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Öğe The ATLAS Fast TracKer system(Iop Publishing Ltd, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliThe ATLAS Fast TracKer (FTK) was designed to provide full tracking for the ATLAS high-level trigger by using pattern recognition based on Associative Memory (AM) chips and fitting in high-speed field programmable gate arrays. The tracks found by the FTK are based on inputs from all modules of the pixel and silicon microstrip trackers. The as-built FTK system and components are described, as is the online software used to control them while running in the ATLAS data acquisition system. Also described is the simulation of the FTK hardware and the optimization of the AM pattern banks. An optimization for long-lived particles with large impact parameter values is included. A test of the FTK system with the data playback facility that allowed the FTK to be commissioned during the shutdown between Run 2 and Run 3 of the LHC is reported. The resulting tracks from part of the FTK system covering a limited eta-phi region of the detector are compared with the output from the FTK simulation. It is shown that FTK performance is in good agreement with the simulation.Öğe Charged-hadron production in pp, p plus Pb, Pb plus Pb, and Xe plus Xe collisions at ?sNN=5 TeV with the ATLAS detector at the LHC(Springer, 2023) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliThis paper presents measurements of charged-hadron spectra obtained in pp, p+Pb, and Pb+Pb collisions at root s or root s(NN) = 5.02TeV, and in Xe+Xe collisions at root s(NN) = 5.44TeV. The data recorded by the ATLAS detector at the LHC have total integrated luminosities of 25 pb(-1), 28 nb(-1), 0.50 nb(-1), and 3 mu b(-1), respectively. The nuclear modification factors RpPb and R-AA are obtained by comparing the spectra in heavy-ion and pp collisions in a wide range of charged-particle transverse momenta and pseudorapidity. The nuclear modification factor RpPb shows a moderate enhancement above unity with a maximum at p(T) approximate to 3 GeV; the enhancement is stronger in the Pb-going direction. The nuclear modification factors in both Pb+Pb and Xe+Xe collisions feature a significant, centrality-dependent suppression. They show a similar distinct p(T)-dependence with a local maximum at p(T) approximate to 2 GeV and a local minimum at p(T) approximate to 7 GeV. This dependence is more distinguishable in more central collisions. No significant eta |-dependence is found. A comprehensive comparison with several theoretical predictions is also provided. They typically describe RAA better in central collisions and in the pT range from about 10 to 100 GeV.Öğe Configuration and performance of the ATLAS b-jet triggers in Run 2(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abed Abud, A.; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliSeveral improvements to the ATLAS triggers used to identify jets containing b-hadrons (b-jets) were implemented for data-taking during Run 2 of the Large Hadron Collider from 2016 to 2018. These changes include reconfiguring the b-jet trigger software to improve primary-vertex finding and allow more stable running in conditions with high pile-up, and the implementation of the functionality needed to run sophisticated taggers used by the offline reconstruction in an online environment. These improvements yielded an order of magnitude better light-flavour jet rejection for the same b-jet identification efficiency compared to the performance in Run 1 (2011-2012). The efficiency to identify b-jets in the trigger, and the conditional efficiency for b-jets that satisfy offline b-tagging requirements to pass the trigger are also measured. Correction factors are derived to calibrate the b-tagging efficiency in simulation to match that observed in data. The associated systematic uncertainties are substantially smaller than in previous measurements. In addition, b-jet triggers were operated for the first time during heavy-ion data-taking, using dedicated triggers that were developed to identify semileptonic b-hadron decays by selecting events with geometrically overlapping muons and jets.