Publications

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.

Resistive Micro-Pattern Gaseous Detectors (MPGDs) are emerging as promising candidates for high-energy physics applications due to their low material budget, simplified operational schemes, and inherent protection for readout electronics. This work presents a comparative study of three resistive MPGD technologies: Micromegas (MM), micro Resistive WELL (μ-RWELL), and Resistive Plate WELL (RPWELL). The detectors were tested with muon and pion beams at the CERN SPS facility. Detection efficiency, cluster size, and rate dependency were characterized, revealing distinct performance features for each technology. The MM detector exhibited high efficiency (98%) across all drift fields, while μ-RWELL reached an 84% efficiency plateau due to a large dead area. RPWELL reached a 96% efficiency plateau with a small dead area and showed efficiency loss at high gain.

The search for resonant mass bumps in invariant-mass distributions remains a cornerstone strategy for uncovering Beyond the Standard Model (BSM) physics at the Large Hadron Collider (LHC). Traditional methods often rely on predefined functional forms and exhaustive computational and human resources, limiting the scope of tested final states and selections. This work presents BumpNet, a machine learning-based approach leveraging advanced neural network architectures to generalize and enhance the Data-Directed Paradigm (DDP) for resonance searches. Trained on a diverse dataset of smoothly-falling analytical functions and realistic simulated data, BumpNet efficiently predicts statistical significance distributions across varying histogram configurations, including those derived from LHC-like conditions. The networks performance is validated against idealized likelihood ratio-based tests, showing minimal bias and strong sensitivity in detecting mass bumps across a range of scenarios. Additionally, BumpNets application to realistic BSM scenarios highlights its capability to identify subtle signals while managing the look-elsewhere effect. These results underscore BumpNets potential to expand the reach of resonance searches, paving the way for more comprehensive explorations of LHC data in future analyses.

Thousands of person years have been invested in searches for new physics (NP), the majority of them motivated by theoretical considerations. Yet, no evidence of beyond the Standard Model physics has been found. This suggests that model-agnostic searches might be an important key to explore NP, and help discover unexpected phenomena which can inspire future theoretical developments. A possible strategy for such searches is identifying asymmetries between data samples that are expected to be symmetric within the Standard Model. We propose exploiting neural networks (NNs) to quickly fit and statistically test the differences between two samples. Our method is based on an earlier work, originally designed for inferring the deviations of an observed dataset from that of a much larger reference dataset. We present a symmetric formalism, generalizing the original one, avoiding fine-tuning of the NN parameters and any constraints on the relative sizes of the samples. Our formalism could be used to detect small symmetry violations, extending the discovery potential of current and future particle physics experiments.

Resistive electrodes are used in gaseous detectors to quench electrical discharges. This helps to protect delicate electrodes and readout electronics and to improve the stability of the detector operation. An RPWELL is a THGEM-based WELL detector with a resistive plate coupled to a conductive anode. Till now, the choice of the resistive plate was limited to a few materials, like LRS Glass and Semitron. These materials have fixed resistivities and, sometimes, thickness and area limitations. This restricts the potential usage of the detector to a rather small range of applications, as well as the possibility of studying in depth the physics processes governing the discharge quenching mechanism. In our present study, we used a new plastic material doped with carbon nanotubes to produce resistive plates with a commercial 3D printer. This method has the flexibility to produce samples of different thicknesses and different resistivity values. We describe here the sample production and characterize the RPWELL performance with different resistive plates. In particular we show the dependence of discharge quenching on the thickness and resistivity of the plate. The dynamics of the charge carriers in the material is proposed as an explanation for the long gain recovery time after a discharge.

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 first operation of a cryogenic Resistive Plate WELL (RPWELL) detector in the saturated vapor of liquid argon is reported. The RPWELL detector was composed of a Thick Gas Electron Multiplier (THGEM) electrode coupled to a metallic anode via Fe₂O₃/YSZ ceramics (Fe₂O₃ in weight equal to 75%), with tunable bulk resistivity in the range 10⁹10¹² Ω·cm. The detector was operated at liquid argon temperature in saturated argon vapor (90 K, 1.2 bar) and characterized in terms of its effective charge gain and stability against discharges. Maximum stable gain of G≈17 was obtained, without discharges. In addition, preliminary results from novel 3D-printed thermoplastic plates doped with carbon nanotubes are presented.

The operation of a cryogenic Resistive WELL (RWELL) in liquid argon vapor is reported for the first time. It comprises a Thick Gas Electron Multiplier (THGEM) structure coupled to a resistive Diamond-Like Carbon (DLC) anode deposited on an insulating substrate. The multiplier was operated at cryogenic temperature (90 K, 1.2 bar) in saturated argon vapor and characterized in terms of charge gain and electrical stability. A comparative study with standard, non-resistive THGEM (a.k.a LEM) and WELL multipliers confirmed the RWELL advantages in terms of discharge quenching i.e. superior gain and stability.

We present a new design and assembly procedure of a large-area gas-avalanche Resistive-Plate WELL (RPWELL) detector. A 50 × 50 cm² prototype was tested in 80 GeV/c muon beam at CERN-SPS, presenting improved performances compared to previous ones: MIP detection efficiency over 96% with 3% uniformity across the entire detector area, a charge gain of ≈ 7.5 × 10³ with a uniformity of 22%, and discharge probability below 10^-6 with a few single hotspots attributed to production imperfections. These results pave the way towards further up-scaling detectors of this kind.

