This paper presents the muon momentum calibration and performance studies for the ATLAS detector based on the pp collisions data sample produced at Formula omitted = 13 TeV at the LHC during Run 2 ...and corresponding to an integrated luminosity of 139 Formula omitted. An innovative approach is used to correct for potential charge-dependent momentum biases related to the knowledge of the detector geometry, using the Formula omitted resonance. The muon momentum scale and resolution are measured using samples of Formula omitted and Formula omitted events. A calibration procedure is defined and applied to simulated data to match the performance measured in real data. The calibration is validated using an independent sample of Formula omitted events. At the Z Formula omitted peak, the momentum scale is measured with an uncertainty at the 0.05% (0.1%) level, and the resolution is measured with an uncertainty at the 1.5% (2%) level. The charge-dependent bias is removed with a dedicated in situ correction for momenta up to 450 GeV with a precision better than 0.03 Formula omitted.
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This paper presents a measurement of fiducial and differential cross-sections for Formula omitted production in proton-proton collisions at Formula omitted TeV with the ATLAS experiment at the Large ...Hadron Collider using a dataset corresponding to an integrated luminosity of 139 fb Formula omitted. Events with exactly one electron, one muon and no hadronic jets are studied. The fiducial region in which the measurements are performed is inspired by searches for the electroweak production of supersymmetric charginos decaying to two-lepton final states. The selected events have moderate values of missing transverse momentum and the 'stransverse mass' variable Formula omitted, which is widely used in searches for supersymmetry at the LHC. The ranges of these variables are chosen so that the acceptance is enhanced for direct Formula omitted production and suppressed for production via top quarks, which is treated as a background. The fiducial cross-section and particle-level differential cross-sections for six variables are measured and compared with two theoretical SM predictions from perturbative QCD calculations.
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The identification of b-jets, referred to as b-tagging, is an important part of many physics analyses in the ATLAS experiment at the Large Hadron Collider and an accurate calibration of its ...performance is essential for high-quality physics results. This publication describes the calibration of the light-flavour jet mistagging efficiency in a data sample of proton-proton collision events at Formula omitted TeV corresponding to an integrated luminosity of 139 fb Formula omitted. The calibration is performed in a sample of Z bosons produced in association with jets. Due to the low mistagging efficiency for light-flavour jets, a method which uses modified versions of the b-tagging algorithms referred to as flip taggers is used in this work. A fit to the jet-flavour-sensitive secondary-vertex mass is performed to extract a scale factor from data, to correct the light-flavour jet mistagging efficiency in Monte Carlo simulations, while simultaneously correcting the b-jet efficiency. With this procedure, uncertainties coming from the modeling of jets from heavy-flavour hadrons are considerably lower than in previous calibrations of the mistagging scale factors, where they were dominant. The scale factors obtained in this calibration are consistent with unity within uncertainties.
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A study of the charge conjugation and parity ( Formula omitted) properties of the interaction between the Higgs boson and Formula omitted-leptons is presented. The study is based on a measurement of ...Formula omitted-sensitive angular observables defined by the visible decay products of Formula omitted-leptons produced in Higgs boson decays. The analysis uses 139 fb Formula omitted of proton-proton collision data recorded at a centre-of-mass energy of Formula omitted TeV with the ATLAS detector at the Large Hadron Collider. Contributions from Formula omitted-violating interactions between the Higgs boson and Formula omitted-leptons are described by a single mixing angle parameter Formula omitted in the generalised Yukawa interaction. Without constraining the Formula omitted signal strength to its expected value under the Standard Model hypothesis, the mixing angle Formula omitted is measured to be Formula omitted, with an expected value of Formula omitted at the 68% confidence level. The pure Formula omitted-odd hypothesis is disfavoured at a level of 3.4 standard deviations. The results are compatible with the predictions for the Higgs boson in the Standard Model.