Öğe Determination of the parton distribution functions of the proton from ATLAS measurements of differential W± and Z boson production in association with jets(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abed Abud, A.; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliThis article presents a new set of proton parton distribution functions, AT-LASepWZVjet20, produced in an analysis at next-to-next-to-leading order in QCD. The new data sets considered are the measurements of W+ and W- boson and Z boson production in association with jets in pp collisions at root s = 8TeV performed by the ATLAS experiment at the LHC with integrated luminosities of 20.2 fb(-1) and 19.9 fb(-1), respectively. The analysis also considers the ATLAS measurements of differential W-+/- and Z boson production at root s = 7 TeV with an integrated luminosity of 4.6 fb(-1) and deep-inelastic-scattering data from e(+/-) p collisions at the HERA accelerator. An improved determination of the sea-quark densities at high Bjorken x is shown, while confirming a strange-quark density similar in size to the up- and down-sea-quark densities in the range x less than or similar to 0.02 found by previous ATLAS analyses.Öğe Evidence for Higgs boson decays to a low-mass dilepton system and a photon in pp collisions at ?s=13 TeV with the ATLAS detector(Elsevier, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliA search for the Higgs boson decaying into a photon and a pair of electrons or muons with an invariant mass m(ll) < 30 GeV is presented. The analysis is performed using 139 fb(-1) of proton-proton collision data, produced by the LHC at a centre-of-mass energy of 13 TeV and collected by the ATLAS experiment. Evidence for the H -> ll(gamma) process is found with a significance of 3.2 over the background-only hypothesis, compared to an expected significance of 2.1 for the Standard Model prediction. The best-fit value of the signal-strength parameter, defined as the ratio of the observed signal yield to the one expected in the Standard Model, is mu = 1.5 +/- 0.5. The Higgs boson production cross-section times the H -> ll(gamma) branching ratio for m(ll) < 30 GeV is determined to be 8.7(-2.7)(+2.8) fb. (C) 2021 The Author. Published by Elsevier B.V.Öğe Exclusive dimuon production in ultraperipheral Pb plus Pb collisions at ?SNN=5.02 TeV with ATLAS(Amer Physical Soc, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliExclusive dimuon production in ultraperipheral collisions (UPC), resulting from photon-photon interactions in the strong electromagnetic fields of colliding high-energy lead nuclei, PbPb(gamma gamma) -> mu(+) mu(-) (Pb-(*Pb-)(()*())), is studied using L-int = 0.48 nb(-1) of root S-NN = 5.02 TeV lead-lead collision data at the LHC with the ATLAS detector. Dimuon pairs are measured in the fiducial region p(T,mu) > 4 GeV, vertical bar eta(mu)vertical bar < 2.4, invariant m(mu mu) > 10 GeV, and p(T,mu mu) <2 GeV. The primary background from single-dissociative processes is extracted from the data using a template fitting technique. Differential cross sections are presented as a function of m(mu mu), absolute pair rapidity (vertical bar y(mu mu)vertical bar), scattering angle in the dimuon rest frame (vertical bar cos v*(mu mu)vertical bar), and the colliding photon energies. The total cross section of the UPC gamma gamma -> mu(+) mu(-) process in the fiducial volume is measured to be sigma(mu mu)(fid) = 34.1 +0.3(stat.)+0.7(syst.) mu b. Generally good agreement is found with calculations from STARlight, which incorporate the leading-order Breit-Wheeler process with no final-state effects, albeit differences between the measurements and theoretical expectations are observed. In particular, the measured cross sections at larger vertical bar y(mu mu)vertical bar are found to be about 10-20% larger in data than in the calculations, suggesting the presence of larger fluxes of photons in the initial state. Modification of the dimuon cross sections in the presence of forward and/or backward neutron production is also studied and is found to be associated with a harder incoming photon spectrum, consistent with expectations.Öğe Higgs boson production cross-section measurements vand their EFT interpretation in the 4l decay channel at ?s = 13 TeV with the ATLAS detector (vol 80, 957, 2020)(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant Ali[Abstract Not Available]Öğe Higgs boson production cross-section measurements vand their EFT interpretation in the 4l decay channel at ?s =13 TeV with the ATLAS detector (vol 80, 10.1140/epjc/s10052-020-8227-9, 2020)(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant Ali[Abstract Not Available]Öğe Jet energy scale and resolution measured in proton-proton collisions at s=13 TeV with the ATLAS detector(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliJet energy scale and resolution measurements with their associated uncertainties are reported for jets using 