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.].

Cryogenic versions of Resistive WELL (RWELL) and Resistive Plate WELL (RPWELL) detectors have been developed, aimed at stable avalanche multiplication of ionization electrons in the vapor phase of LAr (dual-phase TPC). In the RWELL, a thin resistive DLC layer deposited on top of an insulator is inserted in between the electron multiplier (THGEM) and the readout anode; in the RPWELL, a resistive ferrite plate is directly coupled to the THGEM. Radiation-induced ionization electrons in the liquid are extracted into the gaseous phase. They drift into the THGEM's holes where they undergo charge multiplication. Embedding resistive materials into the multiplier proved to enhance operation stability due to the mitigation of electrical discharges thus allowing operation at higher charge gain compared to standard THGEM (a.k.a. LEM) multipliers. We present the detector concepts and report on the main preliminary results.

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.].

The Thick Gas Electron Multiplier (THGEM) is a robust high-gain gas-avalanche electron multiplier a building block of a variety of radiation detectors. It can be manufactured economically by standard printed-circuit drilling and etching technology. We present a detailed review of the THGEM and its derivatives. We focus on the physics phenomena that govern their operation and performances under different operation conditions. Technological aspects associated with the production of these detectors and their current and potential applications are discussed.

Characterization of diamond-like carbon (DLC) coatings at cryogenic temperatures (down to 77 K) is presented, covering the electrical resistivity range of practical interest to gaseous and liquid particle instrumentation: 10⁻¹−10⁵ MΩ/□. The good behaviour observed in terms of linearity, surface uniformity and stability with time and transported charge add to other well-known characteristics like low chemical reactivity and tolerance to radiation. The observed temperature dependence and stability of electrical properties with transported charge is consistent with a conductivity mechanism based on 2-dimensional variable-range electron hopping, as expected for the surface conductivity of thin films made from amorphous carbon. First results from a resistive-protected WELL detector (RWELL) built with DLC and operated close to the liquidvapour coexistence point of argon(87.5 K at 1 bar) are presented.

Resistive Plate WELL is a Thick-GEM based WELL detector that utilizes a resistive anode to quench substantially the energy released in discharges. We present a method developed to identify these low-energy discharges and use it to study their effect on the detector gas gain as a function of distance from the discharge location and time.

Digital Hadronic Calorimeters (DHCAL) were suggested for future Colliders as part of the particle-flow concept. Though studied mainly with RPC, studies focusing on sampling elements based on Micro-Pattern Gaseous Detector have shown the potential advantages; they can be operated with environment-friendly gases and reach similar detection efficiency at lower average pad multiplicity. We summarize here the experimental test-beam results of a small-size DHCAL prototype, incorporating six Micromegas and two RPWELL sampling elements, interlaced with steel-absorber plates. It was investigated with 2-6 GeV pion beams at the CERN/PS beam facility. The data permitted validating a GEANT4 simulation framework of a DHCAL, and evaluating the expected pion energy resolution of a full-scale RPWELL-based calorimeter. The pion energy resolution derived for the RPWELL concept is competitive to that of glass RPC and Micromegas sampling techniques.

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.

The occurrence of electrical discharges in gas detectors restricts their dynamic range and degrades their performance. Among the different methods developed to mitigate discharge effects, the use of resistive materials in the detector assembly was found to be very effective. In this work, we present the results of a comparative study of electrical discharges in Thick-GEM-based WELL-type detectors with and without resistive elements. We present a new method to measure discharges in the resistive-detector configurations; it allows demonstrating, for the first time, the occurrence of discharges also in the Resistive-Plate WELL detector configuration. It also provides direct evidence for the Raether limit.

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.].

The muon spectrometer of the ATLAS detector will be significantly upgraded during the Phase-II upgrade in LS3 in order to cope with the operational conditions at the HL-LHC in Run 4 and beyond. A large fraction of the frontend and on- and off-detector readout and trigger electronics for the Resistive Plate Chambers (RPC), Thin Gap Chambers (TGC), and Monitored Drift Tube (MDT) chambers will be replaced to make them compatible with the higher trigger rates and longer latencies necessary for the new level-0 trigger. The MDT chambers will be integrated into the level-0 trigger in order to sharpen the momentum threshold. Additional RPC chambers will be installed in the inner barrel layer to increase the acceptance and robustness of the trigger, and some chambers in high-rate regions will be refurbished. Some of the MDT chambers in the inner barrel layer will be replaced with new small-diameter MDTs. New TGC triplet chambers in the barrel-endcap transition region will replace the current TGC doublets to suppress the high trigger rate from random coincidences in this region. The power system for the RPC, TGC, and MDT chambers and electronics will need to be replaced due to component obsolescence, ageing, and radiation damage. A high-h tagger is under consideration to extend the angular acceptance for muon identification. The Phase-II upgrade concludes the process of adapting the muon spectrometer to the ever increasing performance of the LHC, which started with the Phase-I upgrade New Small Wheel (NSW) project that will replace the Cathode Strip Chambers (CSC) and the MDT chambers of the innermost endcap wheels by Micromegas and small-strip TGCs.

