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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 m W = 80 , 366.5 ± 9.8 ( stat. ) ± 12.5 ( syst. )  MeV  = 80 , 366.5 ± 15.9  MeV, and the width is measured as Γ W = 2202 ± 32 ( stat. ) ± 34 ( syst. )  MeV  = 2202 ± 47  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 m W is compatible with and supersedes the previous measurement performed using the same data.

This paper presents the reconstruction of missing transverse momentum ( p T miss ) in proton–proton collisions, at a center-of-mass energy of 13 TeV. This is a challenging task involving many detector inputs, combining fully calibrated electrons, muons, photons, hadronically decaying τ -leptons, hadronic jets, and soft activity from remaining tracks. Possible double counting of momentum is avoided by applying a signal ambiguity resolution procedure which rejects detector inputs that have already been used. Several p T miss ‘working points’ are defined with varying stringency of selections, the tightest improving the resolution at high pile-up by up to 39% compared to the loosest. The p T miss performance is evaluated using data and Monte Carlo simulation, with an emphasis on understanding the impact of pile-up, primarily using events consistent with leptonic Z decays. The studies use 140 fb - 1 of data, collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. The results demonstrate that p T miss reconstruction, and its associated significance, are well understood and reliably modelled by simulation. Finally, the systematic uncertainties on the soft p T miss component are calculated. After various improvements the scale and resolution uncertainties are reduced by up to 76 % and 51 % , respectively, compared to the previous calculation at a lower luminosity.

A bstract Inclusive and differential cross-sections are measured at particle level for the associated production of a top quark pair and a photon ( t t ¯ γ ). The analysis is performed using an integrated luminosity of 140 fb − 1 of proton-proton collisions at a centre-of-mass energy of 13 TeV collected by the ATLAS detector. The measurements are performed in the single-lepton and dilepton top quark pair decay channels focusing on t t ¯ γ topologies where the photon is radiated from an initial-state parton or one of the top quarks. The absolute and normalised differential cross-sections are measured for several variables characterising the photon, lepton and jet kinematics as well as the angular separation between those objects. The observables are found to be in good agreement with the Monte Carlo predictions. The photon transverse momentum differential distribution is used to set limits on effective field theory parameters related to the electroweak dipole moments of the top quark. The combined limits using the photon and the Z boson transverse momentum measured in t t ¯ production in associations with a Z boson are also set.

This paper reports the measurement of Higgs boson production in association with a t (t) over bar pair in the H -> b (b) over bar decay channel. The analysis uses 140 fb(-1) of 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. The final states with one or two electrons or muons are employed. An excess of events over the expected background is found with an observed (expected) significance of 4.6 (5.4) standard deviations. The t (t) over barH cross-section is sigma(t (t) over barH)=411(-92)(+101)fb=411 +/- 54(stat.)(-75)(+85)(syst.)fb for a Higgs boson mass of 125.09 GeV, consistent with the prediction of the Standard Model of 507(-50)(+35) fb. The cross-section is also measured differentially in bins of the Higgs boson transverse momentum within the simplified template cross-section framework.

A search is performed for dark matter particles produced in association with a resonant pair of Higgs bosons using 140 fb−1 of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. This signature is expected in some extensions of the Standard Model predicting the production of dark matter particles, and is interpreted in terms of a dark Higgs model containing a Z′ mediator in which the dark Higgs boson s decays into a pair of Higgs bosons. The dark Higgs boson is reconstructed through final states with at least three b-tagged jets, produced by the pair of Higgs boson decays, in events with significant missing transverse momentum consistent with the presence of dark matter. The observed data are found to be in good agreement with Standard Model predictions, constraining scenarios with dark Higgs boson masses within the range of 250 to 400 GeV and Z′ mediators up to 2.3 TeV.

