Publications

Event reconstruction at the LHC, the task of assigning observed physics objects to their true origins, is a central challenge for precision measurements and searches. Many existing machine learning approaches address this problem but rely on a single event topology, restricting their applicability to realistic analyses where multiple signal and background processes with different structures are present. To overcome this, we present TIGER, a novel hierarchical graph network that is fundamentally topology agnostic. By incorporating only the common underlying structure of sequential two-body decays, our model can reconstruct complex events without process-specific assumptions. This flexible architecture supports multitask learning, enabling simultaneous event reconstruction and classification. TIGER thus provides a powerful and generalizable tool for physics analysis at the LHC.

We extend the particle-flow neural assisted simulations (Parnassus) framework of fast simulation and reconstruction to entire collider events. In particular, we use two generative artificial intelligence tools, continuous normalizing flows and diffusion models, to create a set of reconstructed particle-flow objects conditioned on truth-level particles from CMS Open Simulations. While previous work focused on jets, our updated methods now can accommodate all particle-flow objects in an event along with particle-level attributes like particle type and production vertex coordinates. This approach is fully automated, entirely written in Python, and GPU-compatible. Using a variety of physics processes at the LHC, we show that the extended Parnassus is able to generalize beyond the training dataset and outperforms the standard, public tool Delphes.

We extend the re-simulation-based self-supervised learning approach to learning representations of hadronic jets in colliders by exploiting the Markov property of the standard simulation chain. Instead of masking, cropping, or other forms of data augmentation, this approach simulates pairs of events where the initial portion of the simulation is shared, but the subsequent stages of the simulation evolve independently. When paired with a contrastive loss function, this naturally leads to representations that capture the physics in the initial stages of the simulation. In particular, we force the hard scattering and parton shower to be shared and let the hadronization and interaction with the detector evolve independently. We then evaluate the utility of these representations on downstream tasks.

We present the results of the 'Fast Calorimeter Simulation Challenge 2022'-the CaloChallenge. We study state-of-the-art generative models on four calorimeter shower datasets of increasing dimensionality, ranging from a few hundred voxels to a few tens of thousand voxels. The 31 individual submissions span a wide range of current popular generative architectures, including variational autoencoders (VAEs), generative adversarial networks (GANs), normalizing flows, diffusion models, and models based on conditional flow matching. We compare all submissions in terms of quality of generated calorimeter showers, as well as shower generation time and model size. To assess the quality we use a broad range of different metrics including differences in one-dimensional histograms of observables, KPD/FPD scores, AUCs of binary classifiers, and the log-posterior of a multiclass classifier. The results of the CaloChallenge provide the most complete and comprehensive survey of cutting-edge approaches to calorimeter fast simulation to date. In addition, our work provides a uniquely detailed perspective on the important problem of how to evaluate generative models. As such, the results presented here should be applicable for other domains that use generative AI and require fast and faithful generation of samples in a large phase space.Report Numbers: HEPHY-ML-24-05, FERMILAB-PUB-24-0728-CMS, TTK-24-43.

In high energy physics, the ability to reconstruct particles based on their detector signatures is essential for downstream data analyses. A particle reconstruction algorithm based on learning hypergraphs (HGPflow) has previously been explored in the context of single jets. In this paper, we expand the scope to full protonproton and electronpositron collision events and study reconstruction quality using metrics at the particle, jet, and event levels. Instead of passing entire events through HGPflow, we train it on smaller partitions for scalability and to avoid potential bias from long-range correlations related to the physics process. We demonstrate that this approach is feasible and that on most metrics, HGPflow outperforms both traditional particle flow algorithms and a machine learning-based benchmark model.

In the context of a gas-sampling Digital Hadronic Calorimeter (DHCAL), we explore the potential of using (GNN) for hadron energy reconstruction and Graph Neural Networks (PID) in future collider experiments. For Particle Identification (PID), we achieved classification efficiencies exceeding 50% for neutrons and pions, with notably higher efficiencies for kaons and protons. Protons exhibited the highest efficiency of 77%, followed by neutral kaons. The energy resolution for these hadrons is studied in the energy range of 1-50 GeV, with a further investigation into the resolution as a function of the incoming particle's angle and readout granularity, focusing on charged pions. Compared to traditional analysis methods, our results indicate that improved performance can be achieved even with coarser detector granularity, potentially making future (DHCAL) systems more cost-effective.

Precision measurement of hadronic final states presents complex experimental challenges. The study explores the concept of a gaseous Digital Hadronic Calorimeter (DHCAL) and discusses the potential benefits of employing Graph Neural Network (GNN) methods for future collider experiments. In particular, we use GNN to describe calorimeter clusters as point clouds or a collection of data points representing a three-dimensional object in space. Combined with Graph Attention Transformers (GATs) and DeepSets algorithms, this results in an improvement over existing baseline techniques for particle identification and energy resolution. We discuss the challenges encountered in implementing GNN methods for energy measurement in digital calorimeters, e.g., the large variety of hadronic shower shapes and the hyper-parameter optimization. We also discuss the dependency of the measured performance on the angle of the incoming particle and on the detector granularity. Finally, we highlight potential future directions and applications of these techniques.

In high-energy physics, particles produced in collision events decay in a format of a hierarchical tree structure, where only the final decay products can be observed using detectors. However, the large combinatorial space of possible tree structures makes it challenging to recover the actual decay process given a set of final particles. To better analyse the hierarchical tree structure, we propose a graph-based deep learning model to infer the tree structure to reconstruct collision events. In particular, we use a compact matrix representation termed as lowest common ancestor generations matrix, to encode the particle decay tree structure. Then, we introduce a perturbative augmentation technique applied to node features, aiming to mimic experimental uncertainties and increase data diversity. We further propose a supervised graph contrastive learning algorithm to utilize the information of inter-particle relations from multiple decay processes. Extensive experiments show that our proposed supervised graph contrastive learning with perturbative augmentation method outperforms state-of-the-art baseline models on an existing physics-based dataset, significantly improving the reconstruction accuracy. This method provides a more effective training strategy for models with the same parameters and makes way for more accurate and efficient high-energy particle physics data analysis.

The reconstruction of particle tracks from hits in tracking detectors is a computationally intensive task due to the large combinatorics of detector signals. Recent efforts have proven that ML techniques can be successfully applied to the tracking problem, extending and improving the conventional methods based on feature engineering. However, complex models can be challenging to implement on heterogeneous trigger systems, integrating architectures such as field programmable gate arrays (FPGAs). Deploying the network on an FPGA is feasible but challenging and limited by its resources. An efficient alternative can employ symbolic regression (SR). We propose a novel approach that uses SR to replace a graph-based neural network. Substituting each network block with a symbolic function preserves the graph structure of the data and enables message passing. The technique is perfectly suitable for heterogeneous hardware, as it can be implemented more easily on FPGAs and grants faster execution times on CPU with respect to conventional methods. While the tracking problem is the target for this work, it also provides a proof-of-principle for the method that can be applied to many use cases.