The R(D^(*)) puzzle stands for a ~ 3σ violation of lepton flavor universality between the decay rates of B → D^(*)τν and B → D^(*)ℓν, where ℓ = e, μ. If it is accounted for by new physics, there is no reason in general that the relevant neutrinos are, respectively, ν_τ and ν_ℓ. We study whether the τ related rate could be enhanced by significant contributions to B → D^(*)τν_ℓ from a class of operators in the Standard Model Effective Field Theory (SMEFT). We find the upper bounds from forbidden or rare meson decays imply that the contributions from the lepton flavor violating processes account for no more than about 4% of the required shift. Yet, no fine-tuned flavor alignment is required for the new physics. Searching for the related high-p_T process pp → τ^±μ^∓ can at present put a lower bound on the scale of the lepton flavor violating new physics that is a factor of 2.2 weaker than the bound from meson decays. An exception to our conclusion arises from a specific combination of scalar and tensor SMEFT operators.

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.

We propose exploiting symmetries (exact or approximate) of the Standard Model (SM) to search for physics Beyond the Standard Model (BSM) using the data-directed paradigm (DDP). Symmetries are very powerful because they provide two samples that can be compared without requiring simulation. Focusing on the data, exclusive selections which exhibit significant asymmetry can be identified efficiently and marked for further study. Using a simple and generic test statistic which compares two matrices already provides good sensitivity, only slightly worse than that of the profile likelihood ratio test statistic which relies on the exact knowledge of the signal shape. This can be exploited for rapidly scanning large portions of the measured data, in an attempt to identify regions of interest. We also demonstrate that weakly supervised Neural Networks could be used for this purpose as well.

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.]

We propose a data-directed paradigm (DDP) to search for new physics. Focusing on the data without using simulations, exclusive selections which exhibit significant deviations from known properties of the standard model can be identified efficiently and marked for further study. Different properties can be exploited with the DDP. Here, the paradigm is demonstrated by combining the promising potential of neural networks (NN) with the common bump-hunting approach. Using the NN, the resource-consuming tasks of background and systematic uncertainty estimation are avoided, allowing rapid testing of many final states with only a minor degradation in the sensitivity to bumps relative to standard analysis methods.

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.

In bubble-assisted Liquid Hole Multipliers (LHM), developed for noble-liquid radiation detectors, the stability of the bubble and the electro-mechanical properties of the liquid-to-gas interface play a dominant role in the detector performance. A model is proposed to evaluate the static equilibrium configurations of a bubble sustained underneath a perforated electrode immersed in a liquid. For the first time bubbles were optically observed in LAr; their properties were studied in contact with different material surfaces. This permitted investigating the bubble-electrodynamics via numerical simulations; it was shown that the electric field acts as an additional pressure term on the bubble meniscus. The predictions for the liquid-to-gas interface were successfully validated using X-ray micro-CT in water and in silicone oil at STP. The proposed model and the results of this study are an important milestone towards understanding and optimizing the parameters of LHM-based noble-liquid detectors.

Single-electron spectra are the key ingredient in the efficient detection of single UV-photons. In this work, we investigated the shape of single-photoelectron spectra in single- and double-stage Resistive Plate WELL (RPWELL) detector configurations, operated in Ne/CH4 and Ar/CH4. Discharge-free operation was reached over a broad dynamic range, with charge gains of [range-phrase = -]e4e6. Compared to the usual exponential ones, the observed Polya-like charge spectra pave the way towards higher single-electron detection efficiencies. The latter was evaluated here, using experimental data combined with numerical simulations. The effects of the gas mixtures, electric field configuration, and detector geometry on the Polya spectra and their related "θ"parameter are presented.

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.]

The gas detectors, operated under harsh radiation conditions like the one foreseen at the High Luminosity LHC (HL-LHC), must fulfill a number of stringent quality control criteria. Based on high-voltage current measurements, the X-ray scanning technique has been developed for discovery of various production defects prior to the readout electronics installation. The later usually happens at the last stage of detector assembly. Thus, it allows testing the quality of the chambers, identifying defects and when possible fixing them already at early stage.

Digital and Semi-Digital Hadronic Calorimeters ((S)DHCAL) were suggested for future Colliders as part of the particle-flow concept. Though studied mostly with Resistive Plate Chambers (RPC), studies focusing on Micro-Pattern Gaseous Detector (MPGD)-based sampling elements have shown the potential advantages using such techniques. In 2018, eight 48x48 cm² sampling elements based on resistive Micromegas and Resistive Plate WELL (RPWELL) technologies were assembled. They were tested within a small MPGD-based SDHCAL prototype incorporating in addition three 16x16 cm² Micromegas detectors and steel absorber plates recording hadronic showers of low-energy pions at the CERN/PS beam line. Preliminary results and analysis methodology are presented, using data samples recorded with pions with momenta in the range from 2 to 6 GeV/c.

The bubble-assisted Liquid Hole Multiplier (LHM) is a novel concept for the combined detection of ionization electrons and scintillation photons in noble-liquid time projection chambers. It consists of a perforated electrode immersed in the noble liquid, with heating wires generating a stable vapor bubble underneath. Radiation-induced ionization electrons in the liquid drift into the electrode's holes and cross the liquid-vapor interface into the bubble where they induce electroluminescence (EL). The top surface of the electrode is optionally coated with a CsI photocathode; radiation-induced UV-scintillation photons extract photoelectrons that induce EL in a similar way. EL-photons recorded with an array of photo-sensors, e.g. SiPMs, provide event localization. We present the basic principles of the LHM concept and summarize the results obtained in LXe and LAr.