36-81 fb-1 of proton-proton collision data with a centre-of-mass energy of s=13 TeV collected by the ATLAS detector at the LHC. Jets are reconstructed using two different input types: topo-clusters formed from energy deposits in calorimeter cells, as well as an algorithmic combination of charged-particle tracks with those topo-clusters, referred to as the ATLAS particle-flow reconstruction method. The anti-kt jet algorithm with radius parameter R=0.4 is the primary jet definition used for both jet types. This result presents new jet energy scale and resolution measurements in the high pile-up conditions of late LHC Run 2 as well as a full calibration of particle-flow jets in ATLAS. Jets are initially calibrated using a sequence of simulation-based corrections. Next, several in situ techniques are employed to correct for differences between data and simulation and to measure the resolution of jets. The systematic uncertainties in the jet energy scale for central jets (|eta|<1.2) vary from 1% for a wide range of high-pT jets (250<2000 GeV), to 5% at very low pT (20 GeV) and 3.5% at very high pT (>2.5 TeV). The relative jet energy resolution is measured and ranges from (24 +/- 1.5)% at 20 GeV to (6 +/- 0.5)% at 300 GeV.Öğe Longitudinal Flow Decorrelations in Xe plus Xe Collisions at ?sNN =5.44 TeV with the ATLAS Detector(Amer Physical Soc, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliThe first measurement of longitudinal decorrelations of harmonic flow amplitudes v(n) for n = 2-4 in Xe + Xe collisions at root s(NN) = 5.44 TeV is obtained using 3 mu b(-1) of data with the ATLAS detector at the LHC. The decorrelation signal for v(3) and v(4) is found to be nearly independent of collision centrality and transverse momentum (p(T)) requirements on fmal-state particles, but for v(2) a strong centrality and p(T) dependence is seen. When compared with the results from Pb + Pb collisions at. root s(NN) = 5.02 TcV, the longitudinal decorrelation signal in midcentral Xe + Xe collisions is found to be larger for v(2), but smaller for v(3). Current hydrodynamic models reproduce the ratios of the v(n) measured in Xe + Xe collisions to those in Pb + Pb collisions but fail to describe the magnitudes and trends of the ratios of longitudinal flow decorrelations between Xe + Xe and Pb + Pb. The results on the system-size dependence provide new insights and an important lever arm to separate effects of the longitudinal structure of the initial state from other early and late time effects in heavy-ion collisions.Öğe Measurement of b-quark fragmentation properties in jets using the decay B± ? J/?K± in pp collisions at ?s=13 TeV with the ATLAS detector(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliThe fragmentation properties of jets containing b-hadrons are studied using charged B mesons in 139fb(-1) of pp collisions at root s = 13 TeV, recorded with the ATLAS detector at the LHC during the period from 2015 to 2018. The B mesons are reconstructed using the decay of B-+/- into J/psi K-+/-, with the J/psi decaying into a pair of muons. Jets are reconstructed using the anti-k(t) algorithm with radius parameter R = 0.4. The measurement determines the longitudinal and transverse momentum profiles of the reconstructed B hadrons with respect to the axes of the jets to which they are geometrically associated. These distributions are measured in intervals of the jet transverse momentum, ranging from 50 GeV to above 100 GeV. The results are corrected for detector effects and compared with several Monte Carlo predictions using different parton shower and hadronisation models. The results for the longitudinal and transverse profiles provide useful inputs to improve the description of heavy-flavour fragmentation in jets.Öğe Measurement of hadronic event shapes in high-pT multijet final states at ?s = 13 TeV with the ATLAS detector (vol , 188, 2021)(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abed Abud, A.; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliAn error in the normalization of the Aplanarity and D-parameter has been found. Insufficient precision on the eigenvalue calculation can cause lambda(3) to be negative, exclusively in two-jet events, yielding an underestimation of the inclusive two-jet cross section.Öğe Measurement of light-by-light scattering and search for axion-like particles with 2.2 nb-1 of Pb+Pb data with the ATLAS detector(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliThis paper describes a measurement of light-by-light scattering based on Pb+Pb collision data recorded by the ATLAS experiment during Run 2 of the LHC. The study uses 2.2 nb(-1) of integrated luminosity collected in 2015 and 2018 at root sNN = 5.02TeV. