The large rate of multiple simultaneous proton-proton interactions, or pile-up, generated by the Large Hadron Collider in Run 1 required the development of many new techniques to mitigate the adverse ...effects of these conditions. This paper describes the methods employed in the ATLAS experiment to correct for the impact of pile-up on jet energy and jet shapes, and for the presence of spurious additional jets, with a primary focus on the large 20.3 Formula: see text data sample collected at a centre-of-mass energy of Formula: see text. The energy correction techniques that incorporate sophisticated estimates of the average pile-up energy density and tracking information are presented. Jet-to-vertex association techniques are discussed and projections of performance for the future are considered. Lastly, the extension of these techniques to mitigate the effect of pile-up on jet shapes using subtraction and grooming procedures is presented.
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This paper presents measurements of distributions of charged particles which are produced in proton-proton collisions at a centre-of-mass energy of Formula: see text and recorded by the ATLAS ...detector at the LHC. A special dataset recorded in 2012 with a small number of interactions per beam crossing (below 0.004) and corresponding to an integrated luminosity of 160 Formula: see text was used. A minimum-bias trigger was utilised to select a data sample of more than 9 million collision events. The multiplicity, pseudorapidity, and transverse momentum distributions of charged particles are shown in different regions of kinematics and charged-particle multiplicity, including measurements of final states at high multiplicity. The results are corrected for detector effects and are compared to the predictions of various Monte Carlo event generator models which simulate the full hadronic final state.
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Searches for pair-produced scalar leptoquarks are performed using 20 fbFormula: see text of proton-proton collision data provided by the LHC and recorded by the ATLAS detector at Formula: see ...text TeV. Events with two electrons (muons) and two or more jets in the final state are used to search for first (second)-generation leptoquarks. The results from two previously published ATLAS analyses are interpreted in terms of third-generation leptoquarks decaying to Formula: see text and Formula: see text final states. No statistically significant excess above the Standard Model expectation is observed in any channel and scalar leptoquarks are excluded at 95 % CL with masses up to Formula: see text 1050 GeV for first-generation leptoquarks, Formula: see text 1000 GeV for second-generation leptoquarks, Formula: see text 625 GeV for third-generation leptoquarks in the Formula: see text channel, and 200 Formula: see text 640 GeV in the Formula: see text channel.
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Event-shape observables measured using charged particles in inclusive
-boson events are presented, using the electron and muon decay modes of the
bosons. The measurements are based on an integrated ...luminosity of Formula: see text of proton-proton collisions recorded by the ATLAS detector at the LHC at a centre-of-mass energy Formula: see text Formula: see text. Charged-particle distributions, excluding the lepton-antilepton pair from the
-boson decay, are measured in different ranges of transverse momentum of the
boson. Distributions include multiplicity, scalar sum of transverse momenta, beam thrust, transverse thrust, spherocity, and Formula: see text-parameter, which are in particular sensitive to properties of the underlying event at small values of the
-boson transverse momentum. The measured observables are compared with predictions from Pythia 8, Sherpa, and Herwig 7. Typically, all three Monte Carlo generators provide predictions that are in better agreement with the data at high
-boson transverse momenta than at low
-boson transverse momenta, and for the observables that are less sensitive to the number of charged particles in the event.
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Combined analyses of the Higgs boson production and decay rates as well as its coupling strengths to vector bosons and fermions are presented. The combinations include the results of the analyses of ...the Formula: see text and Formula: see text decay modes, and the constraints on the associated production with a pair of top quarks and on the off-shell coupling strengths of the Higgs boson. The results are based on the LHC proton-proton collision datasets, with integrated luminosities of up to 4.7 Formula: see text at Formula: see text TeV and 20.3 Formula: see text at Formula: see text TeV, recorded by the ATLAS detector in 2011 and 2012. Combining all production modes and decay channels, the measured signal yield, normalised to the Standard Model expectation, is Formula: see text. The observed Higgs boson production and decay rates are interpreted in a leading-order coupling framework, exploring a wide range of benchmark coupling models both with and without assumptions on the Higgs boson width and on the Standard Model particle content in loop processes. The data are found to be compatible with the Standard Model expectations for a Higgs boson at a mass of 125.36 GeV for all models considered.
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This article documents the performance of the ATLAS muon identification and reconstruction using the LHC dataset recorded at Formula: see text TeV in 2015. Using a large sample of Formula: see text ...and Formula: see text decays from 3.2 fbFormula: see text of
collision data, measurements of the reconstruction efficiency, as well as of the momentum scale and resolution, are presented and compared to Monte Carlo simulations. The reconstruction efficiency is measured to be close to Formula: see text over most of the covered phase space (Formula: see text and Formula: see text GeV). The isolation efficiency varies between 93 and Formula: see text depending on the selection applied and on the momentum of the muon. Both efficiencies are well reproduced in simulation. In the central region of the detector, the momentum resolution is measured to be Formula: see text (Formula: see text) for muons from Formula: see text (Formula: see text) decays, and the momentum scale is known with an uncertainty of Formula: see text. In the region Formula: see text, the Formula: see text resolution for muons from Formula: see text decays is Formula: see text while the precision of the momentum scale for low-Formula: see text muons from Formula: see text decays is about Formula: see text.
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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 with Formula: see text Formula: see text inside high-momentum jets in dijet events using 20.3 fbFormula: see text of data recorded with the ATLAS detector in
collisions at Formula: see text Formula: see text collisions at the LHC. The jets considered have transverse momenta from 50 Formula: see text up to and beyond 1.5 Formula: see text. 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.
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The decays Formula: see text and Formula: see text are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 and 20.6 fbFormula: see text of
...collisions collected at centre-of-mass energies Formula: see text TeV and 8 TeV, respectively. Signal candidates are identified through Formula: see text and Formula: see text decays. With a two-dimensional likelihood fit involving the Formula: see text reconstructed invariant mass and an angle between the Formula: see text and Formula: see text candidate momenta in the muon pair rest frame, the yields of Formula: see text and Formula: see text, and the transverse polarisation fraction in Formula: see text decay are measured. The transverse polarisation fraction is determined to be Formula: see text, and the derived ratio of the branching fractions of the two modes is Formula: see text, where the first error is statistical and the second is systematic. Finally, a sample of Formula: see text decays is used to derive the ratios of branching fractions Formula: see text and Formula: see text, where the third error corresponds to the uncertainty of the branching fraction of Formula: see text decay. The available theoretical predictions are generally consistent with the measurement.
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The paper presents studies of Bose-Einstein Correlations (BEC) for pairs of like-sign charged particles measured in the kinematic range Formula: see text 100 MeV and Formula: see text 2.5 in proton ...collisions at centre-of-mass energies of 0.9 and 7 TeV with the ATLAS detector at the CERN Large Hadron Collider. The integrated luminosities are approximately 7 Formula: see textbFormula: see text, 190 Formula: see textbFormula: see text and 12.4 nbFormula: see text for 0.9 TeV, 7 TeV minimum-bias and 7 TeV high-multiplicity data samples, respectively. The multiplicity dependence of the BEC parameters characterizing the correlation strength and the correlation source size are investigated for charged-particle multiplicities of up to 240. A saturation effect in the multiplicity dependence of the correlation source size parameter is observed using the high-multiplicity 7 TeV data sample. The dependence of the BEC parameters on the average transverse momentum of the particle pair is also investigated.
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