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A measurement of the top-quark pole mass m t pole is presented in tt <over bar> events with an additional jet, tt <over bar>+ 1-jet, produced in pp collisions at √s = 13TeV. 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 <over bar>+ 1-jet differential cross-section as a function of the inverse of the invariant mass of the tt <over bar> + 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 <over bar> j (2 → 3), where top quarks are taken as stable particles, and the process pp → bb <over bar>l + νl - ν<over bar> j (2 → 7), which includes top-quark decays to the dilepton final state and off-shell effects, are considered. The top-quark mass is extracted using χ 2 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: m t pole = 170.7 ± 0.3 (stat.) ± 1.4 (syst.) ± 0.3 (scale) ±0.2 (PDF ⊕ α S ) GeV, which is in good agreement with other top-quark mass results.

The production of D-+/- and D-s(+/-) charmed mesons is measured using the D-+/-/D-s(+/-) -> phi(mu mu)pi(+/-) decay channel with 137 fb(-1) of root s = 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider during the years 2016-2018. The charmed mesons are reconstructed in the range of transverse momentum 12 < pT < 100 GeV and pseudorapidity vertical bar eta vertical bar < 2.5. The differential cross-sections are measured as a function of transverse momentum and pseudorapidity, respectively, and compared with next-to-leading-order QCD predictions. The predictions are found to be consistent with the measurements in the visible kinematic region within the large theoretical uncertainties.

This paper presents measurements of top-antitop quark pair (t¯t) production inassociation with additional b-jets. The analysis utilises 140 fb −1 of proton–proton collisiondata collected with the ATLAS detector at the Large Hadron Collider at a centre-of-massenergy of 13 TeV. Fiducial cross-sections are extracted in a final state featuring one electronand one muon, with at least three or four b-jets . Results are presented at the particle level forboth integrated cross-sections and normalised differential cross-sections, as functions of globalevent properties, jet kinematics, and b-jet pair properties. Observable quantities characterisingb-jets originating from the top quark decay and additional b-jets are also measured at theparticle level, after correcting for detector effects. The measured integrated fiducial cross-sections are consistent with t¯tb¯b predictions from various next-to-leading-order matrix elementcalculations matched to a parton shower within the uncertainties of the predictions. State-of-the-art theoretical predictions are compared with the differential measurements; none ofthem simultaneously describes all observables. Differences between any two predictions aresmaller than the measurement uncertainties for most observables.

A measurement of the B-0 meson lifetime using B-0 -> J/psi K*(0) decays in data from 13 TeV proton-proton collisions with an integrated luminosity of 140 fb(-1) recorded by the ATLAS detector at the LHC is presented. The measured effective lifetime is tau = 1.5053 +/- 0.0012 (stat.) +/- 0.0035 (syst.) ps. The average decay width extracted from the effective lifetime, using parameters from external sources, is Gamma(d) = 0.6639 +/- 0.0005 (stat.) +/- 0.0016 (syst.) +/- 0.0038 (ext.) ps(-1), where the uncertainties are statistical, systematic and from external sources. The earlier ATLAS measurement of Gamma(s) in the B-s(0) -> J/psi phi decay was used to derive a value for the ratio of the average decay widths Gamma(d) and Gamma(s) for B-0 and B-s(0) mesons respectively, of Gamma(d)/Gamma(s) = 0.9905 +/- 0.0022 (stat.)+/- 0.0036 (syst.)+/- 0.0057 (ext.). Themeasured lifetime, average decay width and decay width ratio are in agreement with theoretical predictions and with measurements by other experiments. This measurement provides the most precise result of the effective lifetime of the B-0 meson to date.

Processes with tau-leptons in the final state are important for Standard Model measurements and searches for physics beyond the Standard Model. The ATLAS experiment at the Large Hadron Collider observes tau-leptons produced in proton-proton collisions only through their decay products. Data analyses involving hadronically decaying tau-leptons face challenges due to backgrounds from jets misidentified as tau-leptons that are not modelled reliably by Monte Carlo simulations. Data-driven methods such as the fake-factor method allow such misidentified backgrounds to be predicted by measuring transfer factors, known as fake factors, in data from dedicated regions. This paper describes a refined technique for determining the fake factors, the Universal Fake Factor method. It evaluates the fake factors for a signal region by using fake factors from samples enriched in different sources of jets misidentified as tau-leptons (light-quark, gluon, b-quark, and pile-up jets). Each fake factor is calculated as a linear combination of fake factors measured in these different enriched samples. For the full Run 2 data set, the systematic uncertainty of the calculated fake factors, evaluated using W(mu nu) enriched event sample, ranges from 15 to 35% depending on the tau-lepton's transverse momentum and charged-particle decay multiplicity.