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy E-T(gamma) > 2.5 GeV, pseudorapidity vertical bar eta(gamma)vertical bar < 2.37, diphoton invariant mass m(gamma gamma) > 5 GeV, and with small diphoton transverse momentum and diphoton acoplanarity. The integrated and differential fiducial cross sections are measured and compared with theoretical predictions. The diphoton invariant mass distribution is used to set limits on the production of axion-like particles. This result provides the most stringent limits to date on axion-like particle production for masses in the range 6-100 GeV. Cross sections above 2 to 70 nb are excluded at the 95% CL in that mass interval.Öğe Measurement of light-by-light scattering and search for axion-like particles with 2.2 nb-1 of Pb+Pb data with the ATLAS detector (vol 2021, 243, 2021)(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abed Abud, A.; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant Ali[Abstract Not Available]Öğe Measurement of single top-quark production in association with a W boson in the single-lepton channel at ?s=8 TeV with the ATLAS detector(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliThe production cross-section of a top quark in association with a W boson is measured using proton-proton collisions at root s = 8 TeV. The dataset corresponds to an integrated luminosity of 20.2 fb(-1,) and was collected in 2012 by the ATLAS detector at the Large Hadron Collider at CERN. The analysis is performed in the single-lepton channel. Events are selected by requiring one isolated lepton (electron or muon) and at least three jets. A neural network is trained to separate the tW signal from the dominant t (t) over bar background. The cross-section is extracted from a binned profile maximum-likelihood fit to a two-dimensional discriminant built from the neural-network output and the invariant mass of the hadronically decaying W boson. The measured crosssection is sigma(tW) = 26 +/- 7 pb, in good agreement with the Standard Model expectation.Öğe Measurement of the CP-violating phase ?s in Bs0 ? J/?? decays in ATLAS at 13 TeV(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliA measurement of the B-0(s) -> J/psi phi decay parameters using 80.5 fb(-1) of integrated luminosity collected with the ATLAS detector from 13 TeV proton-proton collisions at the LHC is presented. The measured parameters include the CP-violating phase phi(s), the width difference Delta Gamma(s) between the B-s(0) meson mass eigenstates and the average decay width Gamma(s). The values measured for the physical parameters are combined with those from 19.2 fb(-1) of 7 and 8 TeV data, leading to the following: phi(s) = -0.087 +/- 0.036 (stat.) +/- 0.021 (syst.) rad Delta Gamma(s) = 0.0657 +/- 0.0043 (stat.) +/- 0.0037 (syst.) ps(-1) Gamma(s) = 0.6703 +/- 0.0014 (stat.) +/- 0.0018 (syst.) ps(-1) Results for phi(s) and Delta Gamma(s) are also presented as 68% confidence level contours in the phi(s)-Delta Gamma(s) plane. Furthermore the transversity amplitudes and corresponding strong phases are measured. phi(s) and Delta Gamma(s) measurements are in agreement with the Standard Model predictions.Öğe Measurement of the production cross section of pairs of isolated photons in pp collisions at 13 TeV with the ATLAS detector(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abed Abud, A.; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliA measurement of prompt photon-pair production in proton-proton collisions at root s = 13 TeV is presented. The data were recorded by the ATLAS detector at the LHC with an integrated luminosity of 139 fb(-1). Events with two photons in the wellinstrumented region of the detector are selected. The photons are required to be isolated and have a transverse momentum of p(T,gamma 1(2)) > 40 (30) GeV for the leading (sub-leading) photon. The differential cross sections as functions of several observables for the diphoton system are measured and compared with theoretical predictions from state-of-the-art Monte Carlo and fixed-order calculations. The QCD predictions from next-to-next-to-leadingorder calculations and multi-leg merged calculations are able to describe the measured integrated and differential cross sections within uncertainties, whereas lower-order calculations show significant deviations, demonstrating that higher-order perturbative QCD corrections are crucial for this process. The resummed predictions with parton showers additionally provide an excellent description of the low transverse-momentum regime of the diphoton system.