sections are compared to the predictions of a next-to-leading-order (NLO) calculation.

This paper describes topological searches for neutral scalar bosons S⁰ produced in association with a Z⁰ boson via the Bjorken process e⁺e^- → S⁰Z⁰ at centre-of-mass energies of 91 GeV and 183-209 GeV. These searches are based on studies of the recoil mass spectrum of Z⁰ → e⁺e^- and μ⁺μ^-and on a search for S⁰Z⁰ with Z⁰ → νν̄ and S⁰ → e⁺e^- or photons. They cover the decays of the S⁰ 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_SM⁰Z⁰. These results can be interpreted in general scenarios independently of the decay modes of the S⁰. 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.

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 Z⁰. 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.

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.

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^- → h⁰Z⁰ followed by the decay of h⁰ → A⁰A⁰ where A⁰ is the CP-odd Higgs boson. The search is done in the region where the A⁰ mass, m_A, is below the production threshold for bb̄, and the CP-even Higgs boson mass m_h is within the range 45-86 GeV/c². In this kinematic range, the decay of h⁰ → A⁰A⁰ 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^- → h⁰Z⁰ are derived assuming 100% decays of the h⁰ into A⁰A⁰ and 100% decays of the A⁰A⁰ into each of the following final 3states: cc̄cc̄, gggg, τ⁺τ^-τ⁺τ^-, cc̄gg, ggτ⁺τ^- and cc̄τ⁺τ^-. The results are also interpreted in the CP-conserving no-mixing MSSM scenario, where the region 45 ≤ m_h ≤ 85 GeV/c² and 2 ≤ m_A ≤ 9.5 GeV/c² is excluded.

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/c² is obtained on the mass of the Standard Model Higgs boson at the 95% CL.

The decay chain b → B̄ → D^**0 ℓ^-ν̄X, D^**0 → D ^*+π^-, D^*+ → D ⁰π⁺, D⁰ → (Kπ or Kπ is identified in a sample of 3.9 million hadronic Z decays collected with the OPAL detector at LEP. The branching ratio BR (b → B̄) × BR (B̄ → D₁⁰ℓ^-ν̄X) × BR (D ₁⁰ → D^*+π^-) is measured to be (2.64 ± 0.79 (stat) ± 0.39 (syst)) × 10 ^-3 for the J^P = 1⁺ (D₁⁰) state. For decays into the J^P = 2⁺ (D₂ ^*0) state, an upper limit of 1.4 × 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^- → lνlν 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.

The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have collected a total of 2461 pb−¹ of e⁺ e⁻ collision data at centre-of-mass energies between¹89 and 209 GeV. The data are used to search for the Standard Model Higgs boson. The search results of the four collaborations are combined and examined in a likelihood test for their consistency with two hypotheses: the background hypothesis and the signal plus background hypothesis. The corresponding confidences have been computed as functions of the hypothetical Higgs boson mass. A lower bound of¹¹4.4 GeV/c² is established, at the 95% confidence level, on the mass of the Standard Model Higgs boson. The LEP data are also used to set upper bounds on the HZZ coupling for various assumptions concerning the decay of the Higgs boson.

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 √s = 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 Z⁰ 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.

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 √S_ee = 189 - 209 GeV, for W > 5 GeV and at an average Q² of 17.9 GeV². 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^- qq̄, and to BFKL calculations. PHOJET, NLO e⁺e^- → e⁺e^- qq̄, and QPM describe the data reasonably well, whereas the cross-section predicted by a Leading Order BFKL calculation is too large.

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/c² 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.

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.

A search is performed for production of short-lived particles in e⁺e^- → XY, with X → γγ and Y → ff̄, for scalar X and scalar or vector Y. Model-independent limits in the range of 25-60 femtobarns are presented on (e⁺e^- → XY) × B(X → γγ) × B(Y → ff̄) for centre-of-mass energies in the range 205-207 GeV. The data from all LEP centre-of-mass energies 88-209 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.

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.

The b quark forward-backward asymmetry has been measured using hadronic Z⁰ decays collected by the OPAL experiment at LEP. Z⁰ → bb̄ 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 √s = 89.50 GeV, A_FB ^b= 0.0977 ± 0.0036 ± 0.0018 at √s = 91.26 GeV, A_FB ^b= 0.1221 ± 0.0123 ± 0.0025 at √s = 92.91 GeV, 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 sin²θ_w ^eff,e = 0.23205 ± 0.00068.

The process e⁺e^- → e⁺e^-Z/γ* is studied with the OPAL detector at LEP at a centre of mass energy of √s = 189 GeV. The cross-section times the branching ratio of the Z/γ* 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 ŝ, t̂, and û 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 eγ → eZ/γ* are derived.

The hadronic structure function of the photon F₂ ^γ (x, Q²) is measured as a function of Bjorken x and of the photon virtuality Q² 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 F₂ ^γ are extended to an average Q² of (Q²) = 780 GeV² using data in the kinematic range 0.15 < × < 0.98. The Q² evolution of F₂ ^γ is studied for 12.1 < (Q²) < 780 GeV² using three ranges of x. As predicted by QCD, the data show positive scaling violations in F₂ ^γ with F₂ ^γ (Q²)/α = (0.08 ± 0.02_-0.03 ^+0.05)+(0.13 ± 0.01 _-0.01 ^+0.01) 1n Q², where Q² is in GeV², for the central x region 0.10-0.60. Several parameterisations of F₂ ^γ are in qualitative agreement with the measurements whereas the quark-parton model prediction fails to describe the data.

A search for first generation scalar and vector leptoquarks (LQ) as well as for squarks (q̃)in R-parity violating SUSY models with the direct decay of the q̃ 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 √S_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/q̄tild; → eq, νq, 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 λ to fermions larger than √4πα_em, the mass limits range from 121 GeV/c² to 175 GeV/c² (149 GeV/c² to 188 GeV/c²) depending on the branching ratio α of the scalar (vector) leptoquark state. Furthermore, limits are set on the Yukawa couplings λ for leptoquarks and λ_1jk for squarks, and on β as a function of the scalar leptoquark/squark mass.

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.

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 × 10⁶ selected Z decays. An interpretation of the data in terms of contributions from pure Z exchange and from γ/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 and 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, Γ_z = (2.4948 ± 0.0041) GeV, σ⁰_h = (41.501 ± 0.055) nb, R_ℓ = 20.823 ± 0.044, and A^0,ℓ_FB = 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, Γ_inv/Γ_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_v = 2.984 ± 0.013. Interpreting the data within the context of the Standard Model allows the mass of the top quark, m_t = (162⁺²⁹_-16) GeV, to be determined through its influence on 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: α_s(m_z) = 0.127 ± 0.005 and m_H = (390⁺⁷⁵⁰_-280)GeV.

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.

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.

About 4.4 million hadronic decays of Z bosons, recorded by the OPAL detector at LEP at a centre-of-mass energy of around √s = 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 ^+0.86_-1.20, 〈n_d〉 = 21.44 ± 0.63 ^+1.46_-1.17, 〈n_s〉 = 20.02 ± 0.13 +^0.39_-0.37, where statistical arid 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, α^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.), C_F = 1.34 ± 0.13(stat.) ± 0.22(syst.), α_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 α_s(M_z).

Measurements of the τ lepton polarization and forward-backward polarization asymmetry near the Z⁰ resonance using the OPAL detector are described. The measurements are based on analyses of τ → eν_eν_τ, τ → μν_μν_τ, τ → πν_τ, τ → ρν_τ, and τ → a₁ν_τ decays from a sample of 144,810 e⁺e^- → τ⁺τ^- candidates corresponding to an integrated luminosity of 151 pb^-1. Assuming that the τ lepton decays according to V-A theory, we measure the average τ polarization near √s = Mz to be 〈P_τ〉 = (-14.10 ± 0.73 ± 0.55)% and the τ polarization forward-backward asymmetry to be A^FB_pol = (-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-Z⁰ interference and photonic radiative corrections, these results can be expressed in terms of the lepton neutral current asymmetry parameters: A_τ = 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_ℓ = 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 script O sign(α²_S) calculations for massive quarks, this difference was used to determine the b quark mass in the MS 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 m̄_b(m_z) = (2.67 ± 0.03(stat.)^+0.29_-0.37(syst.)±0.19(theo.)) GeV. Evolving this value to the b quark mass scale itself yields m̄_b(m̄_b) = (3.95^+0.52_-0.6) 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.

