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Abstract A search for leptoquark pair production decaying into$$te^- \bar{t}e^+$$ $t{e}^{-}\overline{t}{e}^{+}$or$$t\mu ^- \bar{t}\mu ^+$$ $t{\mu }^{-}\overline{t}{\mu }^{+}$in final states with multiple leptons is presented. The search is based on a dataset ofppcollisions at$$\sqrt{s}=13~\text {TeV} $$ $\sqrt{s}=13\text{TeV}$recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb$$^{-1}$$ $_{}^{-1}$. 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 ab-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$$te^{-}$$ $t{e}^{-}$($$t\mu ^{-}$$ $t{\mu }^{-}$), the corresponding lower limit on the scalar mixed-generation leptoquark mass$$m_{\textrm{LQ}_{\textrm{mix}}^{\textrm{d}}}$$ $m_{{\text{LQ}}_{\text{mix}}^{\text{d}}}$is at 1.58 (1.59) TeV and on the vector leptoquark mass$$m_{{\tilde{U}}_1}$$ $m_{{\tilde{U}}_1}$at 1.67 (1.67) TeV in the minimal coupling scenario and at 1.95 (1.95) TeV in the Yang–Mills scenario. 

A search for high-mass resonances decaying into a $\tau$-lepton and a neutrino using proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$is presented. The full run 2 data sample corresponding to an integrated luminosity of $139{\mathrm{fb}}^{-1}$recorded by the ATLAS experiment in the years 2015–2018 is analyzed. The $\tau$-lepton is reconstructed in its hadronic decay modes and the total transverse momentum carried out by neutrinos is inferred from the reconstructed missing transverse momentum. The search for new physics is performed on the transverse mass between the $\tau$-lepton and the missing transverse momentum. No excess of events above the Standard Model expectation is observed and upper exclusion limits are set on the $W^{\prime }\to \tau \nu$production cross section. Heavy $W^{\prime }$vector bosons with masses up to 5.0 TeV are excluded at 95% confidence level, assuming that they have the same couplings as the Standard Model $W$boson. For nonuniversal couplings, $W^{\prime }$bosons are excluded for masses less than 3.5–5.0 TeV, depending on the model parameters. In addition, model-independent limits on the visible cross section times branching ratio are determined as a function of the lower threshold on the transverse mass of the $\tau$-lepton and missing transverse momentum. © 2024 CERN, for the ATLAS Collaboration2024CERN 

A<sc>bstract</sc> A search for new heavy scalars with flavour-violating decays in final states with multiple leptons andb-tagged jets is presented. The results are interpreted in terms of a general two-Higgs-doublet model involving an additional scalar with couplings to the top-quark and the three up-type quarks (ρ_tt,ρ_tc, andρ_tu). The targeted signals lead to final states with either a same-sign top-quark pair, three top-quarks, or four top-quarks. The search is based on a data sample of proton-proton collisions at$$ \sqrt{s} $$ $\sqrt{s}$= 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb⁻¹. Events are categorised depending on the multiplicity of light charged leptons (electrons or muons), total lepton charge, and a deep-neural-network output to enhance the purity of each of the signals. Masses of an additional scalar bosonm_Hbetween 200 − 630 GeV with couplingsρ_tt= 0.4,ρ_tc= 0.2, andρ_tu= 0.2 are excluded at 95% confidence level. Additional interpretations are provided in models ofR-parity violating supersymmetry, motivated by the recent flavour and (g −2)_μanomalies. 

A<sc>bstract</sc> A search for a new massive charged gauge boson,W′, is performed with the ATLAS detector at the LHC. The dataset used in this analysis was collected from proton-proton collisions at a centre-of-mass energy of$$ \sqrt{s} $$ $\sqrt{s}$= 13 TeV, and corresponds to an integrated luminosity of 139 fb⁻¹. The reconstructedtbinvariant mass is used to search for aW′ boson decaying into a top quark and a bottom quark. The result is interpreted in terms of aW′ boson with purely right-handed or left-handed chirality in a mass range of 0.5–6 TeV. Different values for the coupling of theW′ boson to the top and bottom quarks are considered, taking into account interference with single-top-quark production in thes-channel. No significant deviation from the background prediction is observed. The results are expressed as upper limits on theW′ →tbproduction cross-section times branching ratio as a function of theW′-boson mass and in the plane of the coupling vs theW′-boson mass. 

A<sc>bstract</sc> This paper describes a search for dark photons (γ_d) in proton-proton collisions at$$ \sqrt{s} $$ $\sqrt{s}$= 13 TeV at the Large Hadron Collider (LHC). The dark photons are searched for in the decay of Higgs bosons (H→γγ_d) produced through theZHproduction mode. The transverse mass of the system, made of the photon and the missing transverse momentum from the non-interactingγ_d, presents a distinctive signature as it peaks near the Higgs boson mass. The results presented use the total Run-2 integrated luminosity of 139 fb⁻¹recorded by the ATLAS detector at the LHC. The dominant reducible background processes are estimated using data-driven techniques. A Boosted Decision Tree technique is adopted to enhance the sensitivity of the search. As no excess is observed with respect to the Standard Model prediction, an observed (expected) upper limit on the branching ratio BR(H→γγ_d) of 2.28% ($$ {2.82}_{-0.84}^{+1.33}\% $$ ${2.82}_{-0.84}^{+1.33}\%$) is set at 95% CL for masslessγ_d. For massive dark photons up to 40 GeV, the observed (expected) upper limits on BR(H→γγ_d) at 95% confidence level is found within the [2.19,2.52]% ([2.71,3.11]%) range. 

A<sc>bstract</sc> A search for long-lived particles decaying into hadrons is presented. The analysis uses 139 fb⁻¹ofppcollision data collected at$$ \sqrt{s} $$ $\sqrt{s}$= 13 TeV by the ATLAS detector at the LHC using events that contain multiple energetic jets and a displaced vertex. The search employs dedicated reconstruction techniques that significantly increase the sensitivity to long-lived particles decaying in the ATLAS inner detector. Background estimates for Standard Model processes and instrumental effects are extracted from data. The observed event yields are compatible with those expected from background processes. The results are used to set limits at 95% confidence level on model-independent cross sections for processes beyond the Standard Model, and on scenarios with pair-production of supersymmetric particles with long-lived electroweakinos that decay via a smallR-parity-violating coupling. The pair-production of electroweakinos with masses below 1.5 TeV is excluded for mean proper lifetimes in the range from 0.03 ns to 1 ns. When produced in the decay of$$ m\left(\overset{\sim }{g}\right) $$ $m\left(\tilde{g}\right)$= 2.4 TeV gluinos, electroweakinos with$$ m\left({\overset{\sim }{\chi}}_1^0\right) $$ $m\left({\tilde{\chi }}_1^0\right)$= 1.5 TeV are excluded with lifetimes in the range of 0.02 ns to 4 ns.