Digital and Semi-Digital Hadronic Calorimeters (S)DHCAL were suggested for future Colliders as part of the particle-flow concept. Though studied mostly with RPC-based techniques, investigations have shown that Micro Pattern Gaseous Detector (MPGD)-based sampling elements could outperform in terms of average pad multiplicity or at higher rates. An attractive, industry-produced, robust, particle-tracking detector for large-area coverage, e.g. in (S)DHCAL, could be the novel single-stage Resistive Plate WELL (RPWELL). It is a single-sided THick Gaseous Electron Multiplier (THGEM) coupled to the segmented readout electrode through a sheet of large bulk resistivity. We summarize here the preliminary test-beam results obtained with 6.5 mm thick (incl. electronics) 48×48 cm² RPWELL detectors. Two configurations are considered: a standalone RPWELL detector studied with 150 GeV muons and high-rate pions beams and a RPWELL sampling element investigated within a small-(S)DHCAL prototype consisting of 7 resistive Micro-MEsh Gaseous Structure (MICROMEGAS) sampling elements followed by 5 RPWELL ones. The sampling elements were equipped with a Semi-Digital readout electronics based on the MICROROC chip.

We demonstrate, for the first time, the operation of a bubble-assisted Liquid Hole Multiplier (LHM) in liquid argon. The LHM, sensitive to both radiation-induced ionization electrons and primary scintillation photons, consists of a perforated electrode immersed in the noble liquid, with a stable gas-bubble trapped underneath. Electrons deposited in the liquid or scintillation-induced photoelectrons emitted from a photocathode on the electrode's surface, are collected into the holes; after crossing the liquid-gas interface, they induce electroluminescence within the bubble. After having validated in previous works the LHM concept in liquid xenon, we provide here first preliminary results on its operation in liquid argon. We demonstrate the bubble containment under a Thick Gas Electron Multiplier (THGEM) electrode and provide the detector response to alpha particles, recorded with SiPMs and with a PMT - under electroluminescence and with modest gas multiplication; the imaging capability is also demonstrated.

We present for the first time, discharge-free operation at cryogenic conditions of a Resistive-Plate WELL (RPWELL) detector. It is a single-sided Thick Gaseous Electron Multiplier (THGEM) coupled to a readout anode via a plate of high bulk resistivity. The results of single- and double-stage RPWELL detectors operated in stable conditions in Ne/5%CH₄ at 163 K are summarized. The RPWELL comprised a ferric-based (Fe³⁺) ceramic composite (\u201cFe-ceramic\u201d) as the resistive plate, of volume resistivity ∼ 10¹¹ Ω·cm at this temperature. Gains of ∼ 10⁴ and ∼ 10⁵ were reached with the single-stage RPWELL, with 6 keV X-rays and single UV-photons, respectively. The double-stage detector, a THGEM followed by the RPWELL, reached gains ∼ 10⁵ and ∼ 10⁶ with X-rays and single UV-photons, respectively. The results were obtained with and without a CsI photocathode on the first multiplying element. Potential applications at these cryogenic conditions are discussed.

Numerical simulations were used to study signal formation in a Thick Gaseous Electron Multiplier (THGEM) and in THGEM-based Thick-WELL (THWELL) and Resistive-Plate WELL (RPWELL) detectors. The signal shapes were simulated in mixtures of Argon and Neon with 5% Methane under irradiation with soft x-rays and muons. Anode-induced raw signals were convoluted with the response functions of charge-sensitive and current-sensitive pre-amplifiers. The simulation toolkit was validated by the good agreement reached between the simulated and measured response, with different pre-amplifiers. It indicates that our simulations framework provides valid insight into the inherent complex dynamical processes of the various detectors.

First imaging results in liquid xenon of a Liquid Hole Multiplier (LHM) coupled to a quad-Silicon Photomultiplier (SiPM) array are presented. Ionization electrons deposited in the noble liquid by 5.5 MeV alpha particles, are collected into the holes of a Thick Gas Electron Multiplier (THGEM) electrode having a xenon gas bubble trapped underneath. They drift through the liquid-gas interface, inducing electroluminescence within the bubble. The resulting photons are detected with a Hamamatsu VUV4 quad-SiPM array providing the deposited energy with a charge-only RMS resolution of 6.6%. The image reconstruction resolution was estimated to be ∼ 200 µm (RMS).

The time-dependent gain variation of detectors incorporating Thick Gas Electron Multipliers (THGEM) electrodes was studied in the context of charging-up processes of the electrode's insulating surfaces. An experimental study was performed to examine model-simulation results of the aforementioned phenomena, under various experimental conditions. The results indicate that in a stable detector's environment, the gain stabilization process is mainly affected by the charging-up of the detector's insulating surfaces caused by the avalanche charges. The charging-up is a transient effect, occurring during the detector's initial operation period; it does not affect its long-term operation. The experimental results are consistent with the outcome of model-simulations.