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 √s = 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 κ_γ and κ_Z are determined from these results.

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 h⁰ and A⁰ 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 √s ≈ 189 GeV with previous runs at √s ≈ m_z and √s ≈ 183 GeV. The searches are sensitive to the h⁰, A⁰ → qq̄, gg, τ⁺τ^- and h⁰ → A⁰ A⁰ 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.

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.

Measurements are presented of the polarisation of W⁺W^- boson pairs produced in e⁺e^- collisions, and of CP-violating WWZ and WWγ 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 σ_L/σ_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 σ_TT/σ_total = (78.1 ± 9.0 ± 3.2)%, σ_LL/σ_total = (20.1 ± 7.2 ± 1.8)% and σ_TL/σ_total = (1.8 ± 14.7 ± 3.8)%. In the CP-violating trilinear gauge coupling sector we find κ̃_z = -0.20^+0.10_-0.07, g^z₄= -0.02^+0.32_-0.33 and λ̃_z = -0.18^+0.24_-0.16, 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 × 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 mℓ̃ > 83 GeV and m_X̃10 > 85 GeV for tan β = 2, and mτ̃ > 69 GeV, m_ẽ,μ̃ > 88 GeV and m_X̃10 > 76 GeV for tan β = 20, are established.

The rates are measured per hadronic Z⁰ decay for gluon splitting to bb̄ quark pairs, g_bb̄, and of events containing two bb̄ 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 bb̄ quarks are selected if two of the jets are close in phase-space and contain detached secondary vertices. For the event sample containing two bb̄ 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_bb̄ and g_4b, namely gbb̄ = (3.07 ± 0.53(stat) ± 0.97(syst)) × 10^-3, g_4b = (0.36 ± 0.17(stat) ± 0.27(syst)) × 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 κ = 0.97^+0.20_-0.16, g^z₁ = 0.991+^0.060_-0.057 and λ = -0.110^+0.058_-0.055, 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 √s = 189 GeV with the OPAL detector. The invariant mass distributions from 970 W⁺W^- → qqqq and 1118 W⁺W^- → qqℓν_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 Γ_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 √s from 161 to 183 GeV, to obtain: M_W = 80.432 ± 0.066 (stat.) ± 0.045 (syst.) GeV, Γ_W = 2.04 ± 0.16 (stat.) ± 0.09 (syst.) GeV. The consistency of the direct measurement of M_w with that inferred from other measurements of electroweak parameters provides an important test of the Standard Model of electroweak interactions.

We search for lepton flavour violating events (eμ, eτ and μτ) 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 √s. A single eμ 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 σ (e⁺e^- → eμ, eτ and μτ) as a function of √s at LEP2 energies.

The inclusive branching ratio for the process b → τ^-ν̄_τX) has been measured using hadronic Z decays collected by the OPAL experiment at LEP in the years 1992-2000. The result is: BR(b → τ^-ν̄_τX) = (2.78 ± 0.18 ± 0.51)%. This measurement is consistent with the Standard Model expectation and puts a constraint of tan β/M_H^± < 0.53 GeV^-1 at the 95% confidence level on Type II Two Higgs Doublet Models.

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_uℓv event topology and kinematics. This analysis uses OPAL data collected between 1991 and 1995, which correspond to about four million hadronic Z decays. We 1easure Br(b → X_uℓv) to be (1.63 ± 0.53 ^+0.55_-0.62) × 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: |V_ub| = (4.00 ± 0.65(stat)^+0.67_-0.76(sys) ± 0.19(HQE)) × 10^-3. The last error represents the theoretical uncertainties related to the extraction of |V_ub| from Br(b → X_uℓv) 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 √s ≈ 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.

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*^±π⁺π^-. The D*⁺ mesons are reconstructed in the decay channel D*⁺ → D⁰π⁺ with D⁰ → K^-π⁺. 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)) × Br(D*^/+ → D*⁺π⁺π^-) < 3.1 × 10^-3(95%C.L.).

From data collected around the Z resonance by the OPAL detector at LEP, a sample of B⁰_s decays was obtained using D^-_s ℓ⁺ combinations, where the D^-_s was fully reconstructed in the φπ^-, K*⁰K^- and K⁰_sK^- decay channels or partially reconstructed in the φℓ^-v̄(X) decay channel. These events were used to study B⁰_s oscillation. The flavor (b or b̄) 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 B⁰_s oscillation frequency Δm_s > 1.0 ps^-1 at the 95% confidence level.

Correlations among hadrons with the same electric charge produced in Z⁰ 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.

The branching ratio of the τ lepton to a neutral kaon meson is measured from a sample of approximately 200,000 τ decays recorded by the OPAL detector at centre-of-mass energies near the Z° resonance. The measurement is based on two samples which identify one-prong τ decays with K_L⁰ and K_S⁰ mesons. The combined branching ratios are measured to be B(τ^- → π^-K̄⁰ν_τ) = (9.33 ± 0.68 ± 0.49) × 10^-3, B(τ^- → π^-K̄⁰[≥ 1π⁰]ν_τ) = (3.24 ± 0.74 ± 0.66) × 10^-3, B(τ^- → K^-K⁰[≥ 0π⁰]ν_τ) = (3.30 ± 0.55 ± 0.39) × 10^-3, where the first error is statistical and the second systematic.

A search for the decay B(±) → K(±)K(±)π(±) 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.

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) ≡ Γ (W → cX)/Γ (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 |V(cs)| is determined to be |V(cs)| = 0.969 ± 0.058. (C) 2000 Elsevier Science B.V.

The total hadronic cross-section σ_γγ(W) for the interaction of real photons, γγ → hadrons, is measured for γγ centre-of-mass energies 10 ≤ W ≤ 110 GeV. The cross-section is extracted from a measurement of the process e⁺e^- → e⁺e^- → e^+- + hadrons, using a luminosity function for the photon flux together with form factors for extrapolating to real photons (Q² = 0 GeV²). The data were taken with the OPAL detector at LEP at e⁺e^- centre-of-mass energies √s_ee = 161, 172 and 183 GeV. The cross-section σ_γγ(W) is compared with Regge factorisation and with the energy dependence observed in γp and pp interactions. The data are also compared to models which predict a faster rise of σ_γγ(W) compared to γp and pp interactions due to additional hard γγ interactions not present in hadronic collisions.

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.

From an analysis of the ionisation energy loss of charged particles selected from a sample of 147926 e⁺e^-→τ⁺τ^- candidates recorded in the OPAL detector at e⁺e^- centre-of-mass energies near the Z° resonance, we determine the branching ratios: Br(τ^- → ν_τ K^-π^-π⁺(π⁰)) = 0.343 ± 0.073 ± 0.031 % Br(τ^- → ν_τK^-π^-K⁺(π ⁰)) = 0.159 ± 0.053 ± 0.020 %, where the (π⁰) notation refers to decay modes with or without an accompanying π⁰. The τ^- → ν_τK^-π^-π⁺(π ⁰) final states occurring through τ^- → ν_τK^-K_S⁰(π⁰) are treated as background in this analysis. We also examine the resonant structure of τ^- → ν_τK^-π^-π⁺ candidates. Under the assumption that the resonant structure is dominated by the K₁ resonances, we determine: R = Br(τ^- → ν_τK₁(1270))/Br(τ^- → ν_τK₁(1400)) + Br(τ^- → ν_τK₁(1270)) = 0.71 ± 0.16 ± 0.11. In all results, the first uncertainties are statistical and the second are systematic.

The energy distribution and type of the particle with the highest momentum in quark jets are determined for each of the five quark flavours making only minimal model assumptions. The analysis is based on a large statistics sample of hadronic Z⁰ 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 centre-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 γ_s = 0.422 ± 0.049(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.

Cross-sections and angular distributions for hadronic and lepton pair final states in e⁺e^- collisions at a centre-of-mass energy near 189 GeV, 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 α_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 sneutrino in supersymmetric theories with R-parity violation. A search for the indirect effects of the gravitational interaction in extra dimensions on the μ⁺μ^- and τ⁺τ^- final states is also presented.