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A search for supersymmetry involving the pair production of gluinos decaying via off-shell third-generation squarks into the lightest neutralino Formula omitted is reported. It exploits LHC ...proton-proton collision data at a centre-of-mass energy Formula omitted TeV with an integrated luminosity of 139 fb Formula omitted collected with the ATLAS detector from 2015 to 2018. The search uses events containing large missing transverse momentum, up to one electron or muon, and several energetic jets, at least three of which must be identified as containing b-hadrons. Both a simple kinematic event selection and an event selection based upon a deep neural-network are used. No significant excess above the predicted background is found. In simplified models involving the pair production of gluinos that decay via off-shell top (bottom) squarks, gluino masses less than 2.44 TeV (2.35 TeV) are excluded at 95% CL for a massless Formula omitted Limits are also set on the gluino mass in models with variable branching ratios for gluino decays to Formula omitted Formula omitted and Formula omitted
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A search for a WZ resonance, in the fully leptonic final state (electrons or muons), is performed using 139 fb Formula omitted of data collected at a centre-of-mass energy of 13 TeV by the ATLAS ...detector at the Large Hadron Collider. The results are interpreted in terms of a singly charged Higgs boson of the Georgi-Machacek model, produced by WZ fusion, and of a Heavy Vector Triplet, with the resonance produced by WZ fusion or the Drell-Yan process. No significant excess over the Standard Model prediction is observed and limits are set on the production cross-section times branching ratio as a function of the resonance mass for these processes.
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A search for pair production of doubly charged Higgs bosons ( Formula omitted), each decaying into a pair of prompt, isolated, and highly energetic leptons with the same electric charge, is ...presented. The search uses a proton-proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 fb Formula omitted recorded by the ATLAS detector during Run 2 of the Large Hadron Collider (LHC). This analysis focuses on same-charge leptonic decays, Formula omitted where Formula omitted, in two-, three-, and four-lepton channels, but only considers final states which include electrons or muons. No evidence of a signal is observed. Corresponding upper limits on the production cross-section of a doubly charged Higgs boson are derived, as a function of its mass Formula omitted, at 95% confidence level. Assuming that the branching ratios to each of the possible leptonic final states are equal, Formula omitted, the observed (expected) lower limit on the mass of a doubly charged Higgs boson is 1080 GeV (1065 GeV) within the left-right symmetric type-II seesaw model, which is the strongest limit to date produced by the ATLAS Collaboration. Additionally, this paper provides the first direct test of the Zee-Babu neutrino mass model at the LHC, yielding an observed (expected) lower limit of Formula omitted = 900 GeV (880 GeV).
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The exclusive production of pion pairs in the process Formula omitted has been measured at Formula omitted with the ATLAS detector at the LHC, using Formula omitted of low-luminosity data. The pion ...pairs were detected in the ATLAS central detector while outgoing protons were measured in the forward ATLAS ALFA detector system. This represents the first use of proton tagging to measure an exclusive hadronic final state at the LHC. A cross-section measurement is performed in two kinematic regions defined by the proton momenta, the pion rapidities and transverse momenta, and the pion-pion invariant mass. Cross-section values of Formula omitted and Formula omitted are obtained in the two regions; they are compared with theoretical models and provide a demonstration of the feasibility of measurements of this type.
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A measurement of observables sensitive to effects of colour reconnection in top-quark pair-production events is presented using 139 Formula omitted of 13 TeV proton-proton collision data collected by ...the ATLAS detector at the LHC. Events are selected by requiring exactly one isolated electron and one isolated muon with opposite charge and two or three jets, where exactly two jets are required to be b-tagged. For the selected events, measurements are presented for the charged-particle multiplicity, the scalar sum of the transverse momenta of the charged particles, and the same scalar sum in bins of charged-particle multiplicity. These observables are unfolded to the stable-particle level, thereby correcting for migration effects due to finite detector resolution, acceptance and efficiency effects. The particle-level measurements are compared with different colour reconnection models in Monte Carlo generators. These measurements disfavour some of the colour reconnection models and provide inputs to future optimisation of the parameters in Monte Carlo generators.
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Cross-sections for the production of a Z boson in association with two photons are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV. The data used correspond to an integrated ...luminosity of 139 fb Formula omitted recorded by the ATLAS experiment during Run 2 of the LHC. The measurements use the electron and muon decay channels of the Z boson, and a fiducial phase-space region where the photons are not radiated from the leptons. The integrated Formula omitted cross-section is measured with a precision of 12% and differential cross-sections are measured as a function of six kinematic variables of the Formula omitted system. The data are compared with predictions from MC event generators which are accurate to up to next-to-leading order in QCD. The cross-section measurements are used to set limits on the coupling strengths of dimension-8 operators in the framework of an effective field theory.
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