Charging-up processes affecting gain stability in Thick Gas Electron Multipliers (THGEM) were studied with a dedicated simulation toolkit. Integrated with Garfield++, it provides an effective platform for systematic phenomenological studies of charging-up processes in MPGD detectors. We describe the simulation tool and the fine-tuning of the step-size required for the algorithm convergence, in relation to physical parameters. Simulation results of gain stability over time in THGEM detectors are presented, exploring the role of electrode-thickness and applied voltage on its evolution. The results show that the total amount of irradiated charge through electrode's hole needed for reaching gain stabilization is in the range of tens to hundreds of pC, depending on the detector geometry and operational voltage. These results are in agreement with experimental observations presented previously.

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.].

A study of the localization properties of a single-element Resistive Plate WELL (RP-WELL) detector is presented. The detector comprises of a single-sided THick Gaseous Electron Multiplier (THGEM) coupled to a segmented readout anode through a doped silicate-glass plate of 10¹⁰ Ω·cm bulk resistivity. Operated in ambient\nech gas, the detector has been investigated with 150 GeV muons at CERN-SPS. Signals induced through the resistive plate on anode readout strips were recorded with APV25/SRS electronics. The experimental results are compared with that of Monte Carlo simulations. The effects of various physics phenomena on the position resolution are discussed. The measured position resolution in the present configuration is 0.28 mm RMS - compatible with the holes-pattern of the multiplier. Possible ways for improving the detector position resolution are suggested.

The phenomenon of avalanche-gain variations over time, particularly in Micro Pattern Gaseous Detectors (MPGD) incorporating insulator materials, have been generally attributed to electric-field modifications resulting from "charging-up" effects of the insulator. A robust methodology for characterization of gain-transients in such detectors is presented. It comprises three guidelines: detector initialization, long gain-stabilization monitoring and imposing transients by applying abrupt changes in operation conditions. Using THWELL and RPWELL detectors, we validated the proposed methodology by assessing a charging-up/charging-down model describing the governing processes of gain stabilization. The results provide a deeper insight into these processes, reflected by different transients upon abrupt variations of detector gain or the irradiation rate. This methodology provides a handle for future investigations of the involved physics phenomena in MPGD detectors comprising insulating components.

This Report summarizes the results of the activities of the LHC Higgs Cross Section Working Group in the period 2014-2016. The main goal of the working group was to present the state-of-the-art of Higgs physics at the LHC, integrating all new results that have appeared in the last few years. The first part compiles the most up-to-date predictions of Higgs boson production cross sections and decay branching ratios, parton distribution functions, and off-shell Higgs boson production and interference effects. The second part discusses the recent progress in Higgs effective field theory predictions, followed by the third part on pseudo-observables, simplified template cross section and fiducial cross section measurements, which give the baseline framework for Higgs boson property measurements. The fourth part deals with the beyond the Standard Model predictions of various benchmark scenarios of Minimal Supersymmetric Standard Model, extended scalar sector, Next-to-Minimal Supersymmetric Standard Model and exotic Higgs boson decays. This report follows three previous working-group reports: Handbook of LHC Higgs Cross Sections: 1. Inclusive Observables (CERN-2011-002), Handbook of LHC Higgs Cross Sections: 2. Differential Distributions (CERN-2012-002), and Handbook of LHC Higgs Cross Sections: 3. Higgs properties (CERN-2013-004). The current report serves as the baseline reference for Higgs physics in LHC Run 2 and beyond.

A thin single-element THGEM-based, Resistive-Plate WELL (RPWELL) detector was operated with 150 GeV/c muon and pion beams in Ne/(5%CH₄), Ar/(5%CH₄) and Ar/(7%CO₂); signals were recorded with 1 cm² square pads and SRS/APV25 electronics. Detection efficiency values greater than 98% were reached in all the gas mixtures, at average pad multiplicity of 1.2. The use of the 10⁹ Ω cm resistive plate resulted in a completely discharge-free operation also in intense pion beams. The efficiency remained essentially constant at 9899% up to fluxes of ∼10⁴ Hz/cm², dropping by a few % when approaching 10⁵ Hz/cm². These results pave the way towards cost-effective, robust, efficient, large-scale detectors for a variety of applications in future particle, astro-particle and applied fields. A potential target application is digital hadron calorimetry.

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}.

In-beam evaluation of a fully-equipped medium-size 30 30 cm² Resistive Plate WELL (RPWELL) detector is presented. It consists here of a single element gas-Avalanche multiplier with Semitron ESD225 resistive plate, 1 cm² readout pads and APV25/SRS electronics. Similarly to previous results with small detector prototypes, stable operation at high detection efficiency (> 98%) and low average pad multiplicity (1:2) were recorded with 150 GeV muon and high-rate pion beams, in Ne=(5%CH4), Ar=(5%CH4) and Ar=(7%CO2). This is an important step towards the realization of robust detectors suitable for applications requiring large-Area coverage; among them Digital Hadron Calorimetry.

This document determines the constraints placed by the ATLAS experiment on general gauge mediated (GGM) supersymmetric models. The GGM parameters are chosen in such a way that the constraints from the observed Higgs mass are satisfied. Three varied parameters (μ, M2 and tanβ) determine the phenomenology at the LHC, featuring the lightest wino-higgsino mixture neutralinos and charginos decaying to the gravitino and W, Z, Higgs bosons or photons. Constraints from existing ATLAS searches using the full Run 1 dataset of 20.3 fb⁻¹ at √s=8TeV and targeting a variety of final states with multiple leptons or photons are evaluated. Results of different analyses are statistically combined, providing stringent limits on the three theoretical parameters.