Detector simulation and reconstruction are a significant computational bottleneck in particle physics. We develop particle-flow neural-assisted simulations (parnassus) to address this challenge. Our deep learning model takes as input a point cloud (particles impinging on a detector) and produces a point cloud (reconstructed particles). By combining detector simulations and reconstruction into one step, we aim to minimize resource utilization and enable fast surrogate models suitable for application both inside and outside large collaborations. We demonstrate this approach using a publicly available dataset of jets passed through the full simulation and reconstruction pipeline of the Compact Muon Solenoid (CMS) experiment. We show that parnassus accurately mimics the CMS particle flow algorithm on the (statistically) same events it was trained on and can generalize to jet momentum and type outside of the training distribution.

The computational intensity of detector simulation and event reconstruction poses a significant difficulty for data analysis in collider experiments. This challenge inspires the continued development of machine learning techniques to serve as efficient surrogate models. We propose a fast emulation approach that combines simulation and reconstruction. In other words, a neural network generates a set of reconstructed objects conditioned on input particle sets. To make this possible, we advance set-conditional set generation with diffusion models. Using a realistic, generic, and public detector simulation and reconstruction package (COCOA), we show how diffusion models can accurately model the complex spectrum of reconstructed particles inside jets.

Denoising diffusion models have gained prominence in various generative tasks, prompting their exploration for the generation of calorimeter responses. Given the computational challenges posed by detector simulations in high-energy physics experiments, the necessity to explore new machine-learning-based approaches is evident. This study introduces a novel graph-based diffusion model designed specifically for rapid calorimeter simulations. The methodology is particularly well-suited for low-granularity detectors featuring irregular geometries. We apply this model to the ATLAS dataset published in the context of the Fast Calorimeter Simulation Challenge 2022, marking the first application of a graph diffusion model in the field of particle physics.

Accurately reconstructing particles from detector data is a critical challenge in experimental particle physics, where the spatial resolution of calorimeters has a crucial impact. This study explores the integration of super-resolution techniques into an LHC-like reconstruction pipeline to effectively enhance the granularity of calorimeter data and suppress noise. We find that this software preprocessing step can significantly improve reconstruction quality without physical changes to detectors. To demonstrate the impact of our approach, we propose a novel particle flow model that offers enhanced particle reconstruction quality and interpretability. These advancements underline the potential of super-resolution to impact both current and future particle physics experiments.

Searches for new resonances are performed using an unsupervised anomaly-detection technique. Events with at least one electron or muon are selected from 140 fb⁻¹ of pp collisions at √s ¼ 13 TeV 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 simulation of particle physics data is a fundamental but computationally intensive ingredient for physics analysis at the large Hadron collider, where observational set-valued data is generated conditional on a set of incoming particles. To accelerate this task, we present a novel generative model based on a graph neural network and slot-attention components, which exceeds the performance of pre-existing baselines.

A configurable calorimeter simulation for AI (CoCoA) applications is presented, based on the Geant4 toolkit and interfaced with the Pythia event generator. This open-source project is aimed to support the development of machine learning algorithms in high energy physics that rely on realistic particle shower descriptions, such as reconstruction, fast simulation, and low-level analysis. Specifications such as the granularity and material of its nearly hermetic geometry are user-configurable. The tool is supplemented with simple event processing including topological clustering, jet algorithms, and a nearest-neighbors graph construction. Formatting is also provided to visualise events using the Phoenix event display software.

This paper presents direct searches for lepton flavour violation in Higgs boson decays, H → eτ and H → μτ, performed using data collected with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy s = 13 TeV, corresponding to an integrated luminosity of 138 fb ⁻¹. Leptonic (τ → ℓν_ℓ ν_τ) and hadronic (τ → hadrons ν_τ) decays of the τ-lepton are considered. Two background estimation techniques are employed: the MC-template method, based on data-corrected simulation samples, and the Symmetry method, based on exploiting the symmetry between electrons and muons in the Standard Model backgrounds. No significant excess of events is observed and the results are interpreted as upper limits on lepton-flavour-violating branching ratios of the Higgs boson. The observed (expected) upper limits set on the branching ratios at 95% confidence level, B (H → eτ) < 0.20% (0.12%) and B (H → μτ) < 0.18% (0.09%), are obtained with the MC-template method from a simultaneous measurement of potential H → eτ and H → μτ signals. The best-fit branching ratio difference, B (H → μτ) → B (H → eτ), measured with the Symmetry method in the channel where the τ-lepton decays to leptons, is (0.25 ± 0.10)%, compatible with a value of zero within 2.5σ. [Figure not available: see fulltext.].

The task of reconstructing particles from low-level detector response data to predict the set of final state particles in collision events represents a set-to-set prediction task requiring the use of multiple features and their correlations in the input data. We deploy three separate set-to-set neural network architectures to reconstruct particles in events containing a single jet in a fully-simulated calorimeter. Performance is evaluated in terms of particle reconstruction quality, properties regression, and jet-level metrics. The results demonstrate that such a high-dimensional end-to-end approach succeeds in surpassing basic parametric approaches in disentangling individual neutral particles inside of jets and optimizing the use of complementary detector information. In particular, the performance comparison favors a novel architecture based on learning hypergraph structure, HGPflow, which benefits from a physically-interpretable approach to particle reconstruction.

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 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. [Figure not available: see fulltext.].

First-principle simulations are at the heart of the high-energy physics research program. They link the vast data output of multi-purpose detectors with fundamental theory predictions and interpretation. This review illustrates a wide range of applications of modern machine learning to event generation and simulation-based inference, including conceptional developments driven by the specific requirements of particle physics. New ideas and tools developed at the interface of particle physics and machine learning will improve the speed and precision of forward simulations, handle the complexity of collision data, and enhance inference as an inverse simulation problem.

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.

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 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 mX in the range 251 GeV≤mX≤1000 GeV. The observed (expected) limits on the cross section for pp→X→HH range from 640 fb to 44 fb (391 fb to 46 fb) over the considered mass range.

Several observables sensitive to the fragmentation of b quarks into b hadrons are measured using 36 fb-1 of s=13 TeV proton-proton collision data collected with the ATLAS detector at the LHC. Jets containing b hadrons are obtained from a sample of dileptonic tt¯ 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∗.

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.

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 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 [InlineEquation not available: see fulltext.] 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 [InlineEquation not available: see fulltext.], corresponding to an observed (expected) constraint on the charm Yukawa coupling modifier |κc|

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. [Figure not available: see fulltext.].

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.

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. [Figure not available: see fulltext.]

