51�������

Abstract:

A search for the dimuon decay of the Higgs boson is presented based on $pp$ collision data recorded by ATLAS during Run 3 of the Large Hadron Collider, corresponding to an integrated luminosity of $165{\mathrm{fb}}^{-1}$ at $\sqrt{s}=13.6\mathrm{TeV}$ . To enhance the sensitivity, the results are combined with those from Run 2. An excess of events over the background is observed with a significance of $3.4\sigma$ ( $2.5\sigma$ expected). The best-fit signal strength is $\mu =1.4\pm 0.4$ . This result provides evidence for the $H\to \mu \mu$ decay with ATLAS data and offers a direct probe of the Higgs-boson Yukawa coupling to second-generation fermions.

Abstract:

A measurement of the top-quark pole mass mtpole is presented in tt¯ events with an additional jet, tt¯ + 1-jet, produced in pp collisions at s=13 TeV. The data sample, recorded with the ATLAS experiment during Run 2 of the LHC, corresponds to an integrated luminosity of 140 fb−1. Events with one electron and one muon of opposite electric charge in the final state are selected to measure the tt¯ + 1-jet differential cross-section as a function of the inverse of the invariant mass of the tt¯ + 1-jet system. Iterative Bayesian Unfolding is used to correct the data to enable comparison with fixed-order calculations at next-to-leading-order accuracy in the strong coupling. The process pp→tt¯j2→3, where top quarks are taken as stable particles, and the process pp→bb¯l+νl−ν¯j2→7, which includes top-quark decays to the dilepton final state and off-shell effects, are considered. The top-quark mass is extracted using a χ2 fit of the unfolded normalized differential cross-section distribution. The results obtained with the 2 → 3 and 2 → 7 calculations are compatible within theoretical uncertainties, providing an important consistency check. The more precise determination is obtained for the 2 → 3 measurement: mtpole=170.7±0.3stat.±1.4syst.±0.3scale±0.2PDF⊕αS GeV, which is in good agreement with other top-quark mass results.

Abstract:

A search for single production of a vector-like quark Q, which could be either a singlet T, with charge 23, or a Y from a (T, B, Y) triplet, with charge −43, is performed using data from proton-proton collisions at a centre-of-mass energy of 13 TeV. The data correspond to the full integrated luminosity of 140 fb−1 recorded with the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis targets Q → Wb decays where the W boson decays leptonically. The data are found to be consistent with the expected Standard Model background, so upper limits are set on the cross-section times branching ratio, and on the coupling of the Q to the Standard Model sector for these two benchmark models. Effects of interference with the Standard Model background are taken into account. For the singlet T, the 95% confidence level limit on the coupling strength κ ranges between 0.22 and 0.52 for masses from 1150 to 2300 GeV. For the (T, B, Y) triplet, the limits on κ vary from 0.14 to 0.46 for masses from 1150 to 2600 GeV.

Abstract:

A search for pair-production of vector-like leptons is presented, considering their decays into a third-generation Standard Model (SM) quark and a vector leptoquark (U1$$U_1$$) as predicted by an ultraviolet-complete extension of the SM, referred to as the ‘4321’ model. Given the assumed decay of U1$$U_1$$ into third-generation SM fermions, the final state can contain multiple τ$$\tau $$-leptons and b-quarks. This search is based on a dataset of pp collisions at s=13$$\sqrt{s}=13$$ TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of up to 140fb-1$$140~\textrm{fb}^{-1} $$. No significant excess above the SM background prediction is observed, and 95% confidence level limits on the cross-section times branching ratio are derived as a function of the vector-like lepton mass. A lower observed (expected) limit of 910 GeV (970 GeV) is set on the vector-like lepton mass. Additionally, the results are interpreted for a supersymmetric model with an R-parity violating coupling to the third-generation quarks and leptons. Lower observed (expected) limits are obtained on the higgsino mass at 880 GeV (940 GeV) and on the wino mass at 1170 GeV (1170 GeV).