Explore the vast range of titles available.

Processing and scientific analysis of the data taken by the ATLAS experiment requires reliable information describing the event data recorded by the detector or generated in software. ATLAS event processing applications store such descriptive metadata information in the output data files along with the event information. To better leverage the available computing resources during LHC Run3 the ATLAS experiment has migrated its data processing and analysis software to a multi-threaded framework: AthenaMT. Therefore in-file metadata must support concurrent event processing, especially around input file boundaries. The in-file metadata handling software was originally designed for serial event processing. It grew into a rather complex system over the many years of ATLAS operation. To migrate this system to the multi-threaded environment it was necessary to adopt several pragmatic solutions, mainly because of the shortage of available person-power to work on this project in early phases of the AthenaMT development. In order to simplify the migration, first the redundant parts of the code were cleaned up wherever possible. Next the infrastructure was improved by removing reliance on constructs that are problematic during multi-threaded processing. Finally, the remaining software infrastructure was redesigned for thread safety.

The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of20.1 \\rm{fb}{⁻¹}{}of proton-proton collision data at√s̅=8TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay viat̃ → t χ̃₁⁰ort̃→ bχ̃₁^(±) → b W^(\\left{(}{∗}{\\right)}) χ̃₁⁰ , whereχ̃₁⁰( χ̃₁^(±) ) denotes the lightest neutralino (chargino) in supersymmetric models. The search targets a fully-hadronic final state in events with four or more jets and large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits are reported in terms of the top squark and neutralino masses and as a function of the branching fraction oft̃ → t χ̃₁⁰ . For a branching fraction of 100%, top squark masses in the range 270-645 GeV are excluded forχ̃₁⁰masses below 30 GeV. For a branching fraction of 50% to eithert̃ → t χ̃₁⁰ort̃→ bχ̃₁^(±) , and assuming theχ̃₁^(±)mass to be twice theχ̃₁⁰mass, top squark masses in the range 250-550 GeV are excluded forχ̃₁⁰masses below 60 GeV.

We search for evidence of physics beyond the Standard Model in the production of final states with multiple high transverse momentum jets, using 20.3 fb ⁻¹of proton-proton collision data recorded by the ATLAS detector at√s̅ = 8TeV. No excess of events beyond Standard Model expectations is observed, and upper limits on the visible cross-section for non-Standard Model production of multi-jet final states are set. Using a wide variety of models for black hole and string ball production and decay, the limit on the cross-section times acceptance is as low as 0.16 fb at the 95% CL for a minimum scalar sum of jet transverse momentum in the event of about 4.3 TeV. Using models for black hole and string ball production and decay, exclusion contours are determined as a function of the production mass threshold and the gravity scale. These limits can be interpreted in terms of lower-mass limits on black hole and string ball production that range from 4.6 to 6.2 TeV.

A measurement of the splitting scales occuring in thek_(\\mathrm{t}{}{j})et-clustering algorithm is presented for final states containing aZboson. The measurement is done using 20.2 fb ⁻¹of proton-proton collision data collected at a centre-of-mass energy of√s̅ = 8TeV by the ATLAS experiment at the LHC in 2012. The measurement is based on charged-particle track information, which is measured with excellent precision in thep_(\\mathrm{T}{}{r})egion relevant for the transition between the perturbative and the non-perturbative regimes. The data distributions are corrected for detector effects, and are found to deviate from state-of-the-art predictions in various regions of the observables.

Differential cross sections are presented for the prompt and non-prompt production of the hidden-charm states$X(3872)$and$\\psi(2S)$ , in the decay mode$J/\\psi \\pi^+\\pi^-$ , measured using 11.4 fb $^{-1}$of$pp$collisions at$\\sqrt{s} = 8$TeV by the ATLAS detector at the LHC. The ratio of cross-sections$X(3872)/\\psi(2S)$is also given, separately for prompt and non-prompt components, as well as the non-prompt fractions of$X(3872)$and$\\psi(2S)$ . Assuming independent single effective lifetimes for non-prompt$X(3872)$and$\\psi(2S)$production gives$R_B = \\frac{\\mathcal{B}(B \\rightarrow X(3872)\\textrm{ + any}) \\mathcal{B}(X(3872 \\rightarrow J/\\psi\\pi^+\\pi^-)}{\\mathcal{B}(B \\rightarrow \\psi(2S)\\textrm{ + any}) \\mathcal{B}(\\psi(2S) \\rightarrow J/\\psi\\pi^+\\pi^-)} = (3.95 \\pm 0.32 \\mathrm{(stat)} \\pm 0.08\\mathrm{(sys)}) \\times 10^{-2}$ , while separating short- and long-lived contributions, assuming that the short-lived component is due to$B_c$decays, gives$R_B = (3.57 \\pm 0.33\\mathrm{(stat)} \\pm 0.11\\mathrm{(sys)}) \\times 10^{-2}$ , with the fraction of non-prompt$X(3872)$produced via$B_c$decays for$p_\\mathrm{T}(X(3872)) > 10$GeV being$(25 \\pm 13\\mathrm{(stat)} \\pm 2\\mathrm{(sys)} \\pm 5\\mathrm{(spin)})\\%$ . The distributions of the dipion invariant mass in the$X(3872)$and$\\psi(2S)$decays are also measured and compared to theoretical predictions.