Jets with different radius parameters R are an important tool for probing quantum chromodynamics processes at different angular scales. Jets with small R = 0.2 are instrumental in measurements of the substructure of large-R jets resulting from collimated hadronic decays of energetic W, Z, and Higgs bosons, top quarks, and of potential new resonances. This paper presents measurements of the energy scale, resolution, and associated uncertainties of jets with radius parameters R = 0.2 and 0.6, obtained using the ATLAS detector. The results are based on 37 fb(-1) of proton-proton collision data from the Large Hadron Collider at a centre-of-mass energy of root s = 13 TeV. A new in situ method for measuring jet energy scale differences between data and Monte Carlo simulations is presented. The systematic uncertainties in the jet energy scale for central jets (vertical bar eta vertical bar < 1.2) typically vary from 1% to about 5% as a function of vertical bar eta vertical bar at very low transverse momentum, p(T), of around 20 GeV for both R = 0.2 and 0.6 jets. The relative energy resolution ranges from (35 +/- 6)% at p(T) = 20 GeV to (6 +/- 0.5)% at p(T) = 300 GeV for central R = 0.2 jets, and is found to be slightly worse for R = 0.6 jets. Finally, the effect of close-by hadronic activity on the jet energy scale is investigated and is found to be well modelled by the ATLAS Monte Carlo simulations.

A search is presented for a heavy scalar (H) or pseudo-scalar (A) predicted by the two-Higgs-doublet models, where the H/A is produced in association with a top-quark pair (t t<over bar>H/A), and with the H/A decaying into a tt<over bar> pair. The full LHC Run 2 proton-proton collision data collected by the ATLAS experiment is used, corresponding to an integrated luminosity of 139 fb -1 . Events are selected requiring exactly one or two opposite-charge electrons or muons. Data-driven corrections are applied to improve the modelling of the tt<over bar> +jets background in the regime with high jet and b-jet multiplicities. These include a novel multi-dimensional kinematic reweighting based on a neural network trained using data and simulations. An H/A-mass parameterised graph neural network is trained to optimise the signal-to-background discrimination. In combination with the previous search performed by the ATLAS Collaboration in the multilepton final state, the observed upper limits on the tt<over bar>H/A → tt<over bar>tt<over bar> production cross-section at 95% confidence level range between 14 fb and 5.0 fb for an H/A with mass between 400 GeV and 1000 GeV, respectively. Assuming that both the H and A contribute to the tt<over bar>tt<over bar> cross-section, tan β values below 1.7 or 0.7 are excluded for a mass of 400 GeV or 1000 GeV, respectively. The results are also used to constrain a model predicting the pair production of a colour-octet scalar, with the scalar decaying into a tt<over bar> pair.

Inclusive and differential cross-sections are measured at particle level for the associated production of a top quark pair and a photon (tt<over bar>γ). The analysis is performed using an integrated luminosity of 140 fb -1 of proton-proton collisions at a centre-of-mass energy of 13TeV collected by the ATLAS detector. The measurements are performed in the single-lepton and dilepton top quark pair decay channels focusing on tt<over bar>γ topologies where the photon is radiated from an initial-state parton or one of the top quarks. The absolute and normalised differential cross-sections are measured for several variables characterising the photon, lepton and jet kinematics as well as the angular separation between those objects. The observables are found to be in good agreement with the Monte Carlo predictions. The photon transverse momentum differential distribution is used to set limits on effective field theory parameters related to the electroweak dipole moments of the top quark. The combined limits using the photon and the Z boson transverse momentum measured in tt<over bar> production in associations with a Z boson are also set.

Measurements of both the inclusive and differential production cross sections of a top-quark-top-antiquark pair in association with a Z boson (t (t) over barZ) are presented. Final states with two, three or four isolated leptons (electrons or muons) are targeted. The measurements use the data recorded by the ATLAS detector in pp collisions at root s = 13 TeV at the Large Hadron Collider during the years 2015-2018, corresponding to an integrated luminosity of 140 fb(-1). The inclusive cross section is measured to be sigma(t (t) over barZ) = 0.86 +/- 0.04 (stat.) +/- 0.04 (syst.) pb and found to be in agreement with the most advanced Standard Model predictions. The differential measurements are presented as a function of a number of observables that probe the kinematics of the t (t) over barZ system. Both the absolute and normalised differential cross-section measurements are performed at particle level and parton level for specific fiducial volumes, and are compared with NLO+NNLL theoretical predictions. The results are interpreted in the framework of Standard Model effective field theory and used to set limits on a large number of dimension-6 operators involving the top quark. The first measurement of spin correlations in t (t) over barZ events is presented: the results are in agreement with the Standard Model expectations, and the null hypothesis of no spin correlations is disfavoured with a significance of 1.8 standard deviations.

This paper reports the observation of top-quark pair production in proton-lead collisions in the ATLAS experiment at the Large Hadron Collider. The measurement is performed using 165 nb − 1 of p +Pb data collected at √sNN = 8 . 16 TeV in 2016. Events are categorised in two analysis channels, consisting of either events with exactly one lepton (electron or muon) and at least four jets, or events with two opposite-charge leptons and at least two jets. In both channels at least one b -tagged jet is also required. Top-quark pair production is observed with a significance over five standard deviations in each channel. The top-quark pair production cross-section is measured to be σ t¯t =58.1±2.0(stat.) +4.8 −4.4 (syst.) nb, with a total uncertainty of 9%. In addition, the nuclear modification factor is measured to be R pA =1.090±0.039(stat.) +0.094 −0.087 (syst.). The measurements are found to be in good agreement with theory predictions involving nuclear parton distribution functions.