Öğe Measurement of the relative Bc±/B± production cross section with the ATLAS detector at ?s=8 TeV(Amer Physical Soc, 2021) Aaboud, M.; Aad, G.; Abbott, B.; Abbott, D. C.; Abdinov, O.; Abud, A. Abed; Çetin, Serkant AliThe total cross section and differential cross sections for the production of B-c(+/-) mesons, times their branching fraction to J/psi pi(+/-), are measured relative to those for the production of B-+/- mesons, times their branching fraction to J/psi K-+/-. The data used for this study correspond to an integrated luminosity of 20.3 fb(-1) of pp collisions recorded by the ATLAS detector at the Large Hadron Collider in 2012 at a center-of-mass energy of root s = 8 TeV. The measurement is performed differentially in bins of transverse momentum p(T) for 13 GeV < p(T)(B-c(+/-)) < 22 GeV and p(T)(B-c(+/-)) > 22 GeV and in bins of rapidity y for vertical bar y vertical bar < 0.75 and 0.75 < vertical bar y vertical bar < 2.3. The relative cross section times branching fraction for the full range p(T) > 13 GeV and vertical bar y vertical bar < 2.3 is (0.34 +/- 0.04(stat) (+0.06)(-0.02 sys) +/- 0.01(lifetime))%. The differential measurements suggest that the production cross section of the B-c(+/-) decreases faster with p(T) than the production cross section of the B-+/-, while no significant dependence on rapidity is observed.Öğe Measurement of the t(t)over-bart(t)over-bar production cross section in pp collisions at ?s=13 TeV with the ATLAS detector(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliA measurement of four-top-quark production using proton-proton collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the Large Hadron Collider corresponding to an integrated luminosity of 139 fb(-1) is presented. Events are selected if they contain a single lepton (electron or muon) or an opposite-sign lepton pair, in association with multiple jets. The events are categorised according to the number of jets and how likely these are to contain b-hadrons. A multivariate technique is then used to discriminate between signal and background events. The measured four-top-quark production cross section is found to be 26(-15)(+17) fb, with a corresponding observed (expected) significance of 1.9 (1.0) standard deviations over the background-only hypothesis. The result is combined with the previous measurement performed by the ATLAS Collaboration in the multilepton final state. The combined four-top-quark production cross section is measured to be 24(-6)(+7) fb, with a corresponding observed (expected) signal significance of 4.7 (2.6) standard deviations over the background-only predictions. It is consistent within 2.0 standard deviations with the Standard Model expectation of 12.0 +/- 2.4 fb.Öğe Measurements of differential cross-sections in four-lepton events in 13 TeV proton-proton collisions with the ATLAS detector(Springer, 2021) Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Çetin, Serkant AliMeasurements of four-lepton differential and integrated fiducial cross-sections in events with two same-flavour, opposite-charge electron or muon pairs are presented. The data correspond to 139 fb(-1) of root s = 13 TeV proton-proton collisions, collected by the ATLAS detector during Run 2 of the Large Hadron Collider (2015-2018). The final state has contributions from a number of interesting Standard Model processes that dominate in different four-lepton invariant mass regions, including single Z boson production, Higgs boson production and on-shell ZZ production, with a complex mix of interference terms, and possible contributions from physics beyond the Standard Model. The differential cross-sections include the four-lepton invariant mass inclusively, in slices of other kinematic variables, and in different lepton flavour categories. Also measured are dilepton invariant masses, transverse momenta, and angular correlation variables, in four regions of four-lepton invariant mass, each dominated by different processes. The measurements are corrected for detector effects and are compared with state-of-the-art Standard Model calculations, which are found to be consistent with the data. The Z -> 4l branching fraction is extracted, giving a value of (4.41 +/- 0.30) x 10(-6). Constraints on effective field theory parameters and a model based on a spontaneously broken B - L gauge symmetry are also evaluated. Further reinterpretations can be performed with the provided information.