A combination of searches for the single production of vectorlike top quarks (𝑇) is presented. These analyses are based on proton-proton collisions at √𝑠 =13  TeV recorded in 2015–2018 with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 139  fb⁻¹. The 𝑇 decay modes considered in this combination are into a top quark and either a Standard Model Higgs boson or a 𝑍 boson (𝑇 →𝐻⁡𝑡 and 𝑇→𝑍⁢𝑡). The individual searches used in the combination are differentiated by the number of leptons (𝑒, 𝜇) in the final state. The observed data are found to be in good agreement with the Standard Model background prediction. Interpretations are provided for a range of masses and couplings of the vectorlike top quark for benchmark models and generalized representations in terms of 95% confidence level limits. For a benchmark signal prediction of a vectorlike top quark SU(2) singlet with electroweak coupling, 𝜅, of 0.5, masses below 2.1 TeV are excluded, resulting in the most restrictive limits to date.

The mass of the top quark is measured using top-quark-top-antiquark pair events with high transverse momentum top quarks. The dataset, collected with the ATLAS detector in proton–proton collisions at √s=13 TeV delivered by the Large Hadron Collider, corresponds to an integrated luminosity of 140 fb⁻¹. The analysis targets events in the lepton-plus-jets decay channel, with an electron or muon from a semi-leptonically decaying top quark and a hadronically decaying top quark that is sufficiently energetic to be reconstructed as a single large-radius jet. The mean of the invariant mass of the reconstructed large-radius jet provides the sensitivity to the top quark mass and is simultaneously fitted with two additional observables to reduce the impact of the systematic uncertainties. The top quark mass is measured to be m_t=172.95±0.53 GeV, which is the most precise ATLAS measurement from a single channel.

This paper describes an algorithm for reconstructing and identifying a highly collimated hadronically decaying τ-lepton pair with low transverse momentum. When two τ-leptons are highly collimated, their visible decay products might overlap, degrading the reconstruction performance for each of the τ-leptons. A dedicated treatment attempting to tag the τ-lepton pair as a single object is required. The reconstruction algorithm is based on a large radius jet and its associated two leading subjets, and the identification uses a boosted decision tree to discriminate between signatures from τ⁺τ⁻ systems and those arising from QCD jets. The efficiency of the identification algorithm is measured in Zγ events using proton–proton collision data at √s=13 TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018, corresponding to an integrated luminosity of 139 fb⁻¹. The resulting data-to-simulation scale factors are close to unity with uncertainties ranging from 26 to 37%.

A search for a light charged Higgs boson produced in decays of the top quark, t→H±b with H±→cs, is presented. This search targets the production of top-quark pairs tt¯→WbH±b, with W→ℓν (ℓ=e,μ), resulting in a lepton-plus-jets final state characterised by an isolated electron or muon and at least four jets. The search exploits b-quark and c-quark identification techniques as well as multivariate methods to suppress the dominant tt¯ background. The data analysed correspond to 140fb-1 of pp collisions at s=13TeV recorded with the ATLAS detector at the LHC between 2015 and 2018. Observed (expected) 95% confidence-level upper limits on the branching fraction B(t→H±b), assuming B(t→Wb)+B(t→H±(→cs)b)=1.0, are set between 0.066% (0.077%) and 3.6% (2.3%) for a charged Higgs boson with a mass between 60 and 168 GeV.

A search is conducted for a new scalar boson S, with a mass distinct from that of the Higgs boson, decaying promptly into four leptons (ℓ=e, μ) via an intermediate state containing two on-shell, promptly decaying new spin-1 bosons Z_d: S→Z_dZ_d→4ℓ, where the Z_d boson has a mass between 15 and 300 GeV, and the S boson has a mass between either 30 and 115 GeV or 130 and 800 GeV. The search uses proton–proton collision data collected with the ATLAS detector at the Large Hadron Collider with an integrated luminosity of 139 fb⁻¹ at a centre-of-mass energy of √s=13 TeV. No significant excess above the Standard Model background expectation is observed. Upper limits at 95% confidence level are set on the production cross-section times branching ratio, σ(gg→S)×B(S→Z_dZ_d→4ℓ), as a function of the mass of both particles, m_S and m_Zd.

Charged Higgs bosons produced either in top-quark decays or in association with a top quark, subsequently decaying via 𝐻± →𝜏±⁢𝜈𝜏, are searched for in 140  fb−1 of proton-proton collision data at √𝑠 =13  TeV recorded with the ATLAS detector. Depending on whether the top quark is produced together with the 𝐻± decays hadronically or semileptonically, the search targets 𝜏 +jets or 𝜏 +lepton final states, in both cases with a 𝜏-lepton decaying into a neutrino and hadrons. No significant excess over the Standard Model background expectation is observed. For the mass range of 80 ≤𝑚𝐻± ≤3000  GeV, upper limits at 95% confidence level are set on the production cross section of the charged Higgs boson times the branching fraction ℬ⁡(𝐻± →𝜏±⁢𝜈𝜏) in the range 4.5 pb–0.4 fb. In the mass range 80–160 GeV, assuming the Standard Model cross section for 𝑡⁢¯𝑡 production, this corresponds to upper limits between 0.27% and 0.02% on ℬ⁡(𝑡 →𝑏⁢𝐻±) ×ℬ⁡(𝐻± →𝜏±⁢𝜈𝜏).

A search is performed for dark matter particles produced in association with a resonantly produced pair of 𝑏-quarks with 30<𝑚𝑏⁢𝑏<150  GeV using 140  fb−1 of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the LHC. This signature is expected in extensions of the standard model predicting the production of dark matter particles, in particular those containing a dark Higgs boson 𝑠 that decays into 𝑏⁢¯𝑏. The highly boosted 𝑠 →𝑏⁢¯𝑏 topology is reconstructed using jet reclustering and a new identification algorithm. This search places stringent constraints across regions of the dark Higgs model parameter space that satisfy the observed relic density, excluding dark Higgs bosons with masses between 30 and 150 GeV in benchmark scenarios with 𝑍′ mediator masses up to 4.8 TeV at 95% confidence level.

This Letter presents a search for highly ionizing magnetic monopoles in 262  μ⁢b⁻¹ of ultraperipheral Pb+Pb collision data at √𝑠_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. Depending on the model, monopoles with a single Dirac magnetic charge and mass below 80–120 GeV are excluded.

This Letter presents a search for highly ionizing magnetic monopoles in 262  μ⁢b⁻¹ of ultraperipheral Pb+Pb collision data at √𝑠_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. Depending on the model, monopoles with a single Dirac magnetic charge and mass below 80–120 GeV are excluded.

The identification of top quark decays where the top quark has a large momentum transverse to the beam axis, known as top tagging, is a crucial component in many measurements of Standard Model processes and searches for beyond the Standard Model physics at the Large Hadron Collider. Machine learning techniques have improved the performance of top tagging algorithms, but the size of the systematic uncertainties for all proposed algorithms has not been systematically studied. This paper presents the performance of several machine learning based top tagging algorithms on a dataset constructed from simulated proton-proton collision events measured with the ATLAS detector at √_s = 13 TeV. The systematic uncertainties associated with these algorithms are estimated through an approximate procedure that is not meant to be used in a physics analysis, but is appropriate for the level of precision required for this study. The most performant algorithms are found to have the largest uncertainties, motivating the development of methods to reduce these uncertainties without compromising performance. To enable such efforts in the wider scientific community, the datasets used in this paper are made publicly available.