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A search for T and Y vector-like quarks produced in proton-proton collisions at a centre-of-mass energy of 13 TeV and decaying into Wb in the fully hadronic final state is presented. The search uses 139 fb(-1) of data collected by the ATLAS detector at the LHC from 2015 to 2018. The final state is characterised by a hadronically decaying W boson with large Lorentz boost and a b-tagged jet, which are used to reconstruct the invariant mass of the vector-like quark candidate. The main background is QCD multijet production, which is estimated using a data-driven method. Upon finding no significant excess in data, mass limits at 95% confidence level are obtained as a function of the global coupling parameter, kappa. The observed lower limits on the masses of Y quarks with kappa = 0.5 and kappa = 0.7 are 2.0 TeV and 2.4 TeV, respectively. For T quarks, the observed mass limits are 1.4 TeV for kappa = 0.5 and 1.9 TeV for kappa = 0.7.

This paper presents measurements of top-antitop quark pair (t (t) over bar) 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 13TeV. 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) over barb (b) over bar 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.

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.

Three searches for the direct production of tau-sleptons or charginos and neutralinos in final states with at least two hadronically decaying tau-leptons are presented. For chargino and neutralino production, decays via intermediate tau-sleptons or W and h bosons are considered. The analysis uses a dataset of pp collisions corresponding to an integrated luminosity of 139 fb(-1), recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13TeV. No significant deviation from the expected Standard Model background is observed and supersymmetric particle mass limits at 95% confidence level are obtained in simplified models. For direct production of (X) over tilde +/-(1) (X) over tilde -1, chargino masses are excluded up to 970 GeV, while (X) over tilde +/-(1) 1 and (X) over tilde (0)2 masses up to 1160 GeV (330 GeV) are excluded for (X) over tildeC(+/-) (1) (X) over tilde (0)(2) /(X) over tilde (+)(1) (X) over tilde -1 production with subsequent decays via t -sleptons ( W and h bosons). Masses of tau-sleptons up to 500 GeV are excluded for mass degenerate (tau) over tildeL, R scenarios and up to 425 GeV for (tau) over tilde (L) -only scenarios. Sensitivity to (tau) over tilde (R) -only scenarios from the ATLAS experiment is presented here for the first time, with (tau) over tilde (R) masses excluded up to 350 GeV.

A search for a heavy CP-odd Higgs boson, A, decaying into a Z boson and a heavy CP-even Higgs boson, H, is presented. It uses the full LHC Run 2 dataset of pp collisions at s = 13 TeV collected with the ATLAS detector, corresponding to an integrated luminosity of 140 fb−1. The search for A → ZH is performed in the ℓ+ℓ−tt¯ and νν¯bb¯ final states and surpasses the reach of previous searches in different final states in the region with mH > 350 GeV and mA > 800 GeV. No significant deviation from the Standard Model expectation is found. Upper limits are placed on the production cross-section times the decay branching ratios. Limits with less model dependence are also presented as functions of the reconstructed m(tt¯) and m(bb¯) distributions in the ℓ+ℓ−tt¯ and νν¯bb¯ channels, respectively. In addition, the results are interpreted in the context of two-Higgs-doublet models.

Measurements of inclusive and differential production cross-sections of a topquark-top-antiquark pair in association with a W boson (t (t) over bar W) are presented. They are performed by targeting final states with two same-sign or three isolated leptons (electrons or muons) and are based on root s = 13TeV proton-proton collision data with an integrated luminosity of 140 fb(-1), recorded from 2015 to 2018 with the ATLAS detector at the Large Hadron Collider. The inclusive t (t) over bar W production cross-section is measured to be 880 +/- 80 fb, compared to a reference theoretical prediction of 745 +/- 50 (scale) +/- 13 (2-loop approx.) +/- 19 (PDF, alpha(s)) fb. Differential cross-section measurements characterise this process in detail for the first time. Several particle-level observables are compared with a variety of theoretical predictions, which generally agree well with the normalised differential cross-section results. Additionally, the relative charge asymmetry of t (t) over bar W+ and t (t) over bar W- is measured inclusively to be Arel C = 0.33 +/- 0.05, in very good agreement with the theoretical prediction of 0.322 +/- 0.003 (scale) +/- 0.007 (PDF), as well as differentially.

