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A search for a light charged Higgs boson produced in decays of the top quark, [Formula omitted] with [Formula omitted], is presented. This search targets the production of top-quark pairs [Formula omitted], with [Formula omitted] ( [Formula omitted]), resulting in a lepton-plus-jets final state characterised by an isolated electron or muon and at least four jets. The search exploits b-quark and c-quark identification techniques as well as multivariate methods to suppress the dominant [Formula omitted] background. The data analysed correspond to [Formula omitted] of [Formula omitted] collisions at [Formula omitted] recorded with the ATLAS detector at the LHC between 2015 and 2018. Observed (expected) 95% confidence-level upper limits on the branching fraction [Formula omitted], assuming [Formula omitted], are set between 0.066% (0.077%) and 3.6% (2.3%) for a charged Higgs boson with a mass between 60 and 168 GeV.

Proton-proton collision data recorded by the ATLAS detector in 2011, at a centre-of-mass energy of 7 TeV, have been used for an improved determination of the W-boson mass and a first measurement of the W-boson width at the LHC. Recent fits to the proton parton distribution functions are incorporated in the measurement procedure and an improved statistical method is used to increase the measurement precision. The measurement of the W-boson mass yields a value of [Formula omitted] MeV [Formula omitted] MeV, and the width is measured as [Formula omitted] MeV [Formula omitted] MeV. The first uncertainty components are statistical and the second correspond to the experimental and physics-modelling systematic uncertainties. Both results are consistent with the expectation from fits to electroweak precision data. The present measurement of [Formula omitted] is compatible with and supersedes the previous measurement performed using the same data.

Properties of the underlying-event in pp interactions are investigated primarily via the strange hadrons [Formula omitted], [Formula omitted] and [Formula omitted], as reconstructed using the ATLAS detector at the LHC in minimum-bias pp collision data at [Formula omitted] TeV. The hadrons are reconstructed via the identification of the displaced two-particle vertices corresponding to the decay modes [Formula omitted], [Formula omitted] and [Formula omitted]. These are used in the construction of underlying-event observables in azimuthal regions computed relative to the leading charged-particle jet in the event. None of the hadronisation and underlying-event physics models considered can describe the data over the full kinematic range considered. Events with a leading charged-particle jet in the range of [Formula omitted] GeV are studied using the number of prompt charged particles in the transverse region. The ratio [Formula omitted] as a function of the number of such charged particles varies only slightly over this range. This disagrees with the expectations of some of the considered Monte Carlo models.

This paper describes measurements of the transverse momentum spectra of W and Z bosons produced in proton-proton collisions at centre-of-mass energies of [Formula omitted] TeV and [Formula omitted] TeV with the ATLAS experiment at the Large Hadron Collider. Measurements are performed in the electron and muon channels, [Formula omitted] and [Formula omitted] ( [Formula omitted] or [Formula omitted]), and for W events further separated by charge. The data were collected in 2017 and 2018, in dedicated runs with reduced instantaneous luminosity, and correspond to 255 and 338 [Formula omitted] at [Formula omitted] TeV and 13 TeV, respectively. These conditions optimise the reconstruction of the W-boson transverse momentum. The distributions observed in the electron and muon channels are unfolded, combined, and compared to QCD calculations based on parton shower Monte Carlo event generators and analytical resummation. The description of the transverse momentum distributions by Monte Carlo event generators is imperfect and shows significant differences largely common to [Formula omitted], [Formula omitted] and Z production. The agreement is better at [Formula omitted] TeV, especially for predictions that were tuned to Z production data at [Formula omitted] TeV. Higher-order, resummed predictions based on DYTurbo generally match the data best across the spectra. Distribution ratios are also presented and test the understanding of differences between the production processes.

The ratio of branching ratios of the W boson to muons and electrons, [Formula omitted]/ [Formula omitted], has been measured using [Formula omitted] of pp collision data at [Formula omitted] [Formula omitted] collected with the ATLAS detector at the LHC, probing the universality of lepton couplings. The ratio is obtained from measurements of the [Formula omitted] production cross-section in the ee, [Formula omitted] and [Formula omitted] dilepton final states. To reduce systematic uncertainties, it is normalised by the square root of the corresponding ratio [Formula omitted] for the Z boson measured in inclusive [Formula omitted] and [Formula omitted] events. By using the precise value of [Formula omitted] determined from [Formula omitted] colliders, the ratio [Formula omitted] is determined to be RW/e=0.9995±0.0022(stat)±0.0036(syst)±0.0014(ext).The three uncertainties correspond to data statistics, experimental systematics and the external measurement of [Formula omitted], giving a total uncertainty of 0.0045, and confirming the Standard Model assumption of lepton flavour universality in W-boson decays at the 0.5% level.