The number of charged particles inside jets is a widely used discriminant for identifying the quark or gluon nature of the initiating parton and is sensitive to both the perturbative and non-perturbative components of fragmentation. This paper presents a measurement of the average number of charged particles withp_(\\text{T}{>}{5})00MeV inside high-momentum jets in dijet events using 20.3 fb ⁻¹of data recorded with the ATLAS detector inppcollisions at√s̅=8TeV collisions at the LHC. The jets considered have transverse momenta from 50 GeV up to and beyond 1.5 TeV. The reconstructed charged-particle track multiplicity distribution is unfolded to remove distortions from detector effects and the resulting charged-particle multiplicity is compared to several models. Furthermore, quark and gluon jet fractions are used to extract the average charged-particle multiplicity for quark and gluon jets separately.

A measurement of thett̄Zandtt̄Wproduction cross sections in final states with either two same-charge muons, or three or four leptons (electrons or muons) is presented. The analysis uses a data sample of proton-proton collisions at√s̅ = 13TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015, corresponding to a total integrated luminosity of 3.2 fb ⁻¹ . The inclusive cross sections are extracted using likelihood fits to signal and control regions, resulting inσ_(tt̄Z) = 0.9 ± 0.3pb andσ_(tt̄W) = 1.5 ± 0.8pb, in agreement with the Standard Model predictions.

Measurements of the jet activity intt̄events produced in proton–proton collisions at√s̅=8TeV are presented, using20.3 fb ⁻¹of data collected by the ATLAS experiment at the Large Hadron Collider. The events were selected in the dileptoneμdecay channel with two identifiedb -jets. The numbers of additional jets for various jet transverse momentum ( p_(T) ) thresholds, and the normalised differential cross-sections as a function ofp_(T)for the five highest- p_(T)additional jets, were measured in the jet pseudorapidity range|η|<4.5 . The gap fraction, the fraction of events which do not contain an additional jet in a central rapidity region, was measured for several rapidity intervals as a function of the minimump_(T)of a single jet or the scalar sum ofp_(T)of all additional jets. These fractions were also measured in different regions of the invariant mass of theeμ bb̄system. All measurements were corrected for detector effects, and found to be mostly well-described by predictions from next-to-leading-order and leading-ordertt̄event generators with appropriate parameter choices. The results can be used to further optimise the parameters used in such generators.

This paper describes a measurement of fiducial and differential cross sections of gluon-fusion Higgs boson production in theH→ WW^(∗)→ eνμνchannel, using 20.3 fb ⁻¹of proton-proton collision data. The data were produced at a centre-of-mass energy of√s̅ = 8TeV at the CERN Large Hadron Collider and recorded by the ATLAS detector in 2012. Cross sections are measured from the observedH→ WW^(∗)→ eνμνsignal yield in categories distinguished by the number of associated jets. The total cross section is measured in a fiducial region defined by the kinematic properties of the charged leptons and neutrinos. Differential cross sections are reported as a function of the number of jets, the Higgs boson transverse momentum, the dilepton rapidity, and the transverse momentum of the leading jet. The jet-veto efficiency, or fraction of events with no jets above a given transverse momentum threshold, is also reported. All measurements are compared to QCD predictions from Monte Carlo generators and fixed-order calculations, and are in agreement with the Standard Model predictions.

Combined ATLAS and CMS measurements of the Higgs boson production and decay rates, as well as constraints on its couplings to vector bosons and fermions, are presented. The combination is based on the analysis of five production processes, namely gluon fusion, vector boson fusion, and associated production with aWor aZboson or a pair of top quarks, and of the six decay modesH → ZZ, WW ,γγ, ττ, bb , andμμ . All results are reported assuming a value of 125.09 GeV for the Higgs boson mass, the result of the combined measurement by the ATLAS and CMS experiments. The analysis uses the CERN LHC proton–proton collision data recorded by the ATLAS and CMS experiments in 2011 and 2012, corresponding to integrated luminosities per experiment of approximately 5 fb ⁻¹at√s̅=7TeV and 20 fb ⁻¹at√s̅ = 8TeV. The Higgs boson production and decay rates measured by the two experiments are combined within the context of three generic parameterisations: two based on cross sections and branching fractions, and one on ratios of coupling modifiers. Several interpretations of the measurements with more model-dependent parameterisations are also given. The combined signal yield relative to the Standard Model prediction is measured to be 1.09±0.11. The combined measurements lead to observed significances for the vector boson fusion production process and for theH → ττdecay of5.4and5.5standard deviations, respectively. The data are consistent with the Standard Model predictions for all parameterisations considered.