This paper reports a search for a light CP-odd scalar resonance with a mass of 20 GeV to 90 GeV in 13TeV proton-proton collision data with an integrated luminosity of 140 fb(-1) collected with the ATLAS detector at the Large Hadron Collider. The analysis assumes the resonance is produced via gluon-gluon fusion and decays into a tau(+)tau(-) pair which subsequently decays into a fully leptonic mu(+)nu(mu)(nu) over bar (tau)e(-)(nu) over bar (e)nu(tau) or e(+)nu(e)(nu) over bar (tau)mu(-)(nu) over bar (mu)nu(tau) final state. No significant excess of events above the predicted Standard Model background is observed. The results are interpreted within a flavour-aligned two-Higgs-doublet model, and a model-independent cross-section interpretation is also given. Upper limits at 95% confidence level between 3.0 pb and 68 pb are set on the cross-section for producing a CP-odd Higgs boson that decays into a tau(+)tau(-) pair.

A measurement of the production of W bosons with opposite electric charges in association with two jets is presented based on 140 fb−1 of data collected by the ATLAS detector in proton-proton collisions at s = 13 TeV. The analysis is sensitive to the scattering of W bosons, which is of particular interest in the ATLAS physics programme as it can be used to probe the electroweak symmetry breaking mechanism of the Standard Model. This signal is observed with a significance of 7.1 standard deviations above the background expectation, while 6.2 standard deviations were expected. The measured cross-section is determined in a signal-enriched fiducial volume and is found to be 2.7 ± 0.5 fb, which is consistent with the theoretical prediction of 2.20−0.13+0.14 fb.

A search for R-parity-violating supersymmetry in fnal states with high jet multiplicity is presented. The search uses 140 fb−1 of proton-proton collision data at √ s = 13 TeV collected by the ATLAS experiment during Run 2 of the Large Hadron Collider. The results are interpreted in the context of R-parity-violating supersymmetry models that feature prompt gluino-pair production decaying directly to three jets each or decaying to two jets and a neutralino which subsequently decays promptly to three jets. No signifcant excess over the Standard Model expectation is observed and exclusion limits at the 95% confdence level are extracted. Gluinos with masses up to 1800 GeV are excluded when decaying directly to three jets. In the cascade scenario, gluinos with masses up to 2340 GeV are excluded for a neutralino with mass up to 1250 GeV.

Measurements of integrated and differential cross-sections for electroweak WZ production in association with two jets (W(+/-)Zjj) in proton-proton collisions are presented. The data collected by the ATLAS detector at the Large Hadron Collider from 2015 to 2018 at a centre-of-mass energy of root s = 13 TeV are used, corresponding to an integrated luminosity of 140 fb(-1). The W(+/-)Zjj candidate events are reconstructed using leptonic decay modes of the gauge bosons. Events containing three identified leptons, either electrons or muons, and two jets are selected. Processes involving pure electroweak W(+/-)Zjj production at Born level are separated from W(+/-)Zjj production involving a strong coupling. The measured integrated fiducial cross-section of electroweak W(+/-)Zjj production per lepton flavour is sigma(WZjj-EW -> l 'nu lljj) = 0.368 +/- 0.037 (stat.) +/- 0.059 (syst.) +/- 0.003 (lumi.) fb, where l and l ' are either an electron or a muon. Respective cross-sections of electroweak and strong W(+/-)Zjj production are measured separately for events with exactly two jets or with more than two jets, and in three bins of the invariant mass of the two jets. The inclusive W(+/-)Zjj production cross-section, without separating electroweak and strong production, is also measured to be sigma(WZjj -> l 'nu lljj) = 1.462 +/- 0.063 (stat.) +/- 0.118 (syst.) +/- 0.012 (lumi.) fb, per lepton flavour. The inclusive W(+/-)Zjj production cross-section is measured differentially for several kinematic observables. Finally, the measurements are used to constrain anomalous quartic gauge couplings by extracting 95% confidence level intervals on dimension-8 operators.