This paper presents a new $\tau$-lepton reconstruction and identification procedure at the ATLAS detector at the Large Hadron Collider, which leads to significantly improved performance in the case of physics processes where a highly boosted pair of $\tau$-leptons is produced and one $\tau$-lepton decays into a muon and two neutrinos ($\tau_{\mu}$), and the other decays into hadrons and one neutrino ($\tau_{had}$). By removing the muon information from the signals used for reconstruction and identification of the $\tau_{had}$ candidate in the boosted pair, the efficiency is raised to the level expected for an isolated $\tau_{had}$. The new procedure is validated by selecting a sample of highly boosted $Z\rightarrow\tau_{\mu}\tau_{had}$ candidates from the data sample of $140$${fb}^{-1}$ of proton-proton collisions at $13$ TeV recorded with the ATLAS detector. Good agreement is found between data and simulation predictions in both the $Z\rightarrow\tau_{\mu}\tau_{had}$ signal region and in a background validation region. The results presented in this paper demonstrate the effectiveness of the $\tau_{had}$ reconstruction with muon removal in enhancing the signal sensitivity of the boosted $\tau_{\mu}\tau_{had}$ channel at the ATLAS detector.

A measurement of off-shell Higgs boson production in the $H^*\to ZZ\to 4\ell$ decay channel is presented. The measurement uses 140 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}=13$ TeV collected by the ATLAS detector at the Large Hadron Collider and supersedes the previous result in this decay channel using the same dataset. The data analysis is performed using a neural simulation-based inference method, which builds per-event likelihood ratios using neural networks. The observed (expected) off-shell Higgs boson production signal strength in the $ZZ\to 4\ell$ decay channel at 68% CL is $0.87^{+0.75}_{-0.54}$ ($1.00^{+1.04}_{-0.95}$). The evidence for off-shell Higgs boson production using the $ZZ\to 4\ell$ decay channel has an observed (expected) significance of $2.5\sigma$ ($1.3\sigma$). The expected result represents a significant improvement relative to that of the previous analysis of the same dataset, which obtained an expected significance of $0.5\sigma$. When combined with the most recent ATLAS measurement in the $ZZ\to 2\ell 2\nu$ decay channel, the evidence for off-shell Higgs boson production has an observed (expected) significance of $3.7\sigma$ ($2.4\sigma$). The off-shell measurements are combined with the measurement of on-shell Higgs boson production to obtain constraints on the Higgs boson total width. The observed (expected) value of the Higgs boson width at 68% CL is $4.3^{+2.7}_{-1.9}$ ($4.1^{+3.5}_{-3.4}$) MeV.

Top-quark pair production is observed in lead-lead (Pb+Pb) collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV at the Large Hadron Collider with the ATLAS detector. The data sample was recorded in 2015 and 2018, amounting to an integrated luminosity of 1.9 nb$^{-1}$. Events with exactly one electron and one muon and at least two jets are selected. Top-quark pair production is measured with an observed (expected) significance of 5.0 (4.1) standard deviations. The measured top-quark pair production cross-section is $\sigma_{t\bar{t}} = 3.6\;^{+1.0}_{-0.9}\;\mathrm{(stat.)}\;^{+0.8}_{-0.5}\;\mathrm{(syst.)} ~\mathrm{\mu b}$, with a total relative uncertainty of 31%, and is consistent with theoretical predictions using a range of different nuclear parton distribution functions. The observation of this process consolidates the evidence of the existence of all quark flavors in the pre-equilibrium stage of the quark-gluon plasma at very high energy densities, similar to the conditions present in the early universe.

This article presents a search for a heavy charged Higgs boson produced in association with a top quark and a bottom quark, and decaying into a $W$ boson and a $125$ GeV Higgs boson $h$. The search is performed in final states with one charged lepton, missing transverse momentum, and jets using proton-proton collision data at $\sqrt{s} = 13$ TeV recorded with the ATLAS detector during Run 2 of the LHC at CERN. This data set corresponds to a total integrated luminosity of 140 fb$^{-1}$. The search is conducted by examining the reconstructed invariant mass distribution of the $Wh$ candidates for evidence of a localised excess in the charged Higgs boson mass range from $250$ GeV to $3$ TeV. No significant excess is observed and 95% confidence-level upper limits between $2.8$ pb and $1.2$ fb are placed on the production cross-section times branching ratio for charged Higgs bosons decaying into $Wh$.

The paper presents a search for supersymmetric particles produced in proton-proton collisions at $\sqrt{s}=$ 13 TeV and decaying into final states with missing transverse momentum and jets originating from charm quarks. The data were taken with the ATLAS detector at the Large Hadron Collider at CERN from 2015 to 2018 and correspond to an integrated luminosity of 139 fb$^{-1}$. No significant excess of events over the expected Standard Model background expectation is observed in optimized signal regions, and limits are set on the production cross-sections of the supersymmetric particles. Pair production of charm squarks or top squarks, each decaying into a charm quark and the lightest supersymmetric particle $\tilde{\chi}^0_1$, is excluded at 95% confidence level for squarks with masses up to 900 GeV for scenarios where the mass of $\tilde{\chi}^0_1$ is below 50 GeV. Additionally, the production of leptoquarks with masses up to 900 GeV is excluded for the scenario where up-type leptoquarks decay into a charm quark and a neutrino. Model-independent limits on cross-sections and event yields for processes beyond the Standard Model are also reported.