During LHC Run 2 (20152018) the ATLAS Level-1 topological trigger allowed efficient data-taking by the ATLAS experiment at luminosities up to 2.1× 10³⁴ cm^{- 2}s^{- 1}, which exceeds the design value by a factor of two. The system was installed in 2016 and operated in 2017 and 2018. It uses Field Programmable Gate Array processors to select interesting events by placing kinematic and angular requirements on electromagnetic clusters, jets, τ-leptons, muons and the missing transverse energy. It allowed to significantly improve the background event rejection and signal event acceptance, in particular for Higgs and B-physics processes.

Multidimensional efficiency maps are commonly used in high-energy physics experiments to mitigate the limitations in the generation of large samples of simulated events. Binned efficiency maps are however strongly limited by statistics. We propose a neural network approach to learn ratios of local densities to estimate in an optimal fashion efficiencies as a function of a set of parameters. Graph neural network techniques are used to account for the high dimensional correlations between different physics objects in the event. We show in a specific toy model how this method is applicable to produce accurate multidimensional efficiency maps for heavy-flavor tagging classifiers in HEP experiments, including for processes on which it was not trained.

Jet classification is an important ingredient in measurements and searches for new physics at particle colliders, and secondary vertex reconstruction is a key intermediate step in building powerful jet classifiers. We use a neural network to perform vertex finding inside jets in order to improve the classification performance, with a focus on separation of bottom vs. charm flavor tagging. We implement a novel, universal set-to-graph model, which takes into account information from all tracks in a jet to determine if pairs of tracks originated from a common vertex. We explore different performance metrics and find our method to outperform traditional approaches in accurate secondary vertex reconstruction. We also find that improved vertex finding leads to a significant improvement in jet classification performance.

In High Energy Physics experiments Particle Flow (PFlow) algorithms are designed to provide an optimal reconstruction of the nature and kinematic properties of the particles produced within the detector acceptance during collisions. At the heart of PFlow algorithms is the ability to distinguish the calorimeter energy deposits of neutral particles from those of charged particles, using the complementary measurements of charged particle tracking devices, to provide a superior measurement of the particle content and kinematics. In this paper, a computer vision approach to this fundamental aspect of PFlow algorithms, based on calorimeter images, is proposed. A comparative study of the state of the art deep learning techniques is performed. A significantly improved reconstruction of the neutral particle calorimeter energy deposits is obtained in a context of large overlaps with the deposits from charged particles. Calorimeter images with augmented finer granularity are also obtained using super-resolution techniques.

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 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 2.0σ (1.7σ). The observed upper limit on the cross section times branching ratio for pp→H→μμ is 2.2 times the SM prediction at 95% confidence level, while the expected limit on a H→μμ 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 μ=1.2±0.6.

The integrated fiducial cross-section and unfolded differential jet mass spectrum of high transverse momentum Z→bb\u203e decays are measured in Zγ events in protonproton collisions at s=13TeV. 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 36.1fb⁻¹. Photons are required to have a transverse momentum p_T>175GeV. The Z→bb\u203e decay is reconstructed using a jet with p_T>200GeV, 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 protonproton collisions. Two different but related measurements are performed using two jet grooming definitions for reconstructing the Z→bb\u203e decay: trimming and soft drop. These algorithms differ in their experimental and phenomenological implications regarding jet mass reconstruction and theoretical precision. To identify Z bosons, 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.]

Many problems in machine learning can be cast as learning functions from sets to graphs, or more generally to hypergraphs; in short, Set2Graph functions. Examples include clustering, learning vertex and edge features on graphs, and learning features on triplets in a collection. A natural approach for building Set2Graph models is to characterize all linear equivariant set-to-hypergraph layers and stack them with non-linear activations. This poses two challenges: (i) the expressive power of these networks is not well understood; and (ii) these models would suffer from high, often intractable computational and memory complexity, as their dimension grows exponentially. This paper advocates a family of neural network models for learning Set2Graph functions that is both practical and of maximal expressive power (universal), that is, can approximate arbitrary continuous Set2Graph functions over compact sets. Testing these models on different machine learning tasks, mainly an application to particle physics, we find them favorable to existing baselines.

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.

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.].

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 20.3 fb ^{- 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) < 1.43 % , and Br(Z→ μτ) < 1.69 × 10 ^{- 5}.

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.

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) · 10^-4 (kg·day·keV)^-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) (t·y)^-1 from radiogenic neutrons, (1.8 ± 0.3) · 10^-2 (t·y)^-1 from coherent scattering of neutrinos, and less than 0.01 (t·y)^-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 _eff, 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 · 10^-47 cm² at m_χ = 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.

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.

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

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.

The jet energy scale (JES) and its systematic uncertainty are determined for jets measured with the ATLAS detector using protonproton collision data with a centre-ofmass energy of √s = 7 TeV corresponding to an integrated luminosity of 4.7 fb⁻¹. Jets are reconstructed from energy deposits forming topological clusters of calorimeter cells using the anti-k_t algorithm with distance parameters R = 0.4 or R = 0.6, and are calibrated using MC simulations. A residual JES correction is applied to account for differences between data and MC simulations. This correction and its systematic uncertainty are estimated using a combination of in situ techniques exploiting the transverse momentum balance between a jet and a For central jets at lower pT a Z boson, for 20 ≤ p^jet_T < 1000 GeV and pseudorapidities |η| < 4.5. The effect of multiple protonproton interactions is corrected for, and an uncertainty is evaluated using in situ techniques. The smallest JES uncertainty of less than 1 % is found in the central calorimeter region (|η| < 1.2) for jets with 55 ≤ p^jet_T < 500 GeV. For central jets at lower p_T, the uncertainty is about 3 %. A consistent JES estimate is found using measurements of the calorimeter response of single hadrons in protonproton collisions and test-beam data, which also provide the estimate for p^jet_T > 1 TeV. The calibration of forward jets is derived from dijet p_T balance measurements. The resulting uncertainty reaches its largest value of 6 % for low-p_T jets at |η| =4.5. Additional JES uncertainties due to specific event topologies, such as close-by jets or selections of event samples with an enhanced content of jets originating from light quarks or gluons, are also discussed. The magnitude of these uncertainties depends on the event sample used in a given physics analysis, but typically amounts to 0.53 %.

This paper presents a measurement of the cross-section for high transverse momentum W and Z bosons produced in pp collisions and decaying to all-hadronic final states. The data used in the analysis were recorded by the ATLAS detector at the CERN Large Hadron Collider at a centre-of-mass energy of and correspond to an integrated luminosity of . The measurement is performed by reconstructing the boosted W or Z bosons in single jets. The reconstructed jet mass is used to identify the W and Z bosons, and a jet substructure method based on energy cluster information in the jet centre-of-mass frame is used to suppress the large multi-jet background. The cross-section for events with a hadronically decaying W or Z boson, with transverse momentum and pseudorapidity , is measured to be pb and is compared to next-to-leading-order calculations. The selected events are further used to study jet grooming techniques.

