A search for leptoquark pair production decaying into
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Abstract or$$te^- \bar{t}e^+$$ in final states with multiple leptons is presented. The search is based on a dataset of$$t\mu ^- \bar{t}\mu ^+$$ pp collisions at recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb$$\sqrt{s}=13~\text {TeV} $$ . 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$$^{-1}$$ 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 ($$te^{-}$$ ), the corresponding lower limit on the scalar mixed-generation leptoquark mass$$t\mu ^{-}$$ is at 1.58 (1.59) TeV and on the vector leptoquark mass$$m_{\textrm{LQ}_{\textrm{mix}}^{\textrm{d}}}$$ at 1.67 (1.67) TeV in the minimal coupling scenario and at 1.95 (1.95) TeV in the Yang–Mills scenario.$$m_{{\tilde{U}}_1}$$ Free, publicly-accessible full text available August 1, 2025 -
Abstract A study of the charge conjugation and parity ( $$\textit{CP}$$ CP ) properties of the interaction between the Higgs boson and $$\tau $$ τ -leptons is presented. The study is based on a measurement of $$\textit{CP}$$ CP -sensitive angular observables defined by the visible decay products of $$\tau $$ τ -leptons produced in Higgs boson decays. The analysis uses 139 fb $$^{-1}$$ - 1 of proton–proton collision data recorded at a centre-of-mass energy of $$\sqrt{s}= 13$$ s = 13 TeV with the ATLAS detector at the Large Hadron Collider. Contributions from $$\textit{CP}$$ CP -violating interactions between the Higgs boson and $$\tau $$ τ -leptons are described by a single mixing angle parameter $$\phi _{\tau }$$ ϕ τ in the generalised Yukawa interaction. Without constraining the $$H\rightarrow \tau \tau $$ H → τ τ signal strength to its expected value under the Standard Model hypothesis, the mixing angle $$\phi _{\tau }$$ ϕ τ is measured to be $$9^{\circ } \pm 16^{\circ }$$ 9 ∘ ± 16 ∘ , with an expected value of $$0^{\circ } \pm 28^{\circ }$$ 0 ∘ ± 28 ∘ at the 68% confidence level. The pure $$\textit{CP}$$ CP -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.more » « less
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Abstract This paper presents a search for dark matter,
, using events with a single top quark and an energetic$$\chi $$ W boson. The analysis is based on proton–proton collision data collected with the ATLAS experiment at 13 TeV during LHC Run 2 (2015–2018), corresponding to an integrated luminosity of 139 fb$$\sqrt{s}=$$ . The search considers final states with zero or one charged lepton (electron or muon), at least one$$^{-1}$$ b -jet and large missing transverse momentum. In addition, a result from a previous search considering two-charged-lepton final states is included in the interpretation of the results. The data are found to be in good agreement with the Standard Model predictions and the results are interpreted in terms of 95% confidence-level exclusion limits in the context of a class of dark matter models involving an extended two-Higgs-doublet sector together with a pseudoscalar mediator particle. The search is particularly sensitive to on-shell production of the charged Higgs boson state, , arising from the two-Higgs-doublet mixing, and its semi-invisible decays via the mediator particle,$$H^{\pm }$$ a : . Signal models with$$H^{\pm } \rightarrow W^\pm a (\rightarrow \chi \chi )$$ masses up to 1.5 TeV and$$H^{\pm }$$ a masses up to 350 GeV are excluded assuming a value of 1. For masses of$$\tan \beta $$ a of 150 (250) GeV, values up to 2 are excluded for$$\tan \beta $$ masses between 200 (400) GeV and 1.5 TeV. Signals with$$H^{\pm }$$ values between 20 and 30 are excluded for$$\tan \beta $$ masses between 500 and 800 GeV.$$H^{\pm }$$ -
Abstract This paper presents the observation of four-top-quark (
) production in proton-proton collisions at the LHC. The analysis is performed using an integrated luminosity of 140$$t\bar{t}t\bar{t}$$ at a centre-of-mass energy of 13 TeV collected using the ATLAS detector. Events containing two leptons with the same electric charge or at least three leptons (electrons or muons) are selected. Event kinematics are used to separate signal from background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The observed (expected) significance of the measured$$\hbox {fb}^{-1}$$ signal with respect to the standard model (SM) background-only hypothesis is 6.1 (4.3) standard deviations. The$$t\bar{t}t\bar{t}$$ production cross section is measured to be$$t\bar{t}t\bar{t}$$ fb, consistent with the SM prediction of$$22.5^{+6.6}_{-5.5}$$ fb within 1.8 standard deviations. Data are also used to set limits on the three-top-quark production cross section, being an irreducible background not measured previously, and to constrain the top-Higgs Yukawa coupling and effective field theory operator coefficients that affect$$12.0 \pm 2.4$$ production.$$t\bar{t}t\bar{t}$$ -
Abstract This paper reports a search for Higgs boson pair (
hh ) production in association with a vector boson ( ) using 139 fb$$W\; {\text {o}r}\; Z$$ of proton–proton collision data at$$^{-1}$$ recorded with the ATLAS detector at the Large Hadron Collider. The search is performed in final states in which the vector boson decays leptonically ($$\sqrt{s}=13\,\text {TeV}$$ with$$W\rightarrow \ell \nu ,\, Z\rightarrow \ell \ell ,\nu \nu $$ ) and the Higgs bosons each decay into a pair of$$\ell =e, \mu $$ b -quarks. It targetsVhh signals from both non-resonanthh production, present in the Standard Model (SM), and resonanthh production, as predicted in some SM extensions. A 95% confidence-level upper limit of 183 (87) times the SM cross-section is observed (expected) for non-resonantVhh production when assuming the kinematics are as expected in the SM. Constraints are also placed on Higgs boson coupling modifiers. For the resonant search, upper limits on the production cross-sections are derived for two specific models: one is the production of a vector boson along with a neutral heavy scalar resonanceH , in the mass range 260–1000 GeV, that decays intohh , and the other is the production of a heavier neutral pseudoscalar resonanceA that decays into aZ boson andH boson, where theA boson mass is 360–800 GeV and theH boson mass is 260–400 GeV. Constraints are also derived in the parameter space of two-Higgs-doublet models. -