The inclusive production of D^*± mesons in photon-photon collisions has been measured using the OPAL detector at LEP at e⁺e^- centre-of-mass energies √s_ee of 183 and 189 GeV. The D^*+ mesons are reconstructed in their decay to D⁰π⁺ with the D⁰ observed in the two decay modes K^-π⁺ and K^-π⁺π^-π⁺. 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 dσ/dp^D*_T and dσ/d|η^D*| as functions of the D^*± transverse momentum p^D*_T and pseudorapidity η^D* are presented in the kinematic region 2 GeV < p^D*_T < 12 GeV and |η^D*| < 1.5. They are compared to next-to-leading order (NLO) perturbative QCD calculations. The total cross-section for the process e⁺e^- → e⁺e^- cc̄ where the charm quarks are produced in the collision of two quasi-real photons is measured to be σ(e⁺e^- → e⁺e^- cc̄) = 963 ± 110 (stat) ± 86 (sys) ± 224 (extrapolation) pb. A first measurement of the charm structure function F^γ_2,c of the photon is performed in the kinematic range 0.0014 < cursive Greek chi < 0.87 and 5 GeV² < Q² < 100 GeV², and the result is compared to a NLO perturbative QCD calculation.

The correlated production of Λ and Λ̄ baryons has been studied using 4.3 million multihadronic Z° 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_⊥ algorithm. The distributions of rapidity differences from correlated ΛΛ̄ pairs exhibit short-range, 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 between 25 and 58 mrad. At LEP center-of-mass energies around the Z⁰, 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 by direct physical measurement. All measured properites of the luminosity event sample are found to be in agreement with current theoretical expectations. The total systematic measurement uncertainty is 3.4 × 10^-4, significantly below the theoretical error of 5.4 × 10^-4 currently assigned to the QED calculation of the Bhabha acceptance, and contributes negligibly to the total uncertainty in the OPAL measurement of Γ_inv/Γ_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 √s = 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 δww = 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 τ to be 1775.1 ± 1.6(m(cn/stat.)) ± 1.0(m(cn/sys.)) MeV using τ from Z⁰ decays. To test CPT invariance we compare the masses of the positively and negatively charged τ. The relative mass difference is found to be smaller than 3.0 × 10^-3 at the 90% confidence level.

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^- → Zγ. 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 ZγZ and Zγγ 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 inclusive branching fraction of φ mesons from the decay of b hadrons produced in Z decays was measured to be Br(b → φX) = 0.0282 ± 0.0013 (stat.) ± 0.0019 (syst.), using data collected by the OPAL detector at LEP. (C) 2000 Elsevier Science B.V.

We compared the multiplicities of π⁰, η⁰, Κ⁰ 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 π⁰, η or Κ⁰ 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 η 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 photon structure function F^γ₂(cursive Greek chi, Q²) 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, m Q² ranges of 1.5-30.0 GeV² (LEP1) and 7.0-30.0 GeV² (LEP2), probing lower values of cursive Greek chi 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^γ₂ at low-cursive Greek chi as expected from QCD.

The lifetime and oscillation frequency of the B⁰ meson has been measured using B̄⁰ → D*⁺l^-ν̄ decays recorded on the Z⁰ peak with the OPAL detector at LEP. The D*⁺ → D⁰π⁺ decays were reconstructed using an inclusive technique and the production flavour of the B⁰ mesons was determined using a combination of tags from the rest of the event. The results τ(B⁰) = 1.541 ± 0.028 ± 0.023 ps, Δm(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.

The cross-section ratio R_b = σ(e⁺e^- → bb̄)/σ(e⁺e^- → qq̄) and the bottom and charm forward-backward asymmetries A^b_FB and A^c_FB 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^b_FB. In addition, the bottom and charm asymmetries are measured using leptons from semileptonic decays of heavy hadrons and pions from D*⁺ → D⁰π⁺ decays. The results are in agreement with the Standard Model predictions.

The magnitude of the Cabibbo-Kobayashi-Maskawa matrix element V(cb) has been measured using B̄⁰ → D*⁺l^-ν̄ decays recorded on the Z⁰ peak using the OPAL detector at LEP. The D*⁺ → D⁰π⁺ decays were reconstructed both in the particular decay modes D⁰ → K^-π⁺ and D⁰ → K^-π⁺π⁰ and via an inclusive technique. The product of |V(cb)| and the decay form factor of the B̄⁰ → D*⁺l^-ν̄ 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(B̄⁰ → D*⁺l^-ν̄) was also measured to be (5.26 ± 0.20 ± 0.46)%. (C) 2000 Published by Elsevier Science B.V.

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 ρ₀₀ of ρ(770)^± and w(782) mesons produced in Z⁰ 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 < x_E < 0.6, the measured ρ₀₀ values of the ρ^± and the w are 0.373 ± 0.052 and 0.142 ± 0.114, respectively. These results are compared to measurements of other vector mesons.

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-vv̄), quark and lepton pairs, (qq̄l⁺l^-, qq̄vv̄) and the all-hadronic final state (qq̄qq̄) are considered. In all states with at least one Z boson decaying hadronically, qq̄ and bb̄ final states are considered separately using lifetime and event-shape tags, thereby improving the cross-section measurement. At √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 √s = 183 GeV the 95% C.L. upper limit is 0.55 pb. Limits on anomalous ZZγ and ZZZ couplings are derived. (C) 2000 Published by Elsevier Science B.V.

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 173 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 ℓ^±_R→ℓ^± ^-0λ1, we exclude at 95%. CL: right-handed smuons with masses below 65 GeV for mμ̃ - m_-0λ1 > 2 GeV and right-handed staus with masses below 64 GeV for m τ̃ - m_-0λ1 > 10 GeV. Right-handed selectrons are excluded at 95% CL for masses below 77 GeV for mο̃ - m_-0λ1 > 5 GeV within the framework of the Minimal Supersymmetric Standard Model assuming μ < -100 GeV and tan 3 = 1.5.

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 Z⁰ 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.

The rate of secondary charm-quark-pair production has been measured in 4.4 million hadronic Z⁰ decays collected by OPAL. By selecting events with three jets and tagging charmed hadrons in the 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)x10^-2.

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 √s ≈ 189 GeV were used to search for Higgs boson production in the SM process e⁺e^- → H⁰Z⁰ and the MSSM processes e⁺e^- → h⁰Z⁰ and e⁺e^- → A⁰h⁰. The searches are sensitive to the bb̄ and τ⁺T^- decay modes of the Higgs bosons, and also to the MSSM decay mode h⁰ → A⁰A⁰. 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_u > 74.8 GeV and m_A > 76.5 GeV, assuming tan 3 > 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 3 between 0.72 and 2.10 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-rnass 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 ℓ̃^±_R → ℓ^±_χ̃01, where _χ̃01 is the lightest neutralino, we exclude at 95% CL: right-handed smuons with masses below 82.3 GeV for m_μ̃- - m_χ̃01 > 3 GeV and right-handed staus with masses below 81.0 GeV for m_τ̃- -m_χ̃01 > 8 GeV. Right-handed selectrons are excluded at 95% CL for masses below 87.1 GeV for m_ẽ - m_χ̃01 > 5 GeV, within the framework of the Minimal Supersymmetric Standard Model assuming μ, < - 100 GeV and tan β= 1.5. Charged Higgs bosons, H^±, are excluded at 95% CL for masses below 82.8 GeV, assuming a 100% branching ratio for the decay H^± → τ ^± v_τ.

The lifetimes of the B⁺ and B⁰ mesons, and their ratio, have been measured in the OPAL experiment using 2.4 million hadronic Z⁰ decays recorded at LEP. Z⁰ → bb̄ 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 τ_Β+ = 1.643 ± 0.037 ± 0.025 ps τ_Β0 = 1.523 ± 0.057 ± 0.053 ps τ_Β+/τ_Β0 = 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 Z⁰ decays has been used to search for CP and CPT violating effects by comparison of inclusive b and b hadron decays. No evidence for such effects is seen. The CP violation parameter Re(∈_Β) is measured to be Re(∈_Β) = 0.001 ± 0.014 ± 0.003 and the fractional difference between b and b̄ hadron lifetimes is measured to be (Δτ/τ)_b ≡ τ(b hadron) - τ(b̄ hadron)/τ(average) = -0.001 ± 0.012 ± 0.008.