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^-1of proton-proton collisions at √s = 8TeV 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-tolightest neutralinos, both decaying into the lightest neutralino via an intermediate tau slepton, masses up to 410 GeV are excluded for a massless lightest neutralino.

Measurements of normalized differential cross sections for top-quark pair production are presented as a function of the top-quark transverse momentum, and of the mass, transverse momentum, and rapidity of the t¯t system, in proton-proton collisions at a center-of-mass energy of √s = 7 TeV. The data set corresponds to an integrated luminosity of 4.6 fb^-1, recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton + jets channel, requiring exactly one lepton and at least four jets with at least one of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector efficiency and resolution effects and are compared to several Monte Carlo simulations and theory calculations. The results are in fair agreement with the predictions in a wide kinematic range. Nevertheless, data distributions are softer than predicted for higher values of the mass of the t¯t system and of the top-quark transverse momentum. The measurements can also discriminate among different sets of parton distribution functions.

We present the first results of searches for axions and axionlike particles with the XENON100 experiment. The axion-electron coupling constant, gAe, has been probed by exploiting the axioelectric effect in liquid xenon. A profile likelihood analysis of 224.6 live days × 34-kg exposure has shown no evidence for a signal. By rejecting gAe larger than 7.7×10-12 (90% C.L.) in the solar axion search, we set the best limit to date on this coupling. In the frame of the DFSZ and KSVZ models, we exclude QCD axions heavier than 0.3 and 80eV/c2, respectively. For axionlike particles, under the assumption that they constitute the whole abundance of dark matter in our galaxy, we constrain gAe to be lower than 1×10-12 (90% C.L.) for masses between 5 and 10keV/c2.

Results from a search for supersymmetry in events with four or more leptons including electrons, muons and taus are presented. The analysis uses a data sample corresponding to 20.3 fb-1 of proton-proton collisions delivered by the Large Hadron Collider at s=8 TeV and recorded by the ATLAS detector. Signal regions are designed to target supersymmetric scenarios that can be either enriched in or depleted of events involving the production of a Z boson. No significant deviations are observed in data from standard model predictions and results are used to set upper limits on the event yields from processes beyond the standard model. Exclusion limits at the 95% confidence level on the masses of relevant supersymmetric particles are obtained. In R-parity-violating simplified models with decays of the lightest supersymmetric particle to electrons and muons, limits of 1350 and 750 GeV are placed on gluino and chargino masses, respectively. In R-parity-conserving simplified models with heavy neutralinos decaying to a massless lightest supersymmetric particle, heavy neutralino masses up to 620 GeV are excluded. Limits are also placed on other supersymmetric scenarios.

A search for the direct production of charginos and neutralinos in final states with three leptons and missing transverse momentum is presented. The analysis is based on 20.3 fb^-1 of vs = 8TeV proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations and limits are set in R-parity-conserving phenomenological Minimal Supersymmetric Standard Models and in simplified supersymmetric models, significantly extending previous results. For simplified supersymmetric models of direct chargino (∼X±₁) and next-to-lightest neutralino (X∼⁰ ₂) production with decays to lightest neutralino (∼X⁰ ₁) via either all three generations of sleptons, staus only, gauge bosons, or Higgs bosons, ∼X± ₁ and ∼X⁰ ₂ masses are excluded up to 700GeV, 380GeV, 345GeV, or 148GeV respectively, for a massless ∼X⁰ ₁.

The XENON100 dark matter experiment uses liquid xenon in a time projection chamber (TPC) to measure xenon nuclear recoils resulting from the scattering of dark matter weakly interacting massive particles (WIMPs). In this paper, we report the observation of single-electron charge signals which are not related to WIMP interactions. These signals, which show the excellent sensitivity of the detector to small charge signals, are explained as being due to the photoionization of impurities in the liquid xenon and of the metal components inside the TPC. They are used as a unique calibration source to characterize the detector. We explain how we can infer crucial parameters for the XENON100 experiment: the secondary-scintillation gain, the extraction yield from the liquid to the gas phase and the electron drift velocity.

The XENON100 experiment, situated in the Laboratori Nazionali del Gran Sasso, aims at the direct detection of dark matter in the form of weakly interacting massive particles (WIMPs), based on their interactions with xenon nuclei in an ultra low background dual-phase time projection chamber. This paper describes the general methods developed for the analysis of the XENON100 data. These methods have been used in the 100.9 and 224.6 live days science runs from which results on spin-independent elastic, spin-dependent elastic and inelastic WIMP-nucleon cross-sections have already been reported.

This paper presents a measurement of the top quark pair (tt̄) production charge asymmetry AC using 4.7 fb^-1 of proton-proton collisions at a centre-of-mass energy √s = 7TeV collected by the ATLAS detector at the LHC. A tt̄-enriched sample of events with a single lepton (electron or muon), missing transverse momentum and at least four high transverse momentum jets, of which at least one is tagged as coming from a b-quark, is selected. A likelihood fit is used to reconstruct the tt̄ event kinematics. A Bayesian unfolding procedure is employed to estimate AC at the parton-level. The measured value of the tt̄ production charge asymmetry is AC = 0.006 ± 0.010, where the uncertainty includes both the statistical and the systematic components. Differential AC measurements as a function of the invariant mass, the rapidity and the transverse momentum of the tt̄- system are also presented. In addition, AC is measured for a subset of events with large tt̄ velocity, where physics beyond the Standard Model could contribute. All measurements are consistent with the Standard Model predictions.

TheXENON100 experiment, installed underground at the LaboratoriNazionali del Gran Sasso, aims to directly detect dark matter in the form of weakly interacting massive particles (WIMPs) via their elastic scattering off xenon nuclei. This paper presents a study on the nuclear recoil background of the experiment, taking into account neutron backgrounds from (?, n) reactions and spontaneous fission due to natural radioactivity in the detector and shield materials, as well as muon-induced neutrons.Based on MonteCarlo simulations and using measured radioactive contaminations of all detector components, we predict the nuclear recoil backgrounds for the WIMP search results published by theXENON100 experiment in 2011 and 2012, 0.11_-0.04^+0.08 events and 0.17_-0.07^+0.12 events, respectively, and conclude that they do not limit the sensitivity of the experiment.

We present new experimental constraints on the elastic, spin-dependent WIMP-nucleon cross section using recent data from the XENON100 experiment, operated in the Laboratori Nazionali del Gran Sasso in Italy. An analysis of 224.6 live days×34 kg of exposure acquired during 2011 and 2012 revealed no excess signal due to axial-vector WIMP interactions with Xe129 and Xe131 nuclei. This leads to the most stringent upper limits on WIMP-neutron cross sections for WIMP masses above 6 GeV/c2, with a minimum cross section of 3.5×10^-40 cm2 at a WIMP mass of 45 GeV/c2, at 90% confidence level.