This paper reports a search for a light CP-odd scalar resonance with a mass of 20 GeV to 90 GeV in 13 TeV 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 $\mu^{+}\nu_{\mu} \bar{\nu}_{\tau} e^{-} \bar{\nu}_{e} \nu_{\tau}$ or $e^{+}\nu_{e}\bar{\nu}_{\tau} \mu^-\bar{\nu}_{\mu}\nu_{\tau}$ 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.

This paper presents a search for supersymmetric particles in models with highly compressed mass spectra, in events consistent with being produced through vector boson fusion. The search uses 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV collected by the ATLAS experiment at the Large Hadron Collider. Events containing at least two jets with a large gap in pseudorapidity, large missing transverse momentum, and no reconstructed leptons are selected. A boosted decision tree is used to separate events consistent with the production of supersymmetric particles from those due to Standard Model backgrounds. The data are found to be consistent with Standard Model predictions. The results are interpreted using simplified models of $R$-parity-conserving supersymmetry in which the lightest supersymmetric partner is a bino-like neutralino with a mass similar to that of the lightest chargino and second-to-lightest neutralino, both of which are wino-like. Lower limits at 95% confidence level on the masses of next-to-lightest supersymmetric partners in this simplified model are established between 117 and 120 GeV when the lightest supersymmetric partners are within 1 GeV in mass.

A search for the production of top-quark pairs with the same electric charge ($tt$ or $\bar{t}\bar{t}$) is presented. The analysis uses proton-proton collision data at $\sqrt{s}=13$ TeV, recorded by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb$^{-1}$. Events with two same-charge leptons and at least two $b$-tagged jets are selected. Neural networks are employed to define two selections sensitive to additional couplings beyond the Standard Model that would enhance the production rate of same-sign top-quark pairs. No significant signal is observed, leading to an upper limit on the total production cross-section of same-sign top-quark pairs of 1.6 fb at 95$\%$ confidence level. Corresponding limits on the three Wilson coefficients associated with the ${\cal O}_{tu}^{(1)}$, ${\cal O}_{Qu}^{(1)}$, and ${\cal O}_{Qu}^{(8)}$ operators in the Standard Model Effective Field Theory framework are derived.

Inclusive cross-sections for top-quark pair production in association with charm quarks are measured with proton-proton collision data at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 140 fb$^{-1}$, collected with the ATLAS experiment at the LHC between 2015 and 2018. The measurements are performed by requiring one or two charged leptons (electrons and muons), two $b$-tagged jets, and at least one additional jet in the final state. A custom flavor-tagging algorithm is employed for the simultaneous identification of $b$-jets and $c$-jets. In a fiducial phase space that replicates the acceptance of the ATLAS detector, the cross-sections for $t\bar{t}+ {\geq} 2c$ and $t\bar{t}+1c$ production are measured to be $1.28^{+0.27}_{-0.24}\;\text{pb}$ and $6.4^{+1.0}_{-0.9}\;\text{pb}$, respectively. The measurements are primarily limited by uncertainties in the modeling of inclusive $t\bar{t}$ and $t\bar{t}+b\bar{b}$ production, in the calibration of the flavor-tagging algorithm, and by data statistics. Cross-section predictions from various $t\bar{t}$ simulations are largely consistent with the measured cross-section values, though all underpredict the observed values by 0.5 to 2.0 standard deviations. In a phase-space volume without requirements on the $t\bar{t}$ decay products and the jet multiplicity, the cross-section ratios of $t\bar{t}+ {\geq} 2c$ and $t\bar{t}+1c$ to total $t\bar{t}+\text{jets}$ production are determined to be $(1.23 \pm 0.25) \%$ and $(8.8 \pm 1.3) \%$.

This Letter presents a search for highly ionizing magnetic monopoles in 262$~\mu$b$^{-1}$ of ultraperipheral Pb+Pb collision data at $\sqrt{s_{_\textrm{NN}}}=5.36$ TeV collected by the ATLAS detector at the LHC. A new methodology that exploits the properties of clusters of hits reconstructed in the innermost silicon detector layers is introduced to study highly ionizing particles in heavy-ion data. No significant excess above the background, which is estimated using a data-driven technique, is observed. Using a nonperturbative semiclassical model, upper limits at 95% confidence level are set on the cross-section for pair production of monopoles with a single Dirac magnetic charge in the mass range of 20-150 GeV. The search significantly improves on the previous cross-section limits for production of low-mass monopoles in ultraperipheral Pb+Pb collisions.