This paper describes the data acquisition and high level trigger system of the ATLAS experiment at the Large Hadron Collider at CERN, as deployed during Run 1. Data flow as well as control, configuration and monitoring aspects are addressed. An overview of the functionality of the system and of its performance is presented and design choices are discussed.

Hadrons have finite interaction size with dense material, a basic feature common to known forms of hadronic calorimeters (HCAL). We argue that substructure variables cannot use HCAL information to access the microscopic nature of jets much narrower than the hadronic shower size, which we call superboosted massive jets. It implies that roughly 15% of their transverse energy profile remains inaccessible due to the presence of long-lived neutral hadrons. This part of the jet substructure is also subject to order-one fluctuations. We demonstrate that the effects of the fluctuations are not reduced when a global correction to jet variables is applied. The above leads to fundamental limitations in the ability to extract intrinsic information from jets in the superboosted regime. The neutral fraction of a jet is correlated with its flavor. This leads to an interesting and possibly useful difference between superboosted W/. Z/. h/. t jets and their corresponding backgrounds. The QCD jets that form the background to the signal superboosted jets might also be qualitatively different in their substructure as their mass might lie at or below the Sudakov mass peak. Finally, we introduce a set of zero-cone longitudinal jet substructure variables and show that while they carry information that might be useful in certain situations, they are not in general sensitive to the jet substructure.

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

We present the results of the first in-beam studies of a medium size (10 10 cm ² ) Resistive-Plate WELL (RPWELL): a single-sided THGEM coupled to a pad anode through a resistive layer of high bulk resistivity (∼10⁹ Ωcm). The 6.2 mm thick (excluding readout electronics) single-stage detector was studied with 150 GeV muons and pions. Signals were recorded from 1×1 cm ² square copper pads with APV25-SRS readout electronics. The single-element detector was operated in Ne/(5%CH₄) at a gas gain of a few times 10⁴ , reaching 99% detection efficiency at average pad multiplicity of ∼1.2. Operation at particle fluxes up to ∼10⁴ Hz/cm ² resulted in ∼23% gain drop leading to ∼5% efficiency loss. The striking feature was the discharge-free operation, also in intense pion beams. These results pave the way towards robust, efficient large-scale detectors for applications requiring economic solutions at moderate spatial and energy resolutions.

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.

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.

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.

We introduce a new method to search for the lepton-flavor violating Higgs decays h → τ^±μ(-/+) and h -> τ^±e(-/+) in the leptonic τ decay channel. In particular, the Standard Model background is estimated in a fully data driven way. The method exploits the asymmetry between electrons and muons in the final state of signal events and is sensitive to differences in the rates of the two decays. Using this method, we investigate the LHC sensitivity to these processes. With 20 fb^-1 of data at √s = 8 TeV, we expect a 3 σ sensitivity for observing branching ratios of order 0.9%. The method and the suggested statistical treatment are discussed in detail.

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.

In this work we investigate three variants of single amplification-stage detector elements; they comprise THGEM electrodes closed at their bottom with metallic or resistive anodes to form WELL-type configurations. We present the results of a comparative study of the Thick-WELL (THWELL), Resistive-WELL (RWELL) and Segmented Resistive WELL (SRWELL) focusing on their performance in terms of spark-quenching capability, gain variation with position and counting rate, pulse shapes and signal propagation to neighboring readout pads; the study included both 30 × 30 and 100 × 100 mm² detectors. It was shown that the WELL structures with resistive anodes offer stable operation even in a highly ionizing environment with effective spark quenching, as well as higher gain than the standard THGEM/inductiongap configuration. Cross talk between neighboring readout pads (here 10 × 10 mm² in size) was shown to be effectively eliminated in the segmented detector with a conductive grid underneath the resistive layer. The latter multiplier should allow for the design of very thin detectors, e.g. sampling elements in digital hadronic calorimeters planned for experiments in future linear colliders.

A search for the production of weakly coupled supersymmetric particles decaying into final states with at least two hadronically decaying taus and missing transverse momentum is presented in this poster. The complete 2012 data sample of √s =8 TeV proton-proton collisions is used in the analysis, which was recorded with the ATLAS detector at the CERN Large Hadron Collider corresponding to a total integrated luminosity of 20.7 fb⁻¹. Exclusion limit at 95% confidence level are derived in the context of the phenomenological Minimal Supersymmetric Standard Model and Simplified Model, characterized by the presence of low mass staus.

A simple methodology for evaluating the dynamic-range of gas avalanche detectors in the laboratory is presented and discussed. It comprises two tools: a charge injector of tunable gain which transfers radiation-induced amplified electron swarms to the investigated detector to mimic events with well defined primary-ionization spectra, and a systematic approach for measuring the detector's discharge probability. The methodology, applicable to a broad range of detectors, is applied here for instability studies in various single-stage THGEM and THGEM-WELL structures. The results indicate upon a somewhat larger attainable dynamic range in a single-stage THGEM operated with additional multiplication in the induction gap.

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.