Results from the nuclear recoil calibration of the XENON100 dark matter detector installed underground at the Laboratori Nazionali del Gran Sasso, Italy are presented. Data from measurements with an external AmBe241 neutron source are compared with a detailed Monte Carlo simulation which is used to extract the energy-dependent charge-yield Q_y and relative scintillation efficiency L_eff. A very good level of absolute spectral matching is achieved in both observable signal channels - scintillation S1 and ionization S2 - along with agreement in the two-dimensional particle discrimination space. The results confirm the validity of the derived signal acceptance in earlier reported dark matter searches of the XENON100 experiment.

A search for long-lived particles is performed using a data sample of 4.7 fb⁻¹ from protonproton 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 tanβ=520. 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.

A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb⁻¹ of s=7 TeV protonproton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results.

Measurements are presented of differential crosss√ ections for top quark pair production in pp collisions at s = 7 TeV relative to the total inclusive top quark pair production cross-section. A data sample of 2.05 fb⁻¹ recorded by the ATLAS detector at the Large Hadron Collider is used. Relative differential cross-sections are derived as a function of the invariant mass, the transverse momentum and the rapidity of the top quark pair system. Events are selected in the lepton (electron or muon) + jets channel. The backgroundsubtracted differential distributions are corrected for detector effects, normalized to the total inclusive top quark pair production cross-section and compared to theoretical predictions. The measurement uncertainties range typically between 10 % and 20 % and are generally dominated by systematic effects. No significant deviations from the Standard Model expectations are observed.

A search for the electroweak pair production of charged sleptons and weak gauginos decaying into final states with two leptons is performed using 4.7 fb^-1of proton-proton collision data at √s = 7 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excesses are observed with respect to the prediction from Standard Model processes. In the scenario of direct slepton production, if the sleptons decay directly into the lightest neutralino, left-handed slepton masses between 85 and 195 GeV are excluded at 95% confidence level for a 20 GeV neutralino. Chargino masses between 110 and 340 GeV are excluded in the scenario of direct production of wino-like chargino pairs decaying into the lightest neutralino via an intermediate on-shell charged slepton for a 10 GeV neutralino. The results are also interpreted in the framework of the phenomenological minimal supersymmetric Standard Model.

A search for diphoton events with large missing transverse momentum has been performed using proton-proton collision data at s=7 TeV recorded with the ATLAS detector, corresponding to an integrated luminosity of 4.8 fb^-1. No excess of events was observed above the Standard Model prediction and model-dependent 95% confidence level exclusion limits are set. In the context of a generalised model of gauge-mediated supersymmetry breaking with a bino-like lightest neutralino of mass above 50 GeV, gluinos (squarks) below 1.07 TeV (0.87 TeV) are excluded, while a breaking scale Λ below 196 TeV is excluded for a minimal model of gauge-mediated supersymmetry breaking. For a specific model with one universal extra dimension, compactification scales 1/R

The XENON100 experiment, in operation at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy, was designed to search for evidence of dark matter interactions inside a volume of liquid xenon using a dual-phase time projection chamber. This paper describes the Slow Control System (SCS) of the experiment with emphasis on the distributed architecture as well as on its modular and expandable nature. The system software was designed according to the rules of Object-Oriented Programming and coded in Java, thus promoting code reusability and maximum flexibility during commissioning of the experiment. The SCS has been continuously monitoring the XENON100 detector since mid 2008, remotely recording hundreds of parameters on a few dozen instruments in real time, and setting emergency alarms for the most important variables.

A measurement of event shape variables is presented for large momentum transfer proton-proton collisions using the ATLAS detector at the Large Hadron Collider. Six event shape variables calculated using hadronic jets are studied in inclusive multi-jet events in 35 pb^-1 of integrated luminosity at a center-of-mass energy of √s = 7 TeV. These measurements are compared to predictions by three Monte Carlo event generators containing leading-logarithmic parton showers matched to leading order matrix elements for 2→2 and 2→n (n = 2, , 6) scattering. Measurements of the third-jet resolution parameter, aplanarity, thrust, sphericity, and transverse sphericity are generally well described. The mean value of each event shape variable is evaluated as a function of the average momentum of the two leading jets p_T,1 and p_T,2, with a mean pT approaching 1 TeV.

We report on a search for particle dark matter with the XENON100 experiment, operated at the Laboratori Nazionali del Gran Sasso for 13 months during 2011 and 2012. XENON100 features an ultralow electromagnetic background of (5.3±0.6)×10 ^-3events/(keV _ee× kg×day) in the energy region of interest. A blind analysis of 224.6live days×34kg exposure has yielded no evidence for dark matter interactions. The two candidate events observed in the predefined nuclear recoil energy range of 6.6-30.5keV _nr are consistent with the background expectation of (1.0±0.2) events. A profile likelihood analysis using a 6.6-43.3keV _nr energy range sets the most stringent limit on the spin-independent elastic weakly interacting massive particle-nucleon scattering cross section for weakly interacting massive particle masses above 8GeV/c2, with a minimum of 2×10 ^-45cm2 at 55GeV/c2 and 90% confidence level.

The ATLAS detector at the Large Hadron Collider is used to search for highmass resonances decaying to an electron-positron pair or a muon-antimuon pair. The search is sensitive to heavy neutral Z gauge bosons, Randall-Sundrum gravitons, Z* bosons, techni-mesons, Kaluza-Klein Z/γ bosons, and bosons predicted by Torsion models. Results are presented based on an analysis of pp collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 4.9 fb^-1 in the e ⁺e^- channel and 5.0 fb^-1 in the μ⁺μ^- channel. A Z boson with Standard Model-like couplings is excluded at 95% confidence level for masses below 2.22 TeV. A Randall-Sundrum graviton with coupling k /M̄_Pl = 0.1 is excluded at 95% confidence level for masses below 2.16 TeV. Limits on the other models are also presented, including Technicolor and Minimal Z Models. Copyright CERN.

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 diphoton events with large missing transverse momentum has been performed using 1.07 fb^-1 of proton-proton collision data at s=7 TeV recorded with the ATLAS detector. No excess of events was observed above the Standard Model prediction and 95% Confidence Level (CL) upper limits are set on the production cross section for new physics. The limits depend on each model parameter space and vary as follows: Λ

Pseudorapidity gap distributions in proton-proton collisions at √s = 7 TeV are studied using a minimum bias data sample with an integrated luminosity of 7.1 μb⁻¹. Cross sections are measured differentially in terms of Δη^F, the larger of the pseudorapidity regions extending to the limits of the ATLAS sensitivity, at η = ±4.9, in which no final state particles are produced above a transverse momentum threshold p^cut _T. The measurements span the region 0 < Δη^F < 8 for 200 MeV < p^cut _T < 800 MeV. At small Δη^F, the data test the reliability of hadronisation models in describing rapidity and transverse momentum fluctuations in final state particle production. The measurements at larger gap sizes are dominated by contributions from the single diffractive dissociation process (pp →Xp), enhanced by double dissociation (pp → XY) where the invariant mass of the lighter of the two dissociation systems satisfies M_Y < 7 GeV. The resulting cross section is dσ/dΔη^F ≈1 mb for Δη^F > 3. The large rapidity gap data are used to constrain the value of the Pomeron intercept appropriate to triple Regge models of soft diffraction. The cross section integrated over all gap sizes is compared with other LHC inelastic cross section measurements.

