The luminosity determination for the ATLAS detector at the LHC during Run 2 is presented, with pp collisions at a centre-of-mass energy $$\sqrt{s}=13$$s = 13 TeV. The absolute luminosity scale is determined using van der Meer beam separation scans during dedicated running periods in each year, and extrapolated to the physics data-taking regime using complementary measurements from several luminosity-sensitive detectors. The total uncertainties in the integrated luminosity for each individual year of data-taking range from 0.9% to 1.1%, and are partially correlated between years. After standard data-quality selections, the full Run 2 pp data sample corresponds to an integrated luminosity of $$140.1\pm 1.2$$140.1 ± 1.2 $$\hbox {fb}^{-1}$$fb - 1 , i.e. an uncertainty of 0.83%. A dedicated sample of low-pileup data recorded in 2017–2018 for precision Standard Model physics measurements is analysed separately, and has an integrated luminosity of $$338.1\pm 3.1$$338.1 ± 3.1 $$\hbox {pb}^{-1}$$pb - 1 .

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.

This paper presents the muon momentum calibration and performance studies for the ATLAS detector based on the pp collisions data sample produced at $$\sqrt{s}$$s = 13 TeV at the LHC during Run 2 and corresponding to an integrated luminosity of 139 $${\textrm{fb}}^{-1}$$fb - 1 . An innovative approach is used to correct for potential charge-dependent momentum biases related to the knowledge of the detector geometry, using the $$Z\rightarrow \mu ^{+}\mu ^{-}$$Z → μ + μ - resonance. The muon momentum scale and resolution are measured using samples of $$J/\psi \rightarrow \mu ^{+}\mu ^{-}$$J / ψ → μ + μ - and $$Z\rightarrow \mu ^{+}\mu ^{-}$$Z → μ + μ - events. A calibration procedure is defined and applied to simulated data to match the performance measured in real data. The calibration is validated using an independent sample of $$\Upsilon \rightarrow \mu ^{+}\mu ^{-}$$Υ → μ + μ - events. At the Z $$(J/\psi )$$( J / ψ ) peak, the momentum scale is measured with an uncertainty at the 0.05% (0.1%) level, and the resolution is measured with an uncertainty at the 1.5% (2%) level. The charge-dependent bias is removed with a dedicated in situ correction for momenta up to 450 GeV with a precision better than 0.03 $${\textrm{TeV}}^{-1}$$TeV - 1 .