The Thick Gaseous Electron Multiplier (THGEM) is a simple and robust electrode suitable for large area detectors. In this work the results of extensive comparative studies of the physical properties of different THGEM-based structures are reviewed. The focus is on newly suggested THGEM-like WELL configurations as well as on recently developed characterization methods. The WELL structures are single-sided THGEM electrodes directly coupled to different anode readout electrodes; they differ by the coupling concept of the bottom THGEM electrode to the metallic readout pads. The results are compared to that of traditional double-sided THGEM electrodes followed by induction gaps - in some cases with moderate additional multiplication within the gap. We compare the different configurations in terms of gain, avalanche extension, discharge-rate and magnitude as well as rate capabilities over a broad dynamic range - exploiting a method that mimics highly ionizing particles in the laboratory. We report on recent studies of avalanche distribution in THGEM holes using optical readout.

We present the results of first studies of the Resistive Plate WELL (RPWELL): a single-faced THGEM coupled to a copper anode via a resistive layer of high bulk resistivity. We explored various materials of different bulk resistivity (10⁹-10¹² Ωcm) and thickness (0.4-4 mm). Our most successful prototype, with a 0.6 mm resistive plate of ∼ 10 ⁹ Ωcm, achieved gains of up to 10⁵ with 8 keV x-ray in Ne/5%CH₄; a minor 30% gain drop occurred with a rate increase from 10 to 10⁴ Hz/mm². The detector displayed a full "discharge-free" operation - even when exposed to high primary ionization events. We present the RPWELL detector concept and compare its performance to that of other previously explored THGEM configurations - in terms of gain, its curves, dependence on rate, and the response to high ionization. The robust Resistive Plate WELL concept is a step forward in the Micro-Pattern Gas-Detector family, with numerous potential applications.

Optical recording of avalanche-induced photons is an interesting tool for studying basic physics processes in gaseous detectors. In this work we demonstrate the potential of optical readout in avalanche-propagation investigations in Thick Gas Electron Multipliers (THGEMs) operated with Ne/CF₄ (95/5). We present the results of direct measurements with single- and cascaded-THGEM detectors irradiated with soft x-rays, of the hole-multiplicity and avalanche asymmetry within holes as a function of detector parameters. Further study directions are discussed.

Beam studies of thin single- and double-stage THGEM-based detectors are presented. Several 10 × 10 cm² configurations with a total thickness of 5-6 mm (excluding readout electronics), with 1 × 1 cm ² pads inductively coupled through a resistive layer to APV-SRS readout electronics, were investigated with muons and pions. Detection efficiencies in the 98% range were recorded with an average pad-multiplicity of ∼ 1.1. The resistive anode resulted in efficient discharge damping, with few-volt potential drops; discharge probabilities were ∼ 10^-7 for muons and 10^-6 for pions in the double-stage configuration, at rates of a few kHz/cm². These results, together with the robustness of THGEM electrodes against spark damage and their suitability for economic production over large areas, make THGEM-based detectors highly competitive compared to the other technologies considered for the SiD-DHCAL.

In order to benefit from the expected high luminosity performance that will be provided by the Phase-I upgraded LHC, the first station of the ATLAS muon end-cap system (Small Wheel,SW) will need to be replaced. The New Small Wheel (NSW) will have to operate in a high background radiation region (up to 15 kHz=cm²) while reconstructing muon tracks with high precision, as well as furnishing information for the Level-1 trigger. These performance criteria are demanding. In particular, the precision reconstruction of tracks for offline analysis requires a spatial resolution of about 100microns, and the Level-1 trigger track segments have to be reconstructed online with an angular resolution of approximately 1mrad. The NSW will have two chamber technologies, one primarily devoted to the Level-1 trigger function (small-strip Thin Gap Chambers, sTGC) and one dedicated to precision tracking (Micromegas detectors, MM). The sTGC are primarily deployed for triggering given their single bunch crossing identification capability. The MM detectors have exceptional precision tracking capabilities due to their small gap (5mm) and strip pitch (approximately 0.5mm). Such a precision is crucial to maintain the current ATLAS muon momentum resolution in the high background environment of the upgraded LHC. The MM chambers can, at the same time, confirm the existence of track segments found by the muon end-cap middle station (Big Wheels) online. The sTGC also has the ability to measure offline muon tracks with good precision, so the sTGC-MM chamber technology combination forms a fully redundant detector system for triggering and tracking both for online and offline functions. This detector combination has been designed to be able to also provide excellent performance for the eventual High Luminosity LHC upgrade.

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.

Thick Gas Electron Multipliers (THGEMs) have the potential of constituting thin, robust sampling elements in Digital Hadron Calorimetry (DHCAL) at future colliders. We report on recent beam studies of new single-and double-THGEM-like structures: the multiplier is a Segmented Resistive WELL (SRWELL)-a single-faced THGEM in contact with a segmented resistive layer inductively coupled to readout pads. Several 10×10 cm² configurations with a total thickness of 5-6 mm (excluding electronics) with 1 cm² pads were investigated with muons and pions. The pads were coupled to a scalable readout system APV chip, APV-SRS (Raymond et al. [22]). Detection efficiencies in the 98% range were recorded with an average pad-multiplicity of ∼1.1. The resistive anode resulted in efficient discharge damping, with potential drops of a few volts; the discharge probabilities were ∼10^-7 for muons and ∼10^-6 for pions, at rates of a few kHz/cm² and for detectors in the double-stage configuration. Further optimization work and research on larger detectors are underway.

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

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.

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 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.