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^- → vv̄ + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on σ(e⁺e^- → XY) · BR(X → Yγ) and σ(e⁺e^- → XX) · BR²(X → Yγ) are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos (X = v*, Y = v), to neutralino production (X = χ̃⁰₂, Y = χ̃⁰₁) and to supersymmetric models in which X = χ̃⁰₁ and Y = G̃ is a light gravitino. The case of macroscopic decay lengths of particle X is considered for e⁺e^- → XX, X → Yγ, when M_Y ≈ 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.

We have studied hadronic events from e⁺e^- annihilation data at centre-of-mass energies of √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 α_s, the mean charged particle multiplicity 〈n_ch〉 and the peak position ξ₀ in the ξ_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 α_s, 〈n_ch〉 and ξ₀ are consistent with previous determinations at √s=M_z0.

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 (X̃₁⁺X̃₁^-, X̃₁⁰X̃₂⁰) cross-sections are obtained as a function of the chargino mass (mX̃₁^±), the lightest neutralino mass (mX̃₁⁰) and the second lightest neutralino mass (mX̃₂⁰). Within the Constrained Minimal Supersymmetric Standard Model framework, and for mX̃₁^± - mX̃₁⁰ ≥ 5 GeV, the 95% confidence level lower limits on mX̃₁^± are 93.6 GeV for tan β = 1.5 and 94.1 GeV for tan β = 35. These limits are obtained assuming a universal scalar mass m₀ ≥ 500 GeV. The corresponding limits for all m₀ are 78.0 and 71.7 GeV. The 95% confidence level lower limits on the lightest neutralino mass, valid for any value of tan β are 32.8 GeV for m₀ ≥ 500 GeV and 31.6 GeV for all m₀.

A search for pair produced scalar fermions with couplings that violate R-parity has been performed using a data sample corresponding to an integrated luminosity of 56 pb^-1 at a centre-of-mass energy of √s = 183 GeV collected with the OPAL detector at LEP. An important consequence of R-parity breaking interactions is that the lightest supersymmetric particle is expected to be unstable. Searches for R-parity violating decays of charged sleptons, sneutrinos and stop quarks 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 multileptons, jets plus leptons or multi-jets, with or without missing energy, in the final state. No significant excess of such events has been observed. Limits on the production cross-sections of scalar fermions in R-parity violating scenarios are obtained. Mass exclusion regions are also presented in the framework of the Constrained Minimal Supersymmetric Standard Model.

The structure of both quasi-real and highly virtual photons is investigated using the reaction e⁺e^- → e⁺e^-μ⁺μ^-, 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^γ₂ and the differential cross-section dσ/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 Q² ranging from 1.5 to 400 GeV². The differential cross-section dσ/dx for highly virtual photons is measured for 1.5 < Q² < 30 GeV² and 1.5 < P² < 20 GeV², where Q² and P² are the negative values of the four-momentum squared of the two photons such that Q² > P². Based on azimuthal correlations the QED structure functions F^γ_A and F^γ_B for quasi-real photons are determined for an average Q² of 5.4 GeV².

Gluon jets are identified in hadronic Z⁰ 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 p_T, we observe the charged particle multiplicity ratio of gluon to quark jets to be 2.29±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 < 4 GeV/c and 0.8 < p_T < 3.0 GeV/c, are motivated by the predictions of the Herwig Monte Carlo multihadronic event generator. Additionally, our gluon jet data allow a sensitive test of the phenomenon of non-leading QCD terms known as color reconnection. We test the model of color reconnection implemented in the Ariadne Monte Carlo multihadronic event generator and find it to be disfavored by our data.

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 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 √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 branching ratios of the decay of the τ 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 Z⁰ resonance. The branching ratios are measured to be B(τ^- → 3h^-2h⁺ν_τ) = (0.091 ± 0.014 ± 0.006)% B(τ^- → 3h^-2h⁺π⁰ν_τ) = (0.027 ± 0.018 ± 0.009)% where the first error is statistical and the second systematic.

The branching ratio for the decay τ^- → e^-ν + ̄_eν_τ has been measured using Z⁰ decay data collected by the OPAL experiment at LEP. In total 33073 τ^- → e^-ν + ̄_eν_τ candidates were identified from a sample of 186 197 selected τ decays, giving a branching ratio of B(τ^- → e^-ν + ̄_eν_τ) = (17.81 ± 0.09 (stat) ± 0.06 (syst))%. This result is combined with other measurements to test e - μ and μ - τ universality in charged-current weak interactions. Additionally, the strong coupling constant α_s (m_τ²) has been extracted from B(τ^- → e^-ν + ̄_eν_τ) and evolved to the Z⁰ mass scale, giving α_s(m_Z²) = 0.1204 ± 0.0011 (exp) ± 0.0019 (theory).

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 √S = 189 GeV. From these data, 17 W⁺W^- γ 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^-γ cross-section within a set of geometric and kinematic cuts, σwwγ = 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^- γ and W⁺W^- γZ(o) vertices: -0.070GeV^-2 < a(o)/Λ² < 0.070 GeV^-2, -0.13GeV^-2 < a(c)/Λ2 < 0.19GeV^-2, -0.61GeV^-2 < a(n)/Λ² < 0.57 GeV^-2, where A represents the energy scale for new physics. (C) 1999 Published by Elsevier Science B.V.

Searches for a scalar top quark and a scalar bottom quark have been performed using a data sample of 182 pb^-1 at a centre-of-mass energy of √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 states of the top quark is zero. In the worst case, when the scalar top quark decouples from the Z⁰ 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.

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 Z⁰ 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.

The fraction of Z⁰ → bb̄ events in hadronic Z⁰ decays has been measured by the OPAL experiment using the data collected at LEP between 1992 and 1995. The Z⁰ → bb̄ decays were tagged using displaced secondary vertices, and high momentum electrons and mouns. 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 comparison between real and simulated data samples. A value of R_b ≡ σ(e⁺e^- → bb̄)/ σ (e⁺e^- → hadrons) = 0.2178 ± 0.0011 ± 0.0013 was obtained, where thr first error is statistical and the second systematic. The uncertainty on R_c, the fraction Z⁰ → cc̄ events in hadronic Z⁰ decays, is not included in the errors. The dependence o nR_c is ΔR_b/R_b = -0.056ΔR_c/,R_c where ΔR_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.

The inclusive charm hadron semileptonic branching fractions B(c → e) and B(c → μ) in Z⁰ → cc events have been determined using 4.4 million hadronic Z⁰ 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^+0.009_-0.008 and B(c→ μ) = 0.090 ±0.007^+0.007_-0.006, 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^+0.007_-0.006 is obtained.

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 α_s in three flavour tagged samples (uds, c and b). The event shape distributions were fitted by O(α²_s) calculations of jet production taking into account mass effects for the c and b quarks. We find: α^c_s/α^uds_s = 0.997 ± 0.038 (stat.) ± 0.030 (syst.) ± 0.012 (theory) and α^b_s/α^uds_s = 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 Z⁰ decays is performed. The study is based on the data sample of more than 4 × 10⁶ 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 intermittency 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 (X̃⁺₁ X̃^-₁ ,X̃⁰₁X̃⁰₂) cross-sections are obtained as a function of the chargino mass (ℳ_X̃1±), the lightest neutralino mass (ℳ_X̃10) and the second lightest neutralino mass (ℳ_X̃20). 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, ℳ₀, implying heavy scalar fermions, and for the case of a small ℳ₀ resulting in light scalar fermions and giving the worst-case limits. Within this framework and for ℳ_X̃1± - ℳ_X̃10 ≥ 5 GeV the 95% C.L. lower limits on ℳ_X̃1± for ℳ₀ = 500 GeV are 90.0 and 90.2 GeV for tan β = 1.5 and 35 respectively. These limits for all ℳ₀ (the worst-case are 69.1 and 65.2 GeV for tan β = 1.5 and 35 respectively. Exclusion regions are also presented for neuraealino masses, including as absolute lower limit at 95% C.L. for the mass of the lightest neutralino of 30.1 GeV for ℳ₀ = 500 GeV (24.2 GeV for all ℳ₀), with implications for experiemental 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.

A sample of Z⁰ decays containing b-flavoured hadrons is tagged using leptons, and events having precise proper time measurements are selected. These events are used to study B⁰_s oscillations. The flavour (b or b̄) 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 B⁰_s oscillation frequency Δm_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^- → νν̄ + photon(s). No evidence is observed for new physics contributions to these final states. Using the data at √s = 183 GeV, upper limits on σ(e⁺e^- → XY) · BR(X → Y_γ) and σ(e⁺e^- → XX) · BR²(X → Y_γ) are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos (X = ν*, Y = ν), to neutralino production (X = ^∼0_χ2, Y = ^∼0_χ1) and to supersymmetric models in which X = ^∼0_χ1 and Y = G̃ is a light gravitino.