In experiments that are aimed at detecting astrophysical sources such as neutrino telescopes, one usually performs a search over a continuous parameter space (e.g. the angular coordinates of the sky, and possibly time), looking for the most significant deviation from the background hypothesis. Such a procedure inherently involves a "look elsewhere effect", namely, the possibility for a signal-like fluctuation to appear anywhere within the search range. Correctly estimating the p-value of a given observation thus requires repeated simulations of the entire search, a procedure that may be prohibitively expansive in terms of CPU resources. Recent results from the theory of random fields provide powerful tools which may be used to alleviate this difficulty, in a wide range of applications. We review those results and discuss their implementation, with a detailed example applied for neutrino point source analysis in the IceCube experiment.

Many experiments that aim at the direct detection of dark matter are able to distinguish a dominant background from the expected feeble signals, based on some measured discrimination parameter. We develop a statistical model for such experiments using the profile likelihood ratio as a test statistic in a frequentist approach. We take data from calibrations as control measurements for signal and background, and the method allows the inclusion of data from Monte Carlo simulations. Systematic detector uncertainties, such as uncertainties in the energy scale, as well as astrophysical uncertainties, are included in the model. The statistical model can be used to either set an exclusion limit or to quantify a discovery claim, and the results are derived with the proper treatment of statistical and systematic uncertainties. We apply the model to the first data release of the XENON100 experiment, which allows one to extract additional information from the data, and place stronger limits on the spin-independent elastic weakly interacting massive particles nucleon scattering cross section. In particular, we derive a single limit, including all relevant systematic uncertainties, with a minimum of 2.4×10^-44cm2 for weakly interacting massive particles with a mass of 50GeV/c2.

The XENON100 experiment has completed a dark matter search with 100.9 live days of data, taken from January to June 2010. Events with energies between 8.4 and 44.6keV_nr in a fiducial volume containing 48kg of liquid xenon have been analyzed. A total of three events have been found in the predefined signal region, compatible with the background prediction of (1.8±0.6) events. Based on this analysis we present limits on the WIMP-nucleon cross section for inelastic dark matter. With the present data we are able to rule out the explanation for the observed DAMA/LIBRA modulation as being due to inelastic dark matter scattering off iodine, at a 90% confidence level.

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 τ̃ sleptons are excluded at 95% CL up to a mass of 136 GeV, in GMSB models with N5=3, mmessenger=250 TeV, sign(μ)=1 and tanβ=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 g̃-balls.

We present results from the direct search for dark matter with the XENON100 detector, installed underground at the Laboratori Nazionali del Gran Sasso of INFN, Italy. XENON100 is a two-phase time-projection chamber with a 62 kg liquid xenon target. Interaction vertex reconstruction in three dimensions with millimeter precision allows the selection of only the innermost 48 kg as the ultralow background fiducial target. In 100.9 live days of data, acquired between January and June 2010, no evidence for dark matter is found. Three candidate events were observed in the signal region with an expected background of (1.8±0.6) events. This leads to the most stringent limit on dark matter interactions today, excluding spin-independent elastic weakly interacting massive particle (WIMP) nucleon scattering cross sections above 7.0×10^-45cm2 for a WIMP mass of 50GeV/c2 at 90% confidence level.

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.

This paper describes searches for the pair production of first or second generation scalar leptoquarks using 35pb^-1 of proton-proton collision data recorded by the ATLAS detector at √s=7TeV. Leptoquarks are searched in events with two oppositely-charged muons or electrons and at least two jets, and in events with one muon or electron, missing transverse momentum and at least two jets. After event selection, the observed yields are consistent with the predicted backgrounds. Leptoquark production is excluded at the 95 CL for masses M_LQ

This Letter presents the first search for supersymmetry in final states containing one isolated electron or muon, jets, and missing transverse momentum from √s=7TeV proton-proton collisions at the LHC. The data were recorded by the ATLAS experiment during 2010 and correspond to a total integrated luminosity of 35pb^-1. No excess above the standard model background expectation is observed. Limits are set on the parameters of the minimal supergravity framework, extending previous limits. Within this framework, for A₀=0GeV, tan-β=3, and μ>0 and for equal squark and gluino masses, gluino masses below 700 -GeV are excluded at 95% confidence level.

We describe likelihood-based statistical tests for use in high energy physics for the discovery of new phenomena and for construction of confidence intervals on model parameters. We focus on the properties of the test procedures that allow one to account for systematic uncertainties. Explicit formulae for the asymptotic distributions of test statistics are derived using results ofWilks andWald.We motivate and justify the use of a representative data set, called the \u201cAsimov data set\u201d, which provides a simple method to obtain the median experimental sensitivity of a search or measurement as well as fluctuations about this expectation.

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

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

The ATLAS Inner Detector is a composite tracking system consisting of silicon pixels, silicon strips and straw tubes in a 2 T magnetic field. Its installation was completed in August 2008 and the detector took part in data-taking with single LHC beams and cosmic rays. The initial detector operation, hardware commissioning and in-situ calibrations are described. Tracking performance has been measured with 7. 6 million cosmic-ray events, collected using a tracking trigger and reconstructed with modular pattern-recognition and fitting software. The intrinsic hit efficiency and tracking trigger efficiencies are close to 100%. Lorentz angle measurements for both electrons and holes, specific energy-loss calibration and transition radiation turn-on measurements have been performed. Different alignment techniques have been used to reconstruct the detector geometry. After the initial alignment, a transverse impact parameter resolution of 22. 1±0. 9 μm and a relative momentum resolution σ_p/p=(4. 83±0. 16)×10^-4 GeV^-1×p_T have been measured for high momentum tracks.

The ionization signals in the liquid argon of the ATLAS electromagnetic calorimeter are studied in detail using cosmic muons. In particular, the drift time of the ionization electrons is measured and used to assess the intrinsic uniformity of the calorimeter gaps and estimate its impact on the constant term of the energy resolution. The drift times of electrons in the cells of the second layer of the calorimeter are uniform at the level of 1.3% in the barrel and 2.8% in the endcaps. This leads to an estimated contribution to the constant term of (0.29_0.04 ^0.05)% in the barrel and (0.54_0.04 ^0.06)% in the endcaps. The same data are used to measure the drift velocity of ionization electrons in liquid argon, which is found to be 4.61±0.07 mm/μs at 88.5 K and 1 kV/mm.