The observation of the electroweak production of a W boson and a photon in association with two jets, using pp collision data at the Large Hadron Collider at a centre of mass energy of [Formula omitted] TeV, is reported. The data were recorded by the ATLAS experiment from 2015 to 2018 and correspond to an integrated luminosity of 140 fb [Formula omitted]. This process is sensitive to the quartic gauge boson couplings via the vector boson scattering mechanism and provides a stringent test of the electroweak sector of the Standard Model. Events are selected if they contain one electron or muon, missing transverse momentum, at least one photon, and two jets. Multivariate techniques are used to distinguish the electroweak [Formula omitted] process from irreducible background processes. The observed significance of the electroweak [Formula omitted] process is well above six standard deviations, compared to an expected significance of 6.3 standard deviations. Fiducial and differential cross sections are measured in a fiducial phase space close to the detector acceptance, which are in reasonable agreement with leading order Standard Model predictions from MadGraph5+Pythia8 and Sherpa. The results are used to constrain new physics effects in the context of an effective field theory.

This paper reports a summary of searches for a fermionic dark matter candidate in the context of theoretical models characterised by a mediator particle exchange in the s-channel. The data sample considered consists of pp collisions delivered by the Large Hadron Collider during its Run 2 at a centre-of-mass energy of [Formula omitted] and recorded by the ATLAS detector, corresponding to up to 140 fb [Formula omitted]. The interpretations of the results are based on simplified models where the new mediator particles can be spin-0, with scalar or pseudo-scalar couplings to fermions, or spin-1, with vector or axial-vector couplings to fermions. Exclusion limits are obtained from various searches characterised by final states with resonant production of Standard Model particles, or production of Standard Model particles in association with large missing transverse momentum.

This paper presents a measurement of the production cross-section of a Z boson in association with b- or c-jets, in proton-proton collisions at [Formula omitted] TeV with the ATLAS experiment at the Large Hadron Collider using data corresponding to an integrated luminosity of 140 [Formula omitted]. Inclusive and differential cross-sections are measured for events containing a Z boson decaying into electrons or muons and produced in association with at least one b-jet, at least one c-jet, or at least two b-jets with transverse momentum [Formula omitted] GeV and rapidity [Formula omitted]. Predictions from several Monte Carlo generators based on next-to-leading-order matrix elements interfaced with a parton-shower simulation, with different choices of flavour schemes for initial-state partons, are compared with the measured cross-sections. The results are also compared with novel predictions, based on infrared and collinear safe jet flavour dressing algorithms. Selected [Formula omitted]-jet observables, optimized for sensitivity to intrinsic-charm, are compared with benchmark models with different intrinsic-charm fractions.

A search for leptoquark pair production decaying into [Formula omitted] or [Formula omitted] in final states with multiple leptons is presented. The search is based on a dataset of pp collisions at [Formula omitted] recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb [Formula omitted]. Four signal regions, with the requirement of at least three light leptons (electron or muon) and at least two jets out of which at least one jet is identified as coming from a b-hadron, are considered based on the number of leptons of a given flavour. The main background processes are estimated using dedicated control regions in a simultaneous fit with the signal regions to data. No excess above the Standard Model background prediction is observed and 95% confidence level limits on the production cross section times branching ratio are derived as a function of the leptoquark mass. Under the assumption of exclusive decays into [Formula omitted] ( [Formula omitted]), the corresponding lower limit on the scalar mixed-generation leptoquark mass [Formula omitted] is at 1.58 (1.59) TeV and on the vector leptoquark mass [Formula omitted] at 1.67 (1.67) TeV in the minimal coupling scenario and at 1.95 (1.95) TeV in the Yang-Mills scenario.

Presented is the search for anomalous Higgs boson decays into two axion-like particles (ALPs) using the full Run 2 data set of [Formula omitted] of proton-proton collisions at a centre-of-mass energy of [Formula omitted] recorded by the ATLAS experiment. The ALPs are assumed to decay into two photons, providing sensitivity to recently proposed models that could explain the [Formula omitted] discrepancy. This analysis covers an ALP mass range from 100 to [Formula omitted] and ALP-photon couplings in the range [Formula omitted], and therefore includes signatures with significantly displaced vertices and highly collinear photons. No significant excess of events above the Standard Model background is observed. Upper limits at 95% confidence level are placed on the branching ratio of the Higgs boson to two ALPs in the four-photon final state, and are in the range of [Formula omitted] to [Formula omitted], depending on the hypothesized ALP mass and ALP-photon coupling strength.