Abstract This paper presents a statistical combination of searches targeting final states with two top quarks and invisible particles, characterised by the presence of zero, one or two leptons, at least one jet originating from a
b -quark and missing transverse momentum. The analyses are searches for phenomena beyond the Standard Model consistent with the direct production of dark matter inpp collisions at the LHC, using 139 fb of data collected with the ATLAS detector at a centre-of-mass energy of 13 TeV. The results are interpreted in terms of simplified dark matter models with a spin-0 scalar or pseudoscalar mediator particle. In addition, the results are interpreted in terms of upper limits on the Higgs boson invisible branching ratio, where the Higgs boson is produced according to the Standard Model in association with a pair of top quarks. For scalar (pseudoscalar) dark matter models, with all couplings set to unity, the statistical combination extends the mass range excluded by the best of the individual channels by 50 (25) GeV, excluding mediator masses up to 370 GeV. In addition, the statistical combination improves the expected coupling exclusion reach by 14% (24%), assuming a scalar (pseudoscalar) mediator mass of 10 GeV. An upper limit on the Higgs boson invisible branching ratio of 0.38 ($$^{-\text {1}}$$ ) is observed (expected) at 95% confidence level.$$\text {0.30}^{+\text {0.13}}_{-\text {0.09}}$$ -
Abstract The ATLAS experiment at the Large Hadron Collider has a broad physics programme ranging from precision measurements to direct searches for new particles and new interactions, requiring ever larger and ever more accurate datasets of simulated Monte Carlo events. Detector simulation with Geant4 is accurate but requires significant CPU resources. Over the past decade, ATLAS has developed and utilized tools that replace the most CPU-intensive component of the simulation—the calorimeter shower simulation—with faster simulation methods. Here, AtlFast3, the next generation of high-accuracy fast simulation in ATLAS, is introduced. AtlFast3 combines parameterized approaches with machine-learning techniques and is deployed to meet current and future computing challenges, and simulation needs of the ATLAS experiment. With highly accurate performance and significantly improved modelling of substructure within jets, AtlFast3 can simulate large numbers of events for a wide range of physics processes.more » « less
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Abstract The accurate simulation of additional interactions at the ATLAS experiment for the analysis of proton–proton collisions delivered by the Large Hadron Collider presents a significant challenge to the computing resources. During the LHC Run 2 (2015–2018), there were up to 70 inelastic interactions per bunch crossing, which need to be accounted for in Monte Carlo (MC) production. In this document, a new method to account for these additional interactions in the simulation chain is described. Instead of sampling the inelastic interactions and adding their energy deposits to a hard-scatter interaction one-by-one, the inelastic interactions are presampled, independent of the hard scatter, and stored as combined events. Consequently, for each hard-scatter interaction, only one such presampled event needs to be added as part of the simulation chain. For the Run 2 simulation chain, with an average of 35 interactions per bunch crossing, this new method provides a substantial reduction in MC production CPU needs of around 20%, while reproducing the properties of the reconstructed quantities relevant for physics analyses with good accuracy.more » « less
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Abstract This paper presents a measurement of the electroweak production of two jets in association with a $$Z\gamma $$ Z γ pair, with the Z boson decaying into two neutrinos. It also presents a search for invisible or partially invisible decays of a Higgs boson with a mass of 125 $$\text {GeV}$$ GeV produced through vector-boson fusion with a photon in the final state. These results use data from LHC proton–proton collisions at $$\sqrt{s}$$ s = 13 $$\text {TeV}$$ TeV collected with the ATLAS detector and corresponding to an integrated luminosity of 139 $$\hbox {fb}^{-1}$$ fb - 1 . The event signature, shared by all benchmark processes considered for the measurements and searches, is characterized by a significant amount of unbalanced transverse momentum and a photon in the final state, in addition to a pair of forward jets. Electroweak $$Z\gamma $$ Z γ production in association with two jets is observed in this final state with a significance of 5.2 (5.1 expected) standard deviations. The measured fiducial cross-section for this process is $$1.31\pm 0.29$$ 1.31 ± 0.29 fb. An observed (expected) upper limit of 0.37 ( $$0.34^{+0.15}_{-0.10}$$ 0 . 34 - 0.10 + 0.15 ) at 95% confidence level is set on the branching ratio of a 125 $$\text {GeV}$$ GeV Higgs boson to invisible particles, assuming the Standard Model production cross-section. The signature is also interpreted in the context of decays of a Higgs boson into a photon and a dark photon. An observed (expected) 95% CL upper limit on the branching ratio for this decay is set at 0.018 ( $$0.017^{+0.007}_{-0.005}$$ 0 . 017 - 0.005 + 0.007 ), assuming the Standard Model production cross-section for a 125 $$\text {GeV}$$ GeV Higgs boson.more » « less