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

The data collected by the OPAL experiment at √s = 183 GeV were used to 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 for 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_h0 > 70.5 GeV and m_A0 > 72.0 GeV are obtained for tan β > 1, no and maximal scalar top mixing and soft SUSY-breaking masses of 1 TeV. The range 0.8 < tan β < 1.9 is excluded for minimal scalar top mixing and m_top ≤ 175 GeV. More general scans of the MSSM parameter space are also considered.

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 57pb^-1 at √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 (qq̄l̄ν_l, qq̄qq̄, l̄ν_llν̄_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 Δκ_γ = 0.11+^0.52_-0.37, Δg^z₁ = 0.01^+0.13_-0.12 and λ = -0.10^+0.13_-0.12, 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^- → γγ(γ) 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: Λ₊> 304 GeV and Λ_-> 295 GeV as well as a mass limit for an excited electron, M_e* > 306 GeV assuming equal e*eγ and eeγ couplings. Graviton exchange in the context of theories with higher dimensions is excluded for scales G⁺< 660 GeV and G_-< 634 GeV. No evidence for resonance production is found in the invariant mass spectrum of photon pairs. Limits are obtained for the cross-section times branching ratio for a resonance decaying into two photons and produced in association with another photon.

Using data collected with the OPAL detector at LEP, we have searched for the processes e⁺ e^- → Z⁰ → pe^-, pμ^- 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 Γ(Z⁰→ pe) and Γ(Z⁰→ pμ) are found to be 4.6 and 4.4 keV respectively.

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 √s ≃ 182.7 GeV in the OPAL detector at LEP. Dependence on the modelling of hadronic W decays is avoided by comparing W⁺W^-→qq̄qq̄ events with the non-leptonic component of W⁺W^-→qq̄ℓv̄_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: Δ〈n_ch〉 = 〈n_ch^4q〉 - 2〈n_ch^qqℓv〉 = +0.7 ± 0.6 Δ〈x_p〉 = 〈x_p^4q〉 - 〈x_p^qqℓv〉 = (-0.09 ± 0.09 ± 0.05) × 10^-2 In addition, measurements of rapidity and thrust are performed for W+W^-→ qq̄qq̄ 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.

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^- → qq̄qq̄ and 300 W⁺W^- → qq̄lv̄_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 T_W. A second fit provides a direct measure of the width of the W boson and gives T_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 T_w = 1.84 ± 0.32(stat.) ± 0.20(syst.) GeV.

The spectral functions of the vector current and the axial-vector current have been measured in hadronic τ decays using the OPAL detector at LEP. Within the framework of the Operator Product Expansion a simultaneous determination of the strong coupling constant α_s, 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 α_s(m_τ²) = 0.348 ± 0.009_exp ± 0.019_theo at the τ-mass scale and α_s(m_Z²) = 0.1219 ± 0.0010_exp ± 0.0017_theo at the Z°- mass scale. The values obtained for α_s(m_Z²) using Fixed Order Perturbation Theory or Renormalon Chain Resummation are 2.3% and 4.1% smaller, respectively. The 'running' of the strong coupling between S₀ ≃ 1.3 GeV² and S₀ = m_τ² has been tested from direct fits to the integrated differential hadronic decay rate R_τ(s₀). A test of the saturation of QCD sum rules at the τ-mass scale has been performed.

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 √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 Z⁰ 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 lcpton 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.

The production of charged hadrons and K⁰_S 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 K⁰_S 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 < W < 125 GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in γγ interactions with γ-proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions.

The production of K⁰_S mesons and Λ 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_⊥ (Durham) jet finder (Y_cut = 0.005), the ratios of K⁰_S and Λ 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°, the ratios of relative production rates in gluon and quark jets were determined to be 0.94 ± 0.07 ± 0.07 for K⁰_S and 1.18 ± 0.10 ± 0.17 for Λ. The results of both analyses are compared to the predictions of Monte Carlo models.

Fragmentation functions for charged particles in Z⁰ → qq̄ events have been measured for bottom (b), charm (c) and light (uds) quarks as well as for all flavours together. The results are based on data recorded between 1990 and 1995 using the OPAL detector at LEP. Event samples with different flavour compositions were formed using reconstructed D*^± mesons and secondary vertices. The ξ_P = ln(1/x_p) distributions and the position of their maxima ξ₀ are also presented separately for uds, c and b quark events. The fragmentation function for b quarks is significantly softer than for uds quarks.

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^- →qq̄ℓν̄e, W⁺W^- qq̄qq̄, or (Z⁰/γ)* → qq̄. After demonstrating the existence of Bose-Einstein correlations in W decays, an attempt is made to determine Bose-Einstein correlations for pions originating from the same W boson and from different W bosons, as well as for pions from (Z⁰/γ)* → qq̄ events. The following results are obtained for the individual chaoticity parameters λ, assuming a common source radius R: λ^same= 0.63 ± 0.19 ± 0.14 λ^diff = 0.22 ± 0.53 ±0.14, λ ^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 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.

Cross-sections for hadronic, bb̄ and lepton pair final states in e⁺e^- collisions at √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 states have also been measured. Cross-sections and asymmetries are also presented for data recorded in 1997 at √s=130 and 136 GeV. The results are used to measure the energy dependence of the electromagnetic coupling constant α_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.

In the process e⁺e^- → hadrons, one of the effects of gluon emission is to modify the (1 + cos²θ) 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 σ_L/σ_tot = 0.0127 ± 0.0016 ± 0.0013 at the parton level, in good agreement with the expectation of QCD computed to script O sign(α²_s). 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.

The branching ratios of the τ^- → h^- ν_τ, τ^- → h^- π⁰ν_τ and τ^- → h^- ≥ 2π⁰ν_τ decays have been measured using the 1991-1995 data recorded with the OPAL detector at LEP. These branching ratios are measured simultaneously using three selection criteria and are found to be BR(τ^- → h^- ν^τ) = (11.98 ± 0.13 ± 0.16)% BR(τ^- → h^- π⁰ν_τ) = (25.89 ± 0.17 ± 0.29)% BR(τ^- → h^- ≥ 2π⁰ν_τ) = (9.91 ± 0.31 ± 0.27)% where the first error is statistical and the second is systematic.

Mechanisms responsible for cross-talk in a thin-gap multiwire chamber with pad readout are discussed. Using a simple capacitive coupling model, the cross-talk is calculated as a function of chamber geometry, capacitances and charge integration time. Calculation results are compared and found to be in reasonable agreement with measurements. Suggestions are given for reducing cross-talk in such a chamber.

A search is described for the neutral Higgs bosons h⁰ and A⁰ predicted by models with two scalar field doublets and, in particular, the Minimal Supersymmetric Standard Model (MSSM). The search in the Z⁰h⁰ and h⁰A⁰ production channels is based on data corresponding to an integrated luminosity of 25 pb^-1 from e⁺e^- collisions at centre-of-mass energies between 130 and 172 GeV collected with the OPAL detector at LEP. The observation of a number of candidates consistent with Standard Model background expectations is used in combination with earlier results from data collected at the Z⁰ resonance to set limits on m_h and m_A in general models with two scalar field doublets and in the MSSM. For example, in the MSSM, for tan β > 1, minimal and maximal scalar top quark mixing and soft SUSY-breaking masses of 1 TeV, the 95% confidence level limits m_h > 59.0 GeV and m_A > 59.5 GeV are obtained. For the first time, the MSSM parameter space is explored in a detailed scan.

In e⁺e^- collisions at centre-of-mass energies around 91 GeV, D *⁰ mesons have been reconstructed using data collected with the OPAL detector at LEP. The hadronisation fraction has been measured to be f (c → D *⁰) = 0.218 ± 0.054 ± 0.045 ± 0.007, where the errors correspond to the statistical and systematic errors specific to this analysis, and to systematic uncertainties from externally measured branching fractions, respectively. Together with previous OPAL measurements of the hadronisation fractions of other charmed mesons, this value is used to investigate the relative production of observed vector and pseudoscalar charmed mesons in Z⁰ → cc̄ decays. The production ratio is determined to be P_v^eff = V/(V+P) = 0.57 ± 0.05 . The relative primary production of vector and pseudoscalar mesons, P_v^prim, is studied in the context of the production and decay of orbitally excited charmed resonances. The first measurement of the inclusive D_s *⁺ production rate in hadronic Z⁰ decays is presented.