A bstract This paper presents measurements of top-antitop quark pair ( t t ¯ ) production in association with additional b -jets. The analysis utilises 140 fb − 1 of proton–proton collision data collected with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13 TeV. Fiducial cross-sections are extracted in a final state featuring one electron and one muon, with at least three or four b -jets. Results are presented at the particle level for both integrated cross-sections and normalised differential cross-sections, as functions of global event properties, jet kinematics, and b -jet pair properties. Observable quantities characterising b -jets originating from the top quark decay and additional b -jets are also measured at the particle level, after correcting for detector effects. The measured integrated fiducial cross-sections are consistent with t t ¯ b b ¯ predictions from various next-to-leading-order matrix element calculations matched to a parton shower within the uncertainties of the predictions. State-of-the-art theoretical predictions are compared with the differential measurements; none of them simultaneously describes all observables. Differences between any two predictions are smaller than the measurement uncertainties for most observables.

A search for a light charged Higgs boson pro-duced in decays of the top quark, t → H ±b with H ± → cs, is presented. This search targets the production of top-quarkpairs t ¯t → W bH ±b, with W → ν ( = e, μ), result-ing in a lepton-plus-jets final state characterised by an iso-lated 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 t ¯t back-ground. The data analysed correspond to 140 fb −1 of ppcollisions at √s = 13 TeV recorded with the ATLAS detec-tor at the LHC between 2015 and 2018. Observed (expected) 95% confidence-level upper limits on the branching fraction B(t → H ±b), assuming B(t → W b) + B(t → H ±(→cs)b) = 1.0, are set between 0.066% (0.077%) and 3.6% (2.3%) for a charged Higgs boson with a mass between 60and 168 GeV.

This paper presents a search for massive, charged, long-lived particles with the ATLAS detector at the Large Hadron Collider using an integrated luminosity of 140 fb−1 of proton-proton collisions at $\\sqrt{s}$ = 13 TeV. These particles are expected to move significantly slower than the speed of light. In this paper, two signal regions provide complementary sensitivity. In one region, events are selected with at least one charged-particle track with high transverse momentum, large specific ionisation measured in the pixel detector, and time of flight to the hadronic calorimeter inconsistent with the speed of light. In the other region, events are selected with at least two tracks of opposite charge which both have a high transverse momentum and an anomalously large specific ionisation. The search is sensitive to particles with lifetimes greater than about 3 ns with masses ranging from 200 GeV to 3 TeV. The results are interpreted to set constraints on the supersymmetric pair production of long-lived R-hadrons, charginos and staus, with mass limits extending beyond those from previous searches in broad ranges of lifetime.

Differential cross-sections for top-quark pair production, inclusively and in association with jets, are measured in pp collisions at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC using an integrated luminosity of 140 fb(-1). The events are selected with one charged lepton (electron or muon) and at least four jets. The differential cross-sections are presented at particle level as functions of several jet observables, including angular correlations, jet transverse momenta and invariant masses of the jets in the final state, which characterise the kinematics and dynamics of the top-antitop system and the hard QCD radiation in the system with associated jets. The typical precision is 5%-15% for the absolute differential cross-sections and 2%-4% for the normalised differential cross-sections. Next-to-leading-order and next-to-next-to-leading-order QCD predictions are found to provide an adequate description of the rate and shape of the jet-angular observables. The description of the transverse momentum and invariant mass observables is improved when next-to-next-to-leading-order QCD corrections are included.

High precision single-differential W +/--boson production cross-sections as a function of electron or muon transverse momentum pT or their pseudorapity η, as well as double-differential cross-sections as functions of these variables, are measured in proton-proton collisions at centre-of-mass energies √ s = 5.02TeV and 13TeV. The W -boson charge asymmetry as a function of lepton. is also measured. The data, collected in dedicated runs at reduced instantaneous luminosity with the ATLAS detector at the Large Hadron Collider, correspond to integrated luminosities of 255 pb(-1) at 5.02TeV and 338 pb(-1) at 13TeV. The measurements are in agreement with Standard-Model predictions calculated at next-to-next-to-leading-order in the strong coupling constant as including transverse-momentum resummation at next-to-next-to-leading logarithmic accuracy using several parton distribution functions. The impact of the measured differential cross-sections as a function of lepton. on the determination of these functions is studied using a profiling technique.