When searching for a new resonance somewhere in a possible mass range, the significance of observing a local excess of events must take into account the probability of observing such an excess anywhere in the range. This is the so called "look elsewhere effect". The effect can be quantified in terms of a trial factor, which is the ratio between the probability of observing the excess at some fixed mass point, to the probability of observing it anywhere in the range. We propose a simple and fast procedure for estimating the trial factor, based on earlier results by Davies. We show that asymptotically, the trial factor grows linearly with the (fixed mass) significance.

Models of minimal lepton flavor violation where the seesaw scale is higher than the relevant flavor scale predict that all lepton flavor violation is proportional to the charged lepton Yukawa matrix. If extra vectorlike leptons are within the reach of the LHC, it will be possible to test the resulting predictions in ATLAS/CMS.

Charged Higgs boson may be produced at hadron colliders via the decay of top quarks. At the LHC, the large production cross section of top quarks pairs will make it one of the earliest channels allowing to search for physics beyond the standard model. Assuming the decay H⁺→τ⁺ν, previous searches have so far considered only hadronic τ decays. Here, we show that by using appropriate kinematical variables (transverse mass observables), leptonic τ decays can be used as well, for both observing and possibly measuring the mass of the charged Higgs boson. This can increase the overall experimental sensitivity to charged Higgs bosons at hadron colliders.

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.

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

Statistical methods used for discovery and exclusion of signal at the LHC and the TEVATRON are described. An emphasis is given to the Look Elsewhere Effect, the CL_s controversy and the equivalence between the Bayesian and frequentist Profile Likelihood when using flat priors. For the Look Elsewhere Effect, formulas are derived that allow the estimation of the effect from the simple fixed mass result without the need to perform complicated Monte Carlo simuations.

This contribution presents a novel approach to estimate the Standard Model backgrounds based on modifying Monte Carlo predictions within their systematic uncertainties. The improved background model is obtained by altering the original predictions with successively more complex correction functions in signal-free control selections. Statistical tests indicate when sufficient compatibility with data is reached. In this way, systematic effects are absorbed into the new background model. The same correction is then applied on the Monte Carlo prediction in the signal region. Comparing this method to other background estimation techniques shows improvements with respect to statistical and systematic uncertainties. The proposed method can also be applied in other fields beyond high energy physics.

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.

One of the main goals of the ATLAS experiment is to measure various Higgs boson couplings as accurately as possible. Such a measurement is mandatory for a full understanding of the Higgs sector. One of the most challenging measurements of the Higgs boson properties is the determination of the Yukawa coupling to the top quark. To complement the tt̄H→ tt̄bb̄ channel, which is the most significant in the low Higgs mass region (m _H ∼ 120 GeV), we introduce a feasibility study of the tt̄H channel with the Higgs decaying to a pair of τ leptons. The signal events were reconstructed using the full and the fast simulation of the ATLAS detector. It is shown that both the distributions and the number of expected events after the same cuts agree, and that we can use the fast simulation to complete the analysis. We obtain a significance of 1.6σ for the low luminosity condition (30 fb^-1) and m _H =120 GeV, and 2.0σ for the high luminosity condition (300 fb^-1) and m _H =120 GeV. The observability of Higgs boson in this channel is demonstrated to be very marginal, even in the absence of taking into account tt̄ +jets.

Light charged Higgs bosons (m_H+ < m_top), if exist, are expected to be copiously produced at the LHC via top quark decays t→H⁺b. The dominant decay mode in this mass range is to a τ-lepton and a neutrino. We study the semi-leptonic and di-leptonic channels in which the τ decays leptonically (i.e. to an electron or a muon). The presence of an isolated lepton offers a cleaner experimental signature compared to hadronic τ decays, however reconstruction of the event is more challenging due to the presence of multiple neutrinos in the final state. We define a new transverse mass for the charged Higgs in these channels, which can be used to discriminate between the signal and the main background on a statistical basis.

This paper presents a novel approach to estimate the Standard Model backgrounds based on modifying Monte Carlo predictions within their systematic uncertainties. The improved background model is obtained by altering the original predictions with successively more complex correction functions in signal-free control selections. Statistical tests indicate when sufficient compatibility with data is reached. In this way, systematic effects are absorbed into the new background model. The same correction is then applied on the Monte Carlo prediction in the signal region. Comparing this method to other background estimation techniques shows improvements with respect to statistical and systematic uncertainties. The proposed method can also be applied in other fields beyond high energy physics.

Light charged Higgs bosons ( m_h+ < m _top), if exist, are expected to be produced at the LHC dominantly via top quark decays, t →H⁺b . The dominant decay mode of the charged Higgs boson in this mass range is to a τ-lepton and a neutrino. We study the channel in which the τdecays leptonically (i.e. to an electron or a muon). The presence of an isolated lepton offers a cleaner experimental signature compared to hadronic τdecays, however reconstruction of the event is more challenging due to the presence of three neutrinos in the final state. We define a new transverse mass for the charged Higgs in this channel, which can be used to discriminate between the signal and the main background on a statistical basis.

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.).

A pedestrians guide aimed at the LHC laymen statisticians is presented. It is not meant to replace any text book but to help the confused physicist to understand the jargon and methods used by HEP Phystatisticians.

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.

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.

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.

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.

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 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.

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.).

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.

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 α_s and test its evolution with energy scale. The results are consistent with the running of α_s expected from QCD. Combining all data, the value of α_s(M _z is determined to be α_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 α_s with slightly larger errors: α _s(M_z =0.1223 ± 0.0005(stat.)± 0.0014 (expt.)± 0.0016 (hadr.)^{+0.0054}_0.0036.

Upper limits on the cross-section of the pair-production process e ⁺e_- →h⁰ A⁰, assuming 100 % branching fraction to hadrons, are derived from a new search for the →h⁰ A⁰→ hadrons final state, independently of the hadronic flavour of the decay products. This study, combined with previously published searches for the neutral Higgs bosons h⁰ 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, A⁰→qq̄, gg, τ⁺ τ^- h⁰A⁰A⁰ 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, \tanβ planes, using both direct neutral Higgs boson searches and indirect limits derived from Standard Model high precision measurements. The region 1 ≲ m_h ≲ 55 GeV and 3 ≲ m_A ≲ 63 GeV is excluded at 95 % CL, independently of m_h, m_A, tanβ and for selected values of α which are representative of a complete α -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.

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.

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.

The interplay of prospective experimental information from both the Large Hadron Collider (LHC) and the Linear Collider (LC) in the investigation of the MSSM Higgs sector is analyzed in the SPS 1a and SPS 1b benchmark scenarios. Combining LHC information on the heavy Higgs states of the MSSM with precise measurements of the mass and branching ratios of the lightest CP-even Higgs boson at the LC provides a sensitive consistency test of the MSSM. This allows to set bounds on the trilinear coupling A_t. In a scenario where LHC and LC only detect one light Higgs boson, the prospects for an indirect determination of MA are investigated. In particular, the impact of the experimental errors of the other SUSY parameters is analyzed in detail. We find that a precision of about 20% (30%) can be achieved for M_A = 600(800) GeV.