An investigation of CP violation was performed using a total of 24 candidates for B⁰ → J/ψK⁰_S decay, with a purity of about 60%. These events were selected from 4.4 million hadronic Z⁰ decays recorded by the OPAL detector at LEP. An analysis procedure, involving techniques to reconstruct the proper decay times and tag the produced b-flavours, B⁰ or B̄⁰, has been developed to allow a first direct study of the time dependent CP asymmetry that, in the Standard Model, is sin 2β. The result is sin 2β = 3.2^+1.8 _-2.0 ± 0.5 where the first error is statistical and the second systematic. This result is used to determine probabilities for different values of sin 2β in the physical region from -1 to +1.

An upper limit for the τ-neutrino mass has been determined from the decay τ → 5π^±v_τ using data collected with the OPAL detector from 1991 to 1995 in e⁺e^- collisions at √s ≈ Mz. A limit of 43.2 MeV at 95% CL is obtained using a two-dimensional method in the 5π invariant mass and energy distribution from 22 selected events. Combining this result with OPAL's previously published measurement using τ⁺ τ^- → 3h^±v̄_τ + 3h∓v_τ decays, a new combined limit of m_vτ < 27.6 MeV (95% CL) is obtained.

Inclusive production of the f₀(980), f₂(1270) and φ(1020) resonances has been studied in a sample of 4.3 million hadronic Z⁰ decays from the OPAL experiment at LEP. A coupled channel analysis has been used for the f₀ in simultaneous fits to the resonances in inclusive π⁺π^- and K⁺K^- mass spectra. Fragmentation functions are reported for the three states. Total inclusive rates are measured to be 0.141 ± 0.007 ± 0.011 f₀, 0.155 ± 0.011 ± 0.018 f₂ and 0.091 ± 0.002 ± 0.003 φ mesons per hadronic Z⁰ decay. The production properties of the f₀, including those in three-jet events, are compared with those of the f₂ and φ, and with the Lund string model of hadron production. All measurements are consistent with the hypothesis that the f₀(980) is a conventional qq̄ scalar meson.

A search for decays of the B_c meson was performed using data collected from 1990-1995 with the OPAL detector on or near the Z⁰ peak at LEP. The decay channels B⁺_c→J/ψπ⁺, B⁺_c→J/ψa⁺₁ and B⁺_c→J/ψℓ⁺v were investigated, where ℓ denotes an electron or a muon. Two candidates are observed in the mode B⁺_c→J/ψπ⁺, with an estimated background of (0.63 ± 0.20) events. The weighted mean of the masses of the two candidates is (6.32 ± 0.06) GeV/c², which is consistent with the predicted mass of the B_c meson. One candidate event is observed in the mode B⁺_c→J/ψℓ⁺v, with an estimated background of (0.82 ± 0.19) events. No candidate events are observed in the B⁺_c→J/ψa⁺₁ decay mode, with an estimated background of (1.10 ± 0.22) events. Upper bounds at the 90% confidence level are set on the production rates for these processes.

This paper presents updated measurements of the lifetimes of the B⁰_s meson and the Λ⁰_b baryon using 4.4 million hadronic Z⁰ decays recorded by the OPAL detector at LEP from 1990 to 1995. A sample of B⁰_s decays is obtained using D^-_sl⁺ combinations, where the D^-_s is fully reconstructed in the φπ^-, K*⁰K^- and K^-K⁰_S decay channels and partially reconstructed in the φl^-v̄X decay mode. A sample of Λ⁰_b decays is obtained using Λ⁺_cl^- combinations, where the Λ⁺_c is fully reconstructed in its decay to a pK^-π⁺ final state and partially reconstructed in the Λl⁺vX decay channel. From 172±28 D^-_sl⁺ combinations attributed to B⁰_s decays, the measured lifetime is π(B⁰_s) = 1.50^+0.16_-0.15±0.04 ps, where the errors are statistical and systematic, respectively. From the 129±25 Λ⁺_cl^- combinations attributed to Λ⁰_b decays, the measured lifetime is τ(Λ⁰_b) = 1.29^+0.24_-0.22±0.06 ps, where the errors are statistical and systematic, respectively.

The inclusive production rates and differential cross-sections of photons and mesons with a final state containing photons have been measured with the OPAL detector at LEP. The light mesons covered by the measurements are the π⁰, η, ρ(770)±, ω(782), η(958) and a_o(980)±. The particle multiplicities per hadronic Z⁰ decay, extrapolated to the full energy range, are: 〈n_γ〉 = 20.97 ± 0.02 ± 1.15 〈n_π0〉 = 9.55 ± 0.06 ± 0.75 〈n_η〉 = 0.97 ± 0.03 ± 0.11 〈n_ρ±〉 = 2.40 ± 0.06 ± 0.43 〈n_ω〉 = 1.04 ± 0.04 ± 0.14 〈n_η〉 = 0.14 ± 0.01 ± 0.02 〈n_a0±〉 = 0.27 ± 0.04 ± 0.10 where the first errors are statistical and the second systematic. In general, the results are in agreement with the predictions of the JETSET and HERWIG Monte Carlo models.

Photonic events with large missing energy have been observed in e⁺e^- collisions at centre-of-mass energies of 130, 136, 161 and 172 GeV using the OPAL detector at LEP. Results are presented based on search topologies designed to select events with a single photon and missing transverse energy or events with a pair of acoplanar photons. In both search topologies, cross-section measurements are performed within the kinematic acceptance of the selection. These results are compared with the expectations from the Standard Model processes e⁺e^- → vv̄γ(γ) (single-photon) and e⁺e^- → vv̄γγ(γ) (acoplanar-photons). No evidence is observed for new physics contributions to these final states. Upper limits on σ(e⁺e^- → XY) · BR(X → Yγ) and σ(e⁺e^- → XX) · BR²(X → Yγ) are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos (X = v*, Y = v), to neutralino production (X = χ^̃0₂, Y = χ^̃0₁), and to supersymmetric models in which X = χ^̃0₁ and Y = G̃ is a light gravitino. For the latter scenario, the results of the acoplanar-photons search are used to provide model-dependent lower limits on the mass of the lightest neutralino.

Events containing two charged leptons and significant missing transverse momentum are selected from a data sample corresponding to a total integrated luminosity of 20.6 pb^-1 at centre-of-mass energies of 161 GeV and 172 GeV. The observed number of events, four at 161 GeV and nine at 172 GeV, is consistent with the number expected from Standard Model processes, predominantly arising from W⁺W^-1 production with each W 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 Further event selection criteria are described that optimise the sensitivity to particular new physics channels No evidence for new phenomena is observed and limits on the production of scalar charged lepton pairs and other new particles are presented.

A search for the resonant production of high mass photon pairs associated with a leptonic or hadronic system has been performed using a data sample of 57.7 pb^-1 collected at an average center-of-mass energy of 182.6 GeV with the OPAL detector at LEP. No evidence for contributions from non-Standard Model physics processes was observed. The observed candidates are used to place limits on B(H⁰ → γγ) assuming a Standard Model production rate for Higgs boson masses up to 92 GeV, and on the production cross section for a scalar resonance decaying into di-photons up to a mass of 170 GeV. Upper limits on the product of cross section and branching ratios, σ(e⁺e^- → XY) × B(X → γγ) × B(Y → ff̄), as low as 70 fb are obtained over the range 10 < M_X < 170 GeV for the case where 10 < M_Y < 160 GeV and M_X + M_Y > 90 GeV, independent of the nature of Y provided it decays to a fermion pair and has negligible width. Higgs scalars which couple only to gauge bosons at Standard Model strength are ruled out up to a mass of 90.0 GeV at the 95% confidence level. Limits are also placed on non-minimal Higgs sectors having triplet representations.

