This paper presents the observation of four-top-quark ( $$t\bar{t}t\bar{t}$$t t ¯ t t ¯ ) production in proton-proton collisions at the LHC. The analysis is performed using an integrated luminosity of 140 $$\hbox {fb}^{-1}$$fb - 1 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 $$t\bar{t}t\bar{t}$$t t ¯ t t ¯ signal with respect to the standard model (SM) background-only hypothesis is 6.1 (4.3) standard deviations. The $$t\bar{t}t\bar{t}$$t t ¯ t t ¯ production cross section is measured to be $$22.5^{+6.6}_{-5.5}$$22 . 5 - 5.5 + 6.6 fb, consistent with the SM prediction of $$12.0 \pm 2.4$$12.0 ± 2.4 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 $$t\bar{t}t\bar{t}$$t t ¯ t t ¯ production.

The ATLAS experiment has developed extensive software and distributed computing systems for Run 3 of the LHC. These systems are described in detail, including software infrastructure and workflows, distributed data and workload management, database infrastructure, and validation. The use of these systems to prepare the data for physics analysis and assess its quality are described, along with the software tools used for data analysis itself. An outlook for the development of these projects towards Run 4 is also provided.

Proton–proton collision data recorded by the ATLAS detector in 2011, at a centre-of-mass energy of 7 TeV, have been used for an improved determination of the W-boson mass and a first measurement of the W-boson width at the LHC. Recent fits to the proton parton distribution functions are incorporated in the measurement procedure and an improved statistical method is used to increase the measurement precision. The measurement of the W-boson mass yields a value of $$m_W = 80{,}366.5 \pm 9.8~(\text {stat.}) \pm 12.5~(\text {syst.})$$ m W = 80 , 366.5 ± 9.8 ( stat. ) ± 12.5 ( syst. ) MeV $$= 80{,}366.5 \pm 15.9$$ = 80 , 366.5 ± 15.9 MeV, and the width is measured as $$\Gamma _W = 2202 \pm 32~(\text {stat.}) \pm 34~(\text {syst.})$$ Γ W = 2202 ± 32 ( stat. ) ± 34 ( syst. ) MeV $$= 2202 \pm 47$$ = 2202 ± 47 MeV. The first uncertainty components are statistical and the second correspond to the experimental and physics-modelling systematic uncertainties. Both results are consistent with the expectation from fits to electroweak precision data. The present measurement of $$m_W$$ m W is compatible with and supersedes the previous measurement performed using the same data.

The ATLAS trigger system is a crucial component of the ATLAS experiment at the LHC. It is responsible for selecting events in line with the ATLAS physics programme. This paper presents an overview of the changes to the trigger and data acquisition system during the second long shutdown of the LHC, and shows the performance of the trigger system and its components in the proton-proton collisions during the 2022 commissioning period as well as its expected performance in proton-proton and heavy-ion collisions for the remainder of the third LHC data-taking period (2022–2025).

A search for heavy right-handed Majorana or Dirac neutrinos $$N_{\textrm{R}}$$N R and heavy right-handed gauge bosons $$W_{\textrm{R}}$$W R is performed in events with energetic electrons or muons, with the same or opposite electric charge, and energetic jets. The search is carried out separately for topologies of clearly separated final-state products (“resolved” channel) and topologies with boosted final states with hadronic and/or leptonic products partially overlapping and reconstructed as a large-radius jet (“boosted” channel). The events are selected from pp collision data at the LHC with an integrated luminosity of 139 fb $$^{-1}$$- 1 collected by the ATLAS detector at $$\sqrt{s} = 13$$s = 13 TeV. No significant deviations from the Standard Model predictions are observed. The results are interpreted within the theoretical framework of a left-right symmetric model, and lower limits are set on masses in the heavy right-handed $$W_{\textrm{R}}$$W R boson and $$N_{\textrm{R}}$$N R plane. The excluded region extends to about $$m(W_{\textrm{R}}) = 6.4$$m ( W R ) = 6.4 TeV for both Majorana and Dirac $$N_{\textrm{R}}$$N R neutrinos at $$m(N_{\textrm{R}})<1$$m ( N R ) < 1 TeV. $$N_{\textrm{R}}$$N R with masses of less than 3.5 (3.6) TeV are excluded in the electron (muon) channel at $$m(W_{\textrm{R}})=4.8$$m ( W R ) = 4.8 TeV for the Majorana neutrinos, and limits of $$m(N_{\textrm{R}})$$m ( N R ) up to 3.6 TeV for $$m(W_{\textrm{R}}) = 5.2$$m ( W R ) = 5.2 (5.0) TeV in the electron (muon) channel are set for the Dirac neutrinos. These constitute the most stringent exclusion limits to date for the model considered.

This paper presents the electron and photon energy calibration obtained with the ATLAS detector using 140 fb^-1 of LHC proton-proton collision data recorded at √(s) = 13 TeV between 2015 and 2018. Methods for the measurement of electron and photon energies are outlined, along with the current knowledge of the passive material in front of the ATLAS electromagnetic calorimeter. The energy calibration steps are discussed in detail, with emphasis on the improvements introduced in this paper. The absolute energy scale is set using a large sample of Z-boson decays into electron-positron pairs, and its residual dependence on the electron energy is used for the first time to further constrain systematic uncertainties. The achieved calibration uncertainties are typically 0.05% for electrons from resonant Z-boson decays, 0.4% at E _T ∼ 10 GeV, and 0.3% at E _T ∼ 1 TeV; for photons at E _T ∼ 60 GeV, they are 0.2% on average. This is more than twice as precise as the previous calibration. The new energy calibration is validated using J/ψ → ee and radiative Z-boson decays.