A search for the direct production of charginos and neutralinos in final states with three leptons (electrons or muons) and missing transverse momentum is presented. The analysis uses 13.0 fb−1 of proton-proton collision data delivered by the LHC at √s = 8 TeV and recorded by the ATLAS detector. No excess above the Standard Model expectation is observed in three signal regions that are either enriched or depleted in Z-boson decays. Limits are placed at the 95% confidence level on the masses of the charginos and neutralinos in simplified models and on the parameters of the phenomenological Minimal Supersymmetric Standard Model. In simplified models, chargino masses are excluded up to 580 GeV in the presence of light sleptons and in the range 150300 GeV in case of heavy sleptons for a massless lightest neutralino, significantly extending previous results.

Results of a search for supersymmetry in events with four or more leptons (electrons or muons) are presented. The analysis uses a sample corresponding to 13.0 fb⁻¹ of proton-proton collision data delivered by the LHC at √s = 8 TeV and recorded by the ATLAS detector. Two signal regions are defined with a selection vetoing Z boson production. The first signal region has large missing transverse momentum (Etmiss > 50GeV), where 1 event is observed while 0.25+0.29-0.25 are expected from Standard Model processes. The second requires large total effective mass (meff > 300GeV), where 2 events are observed while 1.2±0.5 are expected from Standard Model processes. Limits are placed on various R-parity violating simplified models, where the lightest supersymmetric particle (chi_1^0) decays promptly to first- and second-generation leptons. Considering one other sparticle at a time, pair-produced and decaying directly to a chi_1^0, charged Winos are excluded up to a mass of ∼ 710 GeV, left-handed sleptons up to ∼ 450 GeV, sneutrinos up to ∼ 410 GeV and gluinos up to ∼ 1300 GeV.

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.

Searches for various super-symmetry signatures were performed using a data sample of approximately 35ipb from pp collisions at sqrts=7TeV collected by the ATLAS detector at the LHC in 2010. No excess was observed above the estimated background in any of the searches. In absence of signal the results were interpreted into limits, most of them most stringent to date.

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.

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.

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.

Long lived charged particles are predicted by many models of physics beyond the standard model (SM). The common signature of such models is a heavy long-lived charged particle with velocity smaller than the speed of light, β

In certain Supersymmetry breaking scenarios, characteristic signatures can beexpected which would not necessarily be found in generic SUSY searches forevents containing high pT multi-jets and large missing transverse energy. Thispaper describes the expected response of the ATLAS detector to four signatures: high-pT photons which may or may not appear to point back to theprimary collision vertex and long-lived charged sleptons and R hadrons. Suchprocesses often have the advantage of small Standard Model backgroundsand their observation could provide unique constraints on the different SUSYbreaking scenarios. Using these signatures discovery potentials are estimatedfor either Gauge-Mediated Supersymmetry Breaking or Split-Supersymmetryscenarios. Using Monte Carlo samples of SUSY and background processescorresponding to integrated luminosity of about 1 fb⁻¹ we study all aspectsof the analysis, including the expected trigger response and offline data reconstruction.

If long lived charged particles exist, and produced at the LHC, they may travel with velocity significantly slower than the speed of light. This unique signature was not considered during the design of the LHC experiments, ATLAS and CMS. As a result, hardware and trigger capabilities need to be evaluated. Model independent approaches for finding long lived particles with the LHC experiments are introduced. They are tested using two bench marks, one in GMSB and one in Split SUSY. The focus is on hardware and trigger issues, as well as reconstruction methods developed by ATLAS and CMS. Both experiments suggest time of flight (TOF) based methods. However, the implementation is different. In ATLAS a first beta estimation is done already at the trigger level. CMS also uses dE/dx to estimate beta.

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

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

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

The ATLAS Muon Spectrometer (ATLAS Collaboration, ATLAS Muon Spectrometer Technical Design Report CERN/LHCC/97-22, ATLAS TDR 10, 1997.) will use dedicated detectors to trigger on muons and to identify the bunch-crossing at the appropriate rate. The Spectrometer has been designed to perform stand-alone triggering and measurement of muon transverse momentum up to 1 TeV with good resolution (from 3% up to 10% at 1 TeV). The magnetic system is composed of three large superconducting air-core toroids instrumented with trigger and high-precision tracking chambers, a central part (barrel) composed of eight coils and two end-cap magnets.The high-precision tracking system is based on Monitored Drift Tube (MDT) and Cathode Strip Chambers (CSC) in the small angle-regions. The Level-1 trigger is provided by Resistive Plate Chambers (RPC) in the barrel and Thin Gap Chambers (TGC) in the end-cap. These detectors will also measure the track coordinates in the magnetic field direction (second coordinate), to complement the precision tracking provided by the MDT which only measure the track coordinates in the bending direction of the magnetic field. The trigger system covers an area of 3650 m² in the barrel and 2900 m 2 in the end-cap. In the barrel region three double-gap RPC stations are used, two in the middle and one in the outer MDT chamber layer. In the end-cap region one triple-gap TGC station is used, in front of the middle MDT station, and two double-gap TGC stations behind it. The mass production of both systems is under way.The systems were involved in extensive beam tests in 20022003, testing their compliance with LHC timing requirements using 25ns beam bunching to emulate the LHC beam structure, aging under critical environment conditions and so on.

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

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