Production of events with hadronic and leptonic final states has been measured in e⁺e^- collisions at centre-of-mass energies of 130-172 GeV, using the OPAL detector at LEP. Cross-sections and leptonic forward-backward asymmetries are presented, both including and excluding the dominant production of radiative Zγ events, and compared to Standard Model expectations. The ratio R_b of the cross-section for bb̄ production to the hadronic cross-section has been measured. In a model-independent fit to the Z lineshape, the data have been used to obtain an improved precision on the measurement of γ-Z interference. The energy dependence of α_em has been investigated. The measurements have also been used to obtain limits on extensions of the Standard Model described by effective four-fermion contact interactions, to search for t-channel contributions from new massive particles and to place limits on gaugino pair production with subsequent decay of the gaugino into a light gluino and a quark pair.

The process e⁺e^- → γγ(γ) is studied using data recorded with the OPAL detector at LEP. The data sample corresponds to a total integrated luminosity of 56.2 pb^-1 taken at a centre-of-mass energy of 183 GeV. 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: Λ₊> 233 GeV and Λ_> 265 GeV as well as a mass limit for an excited electron, M_e* > 227 GeV assuming equal e * eγ and eeγ couplings. No evidence for resonance production is found in the invariant mass spectrum of photon pairs. Limits are obtained for the cross-section times branching ratio for a resonance decaying into two photons.

We present an observation at LEP of the production of χ_c2 mesons in the collisions of two quasi-real photons using the OPAL detector. The χ_c2 mesons are reconstructed in the decay channel χ_c2 → J/ψγ →ℓ⁺ ℓ^- γ (with ℓ = e, μ) using all data taken at e⁺e^- centre-of-mass energies of 91 and 183 GeV, corresponding to integrated luminosities of 167 and 55 pb^-1 respectively. The two-photon width of the χ_c2 is determined to be Γ(χ_c2 → γγ) = 1.76 ± 0.47 ± 0.37 ± 0.15 keV, where the first error is statistical, the second is systematic and the third comes from branching ratio uncertainties.

The longitudinal polarization, the transverse polarization, and the forward-backward asymmetry of Λ baryons, have been measured using a sample of 4.34 million hadronic Z⁰ decays collected with the OPAL detector at LEP between 1990 and 1995. These results are important as an aid to the understanding of hadronizatiou mechanisms. Significant longitudinal polarization has been observed at intermediate and high momentum. For x_E (= 2EΛ/√s) > 0.3, the longitudinal polarization has been measured to be -32.9 ± 5.5 (stat) ± 5.2 (syst)%. We have observed no transverse polarization. A significant forward-backward asymmetry has been measured and can be described by a JETSET model.

We have searched for unstable neutral and charged heavy leptons, N and L^± and for excited states of neutral and charged leptons, ν*, e*, μ*, and τ*, in e⁺e^- collisions at centre-of-mass energies of 170 and 172 GeV using the OPAL detector at LEP. No evidence for their existence was found From the analysis of charged-current decays of pair-produced unstable heavy leptons, and of charged-current and photonic decays of pair-produced excited leptons, lower limits on their masses are derived From the analysis of charged-current and photonic decays of singly-produced excited leptons, upper limits on the ratio of the coupling to the compositeness scale, f/Λ, are determined for masses up to the kinematic limit.

A search for the resonant production of high mass photon pairs associated with a leptonic or hadronic system has been performed using a total data sample of 25.7 pb^-1 taken at centre-of-mass energies between 130 GeV and 172 GeV with the OPAL detector at LEP. The observed number of events is consistent with the expected number from Standard Model processes. The observed candidates are combined with search results from √s ≈ M_z to place limits on B(H⁰ → γγ) within the Standard Model for Higgs boson masses up to 77 GeV, and on the production cross section of any scalar resonance decaying into di-photons. Upper limits on B(H⁰ → γγ) × σ(e⁺e^- → H⁰Z⁰) of 290 - 830 fb are obtained over 40 < M_H < 160 GeV. Higgs scalars which couple only to gauge bosons with Standard Model strength are ruled out up to a mass of 76.5 GeV at the 95% confidence level.

We present measurements of triple gauge boson coupling parameters using data recorded by the OPAL detector at LEP2 at a centre-of-mass energy of 172 GeV. A total of 120 W-pair candidates has been selected in the qq̄qq̄, qq̄ℓv̄_ℓ and ℓv̄_ℓℓ̄v_ℓ decay channels, for an integrated luminosity of 10.4 pb^-1. We use these data to determine several different anomalous coupling parameters using the measured cross-section and the distributions of kinematic variables. We measure α_Βφ=0.35 ^+1.29_-1.07 ± 0.38, α_Wφ=0.00^+0.30_-0.28 ± 0.11, α_W = 0.18^+0.49_-0.47 ± 0.23, Δg^z₁=-0.03^+0.40_-0.37 ± 0.14, Δκ^(H1SZ)_γ = 0.03^+0.55_-0.51 ± 0.20, and Δκ=0.03^+0.49_-0.46 ± 0.21. Combining the α_Wφ result with our previous result obtained from the 161 GeV data sample we measure α_Wφ=-0.08^+0.28_-0.25 ± 0.10. All of these measurements are consistent with the Standard Model.

A search for stable and long-lived massive particles of electric charge |Q/e| = 1 or 2/3, pair-produced in e⁺ e^- collisions at centre-of-mass energies from 130 to 183 GeV, is reported by the OPAL collaboration at LEP. No evidence for production of these particles was observed in a mass range between 45 and 89.5 GeV. Model-independent upper limits on the production cross-section between 0.05 and 0.19 pb have been derived for scalar and spin-1/2 particles with charge ±1. Within the framework of the minimal supersymmetric model (MSSM), this implies a lower limit of 82.5 (83.5) GeV on the mass of long-lived right- (left-)handed scalar muons and scalar taus. Long-lived charged leptons and charginos are excluded for masses below 89.5 GeV. For particles with charge ±2/3 the upper limits on the production cross-section vary between 0.05 and 0.2 pb. All limits, on masses and on cross-sections, are valid at the 95% confidence level for particles with lifetimes longer than 10^-6 s.

This paper describes a search for the Standard Model Higgs boson using data from e⁺e^- collisions collected at center-of-mass energies of 161, 170 and 172 GeV by the OPAL detector at LEP. The data collected at these energies correspond to integrated luminosities of 10.0, 1.0 and 9.4 pb^-1, respectively. The search is sensitive to the main final states from the process in which the Higgs boson is produced in association with a fermion anti-fermion pair, namely four jets, two jets with missing energy, and two jets produced together with a pair of electron, muon or tau leptons. One candidate event is observed, in agreement with the Standard Model background expectation. In combination with previous OPAL searches at center-of-mass energies close to the Z⁰ resonance and the revised previous OPAL searches at 161 GeV , we derive a lower limit of 69.4 GeV for the mass of the Standard Model Higgs boson at the 95% confidence level.

This paper describes the measurement of the W boson mass, M_W, and decay width, Γ_W, from the direct reconstruction of the invariant mass of its decay products in W pair events collected at a mean centre-of-mass energy of √s=172.12 GeV with the OPAL detector at LEP. Measurements of the W pair production cross-section, the W decay branching fractions and properties of the W decay final states are also described. A total of 120 candidate W⁺W^- events has been selected for an integrated luminosity of 10.36 pb^-1. The W⁺W^- production cross-section is measured to be σ_WW = 12.3 ± 1.3(stat.)± 0.3 (syst.) pb, consistent with the Standard Model expectation. The W⁺W^- → qqℓv_ℓ and W⁺W^- → qqqq final states are used to obtain a direct measurement of Γ_W = 1.30^+0.70_-0.55 (stat.) ± 0.18 (syst.) GeV. Assuming the Standard Model relation between M_W and Γ_W, the W boson mass is measured to be M_W = 80.32 ± 0.30(stat.) ± 0.09(syst.) GeV. The event properties of the fully-hadronic decays of W⁺W^- events are compared to those of the semileptonic decays. At the current level of precision there is no evidence for effects of colour reconnection in the observables studied. Combining data recorded by OPAL at √s ∼ 161-172 GeV, the W boson branching fraction to hadrons is determined to be 69.8^+3.0_-3.2 (stat.) ± 0.7 (syst.)%, consistent with the prediction of the Standard Model. The combined mass measurement from direct reconstruction and from the W⁺W^- production cross-sections measured at √s ∼ 161 and √s ∼ 172 GeV is M_W = 80.35 ± 0.24 (stat.) ± 0.07(syst.) GeV.