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

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.

The flavoenzyme Ero1p produces disulfide bonds for oxidative protein folding in the endoplasmic reticulum. Disulfides generated de novo within Ero1p are transferred to protein disulfide isomerase and then to substrate proteins by dithiol-disulfide exchange reactions. Despite this key role of Ero1p, little is known about the mechanism by which this enzyme catalyzes thiol oxidation. Here, we present the X-ray crystallographic structure of Ero1p, which reveals the molecular details of the catalytic center, the role of a CXXCXXC motif, and the spatial relationship between functionally significant cysteines and the bound cofactor. Remarkably, the Ero1p active site closely resembles that of the versatile thiol oxidase module of Erv2p, a protein with no sequence homology to Ero1p. Furthermore, both Ero1p and Erv2p display essential dicysteine motifs on mobile polypeptide segments, suggesting that shuttling electrons to a rigid active site using a flexible strand is a fundamental feature of disulfide-generating flavoenzymes.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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 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.

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 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.

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.

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.

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.

At the Kaon spectrometer KaoS at SIS/GSI, the production of kaons and antikaons in heavy-ion reactions at a beam energy of 1.5 A GeV has been measured for the collision systems Ni+Ni and Au + Au. The K^-/K ⁺ ratio is found to be constant for both systems and as a function of impact parameter but the slopes of K⁺ and K^- spectra differ for all impact parameters. Furthermore, the respective polar angle distributions will be presented as a function of centrality.

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.

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.

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.

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.

Erv2p is an FAD-dependent sulfhydryl oxidase that can promote disulfide bond formation during protein biosynthesis in the yeast endoplasmic reticulum. The structure of Erv2p, determined by X-ray crystallography to 1.5 Å resolution, reveals a helix-rich dimer with no global resemblance to other known FAD-binding proteins or thiol oxidoreductases. Two pairs of cysteine residues are required for Erv2p activity. The first (Cys-Gly-Glu-Cys) is adjacent to the isoalloxazine ring of the FAD. The second (Cys-Gly-Cys) is part of a flexible C-terminal segment that can swing into the vicinity of the first cysteine pair in the opposite subunit of the dimer and may shuttle electrons between substrate protein dithiols and the FAD-proximal disulfide.

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.

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.

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 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.

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 (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.

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.

Using about 3.9 million hadronic Z decays from e⁺e^- collisions recorded by the OPAL detector at LEP at centre-of-mass energies √s ≈ M_Z, the branching ratio for the decay D_s^- → τ^-ν̄_τ has been measured to be BR(D_s^- → τ^-ν̄_τ) = (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.

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.

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.

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.

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.

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).

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.).

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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 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.

A search for direct production of C-even resonances a₂(1320) and f₂(1270) in e⁺e^- annihilation was performed with SND detector at VEPP-2M e⁺e^- collider. The upper limits of electronic widths of these mesons were obtained at 90% confidence level: Γ(a₂(1320) → e⁺e^-) < 0.56 eV, Γ(f₂(1270) → e⁺e^-) < 0.11 eV. (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 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.

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.

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.

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.

Searches for unstable neutral and charged heavy leptons, N and L^±, and for excited states of neutral and charged leptons, v*, e*, μ*, and τ*, have been performed in e⁺e^- collisions using data collected by the OPAL detector at LEP. The data analysed correspond to an integrated luminosity of about 58 pb^-1 at a centre-of-mass energy of 183 GeV, and about 10 pb^-1 each at 161 GeV and 172 GeV. No evidence for new particles was found. Lower limits on the masses of unstable heavy and excited leptons are derived. From the analysis of charged-current, neutral-current, and photonic decays of singly produced excited leptons. upper limits are determined for the ratio of the coupling to the compositeness scale, f/Λ, for masses up to the kinematic limit. For excited leptons, the limits are established independently of the relative values of the coupling constants f and f.

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.

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.

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.

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.

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 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.

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.

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.

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.

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 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.

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 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.

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.

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^{± -0}λ1, 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.

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.

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.

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 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.

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 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.

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.

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 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.

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.

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).

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.

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.

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.

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.

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 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.

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 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.

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.

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.

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 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 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.

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.

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 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.

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 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.

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.

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.

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.

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).

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.

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.

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 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.

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 Standard Model, b quarks produced in e⁺e^- annihilation at the Z⁰ peak have a large average longitudinal polarization of -0.94. Some fraction of this polarization is expected to be transferred to b-flavored baryons during hadronization. The average longitudinal polarization of weakly decaying b baryons, 〈P^Λb_L〉, is measured in approximately 4.3 million hadronic Z⁰ decays collected with the OPAL detector between 1990 and 1995 at LEP. Those b baryons that decay semileptonically and produce a Λ baryon are identified through the correlation of the baryon number of the Λ and the electric charge of the lepton. In this semileptonic decay, the ratio of the neutrino energy to the lepton energy is a sensitive polarization observable. The neutrino energy is estimated using missing energy measurements. From a fit to the distribution of this ratio, the value 〈P^Λb_L〉 = -0.56^+0.20_-0.13 ± 0.09 is obtained, where the first error is statistical and the second systematic.

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.

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 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 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.

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.

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 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.

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.

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.

The OPAL experiment at the CERN LEP collider recently increased the geometrical acceptance of its silicon microvertex detector. The azimuthal coverage is improved by adding one pair of detector modules to each of the two layers, while the polar angle coverage is extended by adding new detector modules in line with the existing ones. This improves the efficiency for high quality tracking in OPAL and in particular for b quark tagging in Higgs boson searches. A description of the detector is given, with emphasis on new or modified elements with respect to the earlier version. Results on the performance of the new detector 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 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.

A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of 10.3 pb^-1 at centre-of-mass energies of √s =170 and 172 GeV with the OPAL detector at LEP. No evidence for these particles has been found. The results are combined with those from previous OPAL chargino and neutralino searches at lower energies to obtain limits. Exclusion regions at 95% C.L. of parameters of the Minimal Supersymmetric Standard Model are determined. Within this framework, for tan β ≥ 1.0, lower mass limits are placed on the lightest chargino and the three lightest neutralinos. The 95% C.L. lower mass limit on the lightest chargino, assuming that it is heavier than the lightest neutralino by more than 10 GeV, is 84.5 GeV for the case of a large universal scalar mass (m₀ > 1 TeV) and 65.7 GeV for the smallest m₀ compatible with current limits on the sneutrino mass and slepton cross-sections. The lower limit on the lightest neutralino mass at 95% C.L. for tan β≥ 1.0 is 24.7 GeV for m₀ = 1 TeV and 13.3 GeV for the minimum m0 scenario. These mass limits are higher for increasing values of tan β. The interpretation of the limits in terms of gluino and scalar quark mass limits is also given.

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.

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.

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.

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.