The production rates of D*^± mesons in charm and bottom events at centre-of-mass energies of about 91 GeV and the partial width of primary cc̄ pairs in hadronic Z⁰ decays have been measured at LEP using almost 4.4 million hadronic Z⁰ decays collected with the OPAL detector between 1990 and 1995. Using a combination of several charm quark tagging methods based on fully and partially reconstructed D*^± mesons, and a bottom tag based on identified muons and electrons, the hadronisation fractions of charm and bottom quarks into D*^± mesons have been found to be f (b → D*⁺X) = 0.173 ± 0.016 ± 0.012 and f (c → D*⁺X) = 0.222 ± 0.014 ± 0.014. The fraction of cc̄ events in hadronic Z⁰ decays, Γcc̄/Γ_had = Γ(Z⁰ → cc̄)/Γ(Z⁰ → hadrons), is determined to be Γ_cc̄/Γ_had = 0.180 ± 0.011 ± 0.012 ± 0.006. In all cases the first error is statistical, and the second one systematic. The last error quoted for Γ_cc̄/Γ_had is due to external branching ratios.

The process e⁺e^- → γγ(γ) is studied using data recorded with the OPAL detector at LEP. The data sample corresponds to a total integrated luminosity of 25.38 pb^-1 taken at centre-of-mass energies of 130-172 GeV. The measured cross-sections agree well with the expectation from QED. In a combined fit using data from all centre-of-mass energies, the angular distribution is used to obtain improved limits on the cut-off parameters: A₊ > 195 GeV and A_ > 210 GeV (95% CL). In addition, limits on non-standard e⁺e^-γ couplings and contact interactions, as well as a 95% CL mass limit for an excited electron, M_e* > 194 GeV for an e⁺e^-γ coupling κ = 1, are determined.

A search is described to detect charged Higgs bosons via the process e⁺e^-→H⁺H^-, using data collected by the OPAL detector at center-of-mass energies of 130-172 GeV with a total integrated luminosity of 25 pb^-1. The decay channels are assumed to be H⁺→qq̄ and H⁺-→ τ⁺v_τ. No evidence for charged Higgs boson production is observed. The lower limit for its mass is determined to be 52 GeV at 95% confidence level, independent of the H⁺→τ⁺v_τ branching ratio.

We report the first observation of Z/γ* production in Compton scattering of quasi-real photons. This is a subprocess of the reaction e⁺e^-→ e⁺e^-Z/γ*, where one of the final state electrons is undetected. Approximately 55pb^-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/γ* 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/γ* 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)eγ* 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.

Bose-Einstein Correlations (BEC) of three identical charged pions were studied in 4 × 10⁶ hadronic Z⁰ decays recorded with the OPAL detector at LEP. The genuine three-pion correlations, corrected for the Coulomb effect, were separated from the known two-pion correlations by a new subtraction procedure. A significant genuine three-pion BEC enhancement near threshold was observed having an emitter source radius of r₃ = 0.580 ± 0.004 (stat.) ± 0.029 (syst.) fm and a strength of λ₃ = 0.504 ± 0.010 (stat.) ± 0.041 (syst.). The Coulomb correction was found to increase the λ₃ value by ∼ 9% and to reduce r₃ by ∼ 6%. The measured λ₃ corresponds to a value of 0.707 ± 0.014 (stat.) ± 0.078 (syst.) when one takes into account the three-pion sample purity. A relation between the two-pion and the three-pion source parameters is discussed.

Experiments at the future Large Hadron Collider (LHC) at CERN will be faced with an extraordinary problem of event selection in real time. Algorithms at different levels of locality are needed, among them local feature extraction algorithms that are typical in many image processing tasks. We report here about implementations of several such local algorithms on a commercial modular image processing system, for a decision frequency of 100 kHz (''second-level''). Such devices can keep up with the required decision frequency for suitable algorithms, for some algorithms custom hardware additions would be necessary.

The fraction of {Mathematical expression}; events in hadronic Z⁰ decays has been measured using the data collected by OPAL in 1992 and 1993. The presence of electrons or muons from semileptonic decays of bottom hadrons and the detection of bottom hadron decay vertices were used together to obtain an event sample enriched in {Mathematical expression} decays. To reduce the systematic error on the measurement of the {Mathematical expression} fraction, the efficiency of the {Mathematical expression} event tagging was obtained from the data by comparing the numbers of events having a bottom signature in either one or both thrust hemispheres. A value of {Mathematical expression} was obtained, where the first error is statistical and the second systematic. The uncertainty on the decay width {Mathematical expression} is not included in these errors. A fractional variation of this width by ±8% about its Standard Model prediction would result in a variation of the measured {Mathematical expression} fraction of ±0.0015.

The first level muon trigger in the endcap region of the ATLAS experiment requires the use of detectors with well defined high segmentation and good time resolution, to cope with high backgrounds at large rapidity. Thin gap chambers operating in a high gain mode have been proposed for this task due to its anode readout capability, that permits a well defined segmentation down to the two wire separation, combined with its good timing properties. An optimization of such a device for timing application has been performed using large (1.22 × 0.74 m²) detectors under different operating conditions. A time resolution better than 3.7 ns has been achieved for a single layer, showing no deterioration under background rates at as high as 170 kHz/cm².

New detectors for fast, real-time, high resolution X-ray imaging at high photon fluxes are described. A thin solid photoconverter is coupled to a multistage gaseous electron multiplier operating at low pressure. The readout electronics connected to the wire electrodes of the chamber provides two-dimensional localization of single registered photons. Prototype detectors were tested with Csl, Ag and Ta converters in the photon energy range of 8-60 keV. Radiographic digital images are presented compared to that of X-ray films. It is shown that compared to the film technique the secondary electron emission detector provides radiographic images of an equivalent contrast at an order of magnitude lower exposure. This novel type of detector is suited for static and dynamic in-line quality control on industrial production lines and for medical imaging.

We report on an experimental study of the properties of a modular lead/scintillating-fiber calorimeter with a fully projective tower geometry. Although the calorimeter structure is monolythic, an effective segmentation into an electromagnetic and an hadronic section is achieved by separating the readout of the fibers running throughout the calorimeter from those starting beyond a depth of ∼ 25 radiation lengths. This feature is used for e/π separation. Discontinuities in the sampling fraction near the boundaries of the modules cause a signal nonuniformity for electrons, which can be corrected. Similar effects observed for high energy hadrons are discussed. Results are given on the energy and position resolution for electrons, on the signal linearity and on the absolute light yield. Pion showers were only partially contained in this detector. The information from a backing calorimeter, consisting of fast thin-gap wire chambers interleaved by iron slabs, significantly improves the energy resolution of the incompletely contained pion showers.

A new class of X-ray imaging gaseous detectors was developed, based on photon conversion in a thin film and the detection of the emitted secondary electrons with a low-pressure avalanche wire chamber. Their main features are parallax-free imaging with a submillimeter spatial resolution over a broad range of X-ray energies, a nanosecond response and a counting rate capability above 1 MHz/mm². The imaging characteristics of a large area secondary electron emission (SEE) detector equipped with CsI, Ag and Ta photoconvertors, in the photon energy range of 6-60 keV are summarized. In localization capability this detector can successfully compete with commercial image intensifier systems and medical film-screen combinations.

A sample of 2610 electron candidates and 2762 muon candidates identified in hadronic Z⁰ decays has been used to measure the average b hadron lifetime. These data were recorded with the OPAL detector during 1990 and 1991. Maximum likelihood fits to the distributions of the lepton impact parameters yield an average b hadron lifetime of {Mathematical expression}, where the first error is statistical and the second systematic. This result is a weighted average over the semileptonic branching fractions and production rates of the b hadrons produced in Z⁰ decays.

Thin multiwire chambers, operating in a high gain mode, are used for electromagnetic presampling in the endcaps of the OPAL detector at LEP. The design, setup and performance of these chambers with respect to energy and space resolution for electromagnetic showers as well as to pion/electron separation is described.

A new type of thin gap multiwire gas detector operating in a high gain mode has been developed for use in the OPAL pole tip calorimeter. The detector thickness is only 6.6 mm and its area is 0.61 m². The induced pad readout provides high output pulses which require no amplification. The set-up for the detector mass production and quality control test is described. Results from a test beam set-up that simulates the OPAL pole tip calorimeter are presented and compared with computer simulations.