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The top-quark mass is measured in the all-hadronic top-antitop quark decay channel using proton-proton collisions at a centre-of-mass energy of√s̅= 8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The data set used in the analysis corresponds to an integrated luminosity of 20.2 fb ⁻¹ . The large multi-jet background is modelled using a data-driven method. The top-quark mass is obtained from template fits to the ratio of the three-jet to the dijet mass. The three-jet mass is obtained from the three jets assigned to the top quark decay. From these three jets the dijet mass is obtained using the two jets assigned to the W boson decay. The top-quark mass is measured to be 173.72±0.55 (stat.)±1.01 (syst.) GeV.

Detailed measurements of$t$ -channel single top-quark production are presented. They use 20.2 fb $^{-1}$of data collected by the ATLAS experiment in proton-proton collisions at a centre-of-mass energy of 8 TeV at the LHC. Total, fiducial and differential cross-sections are measured for both top-quark and top-antiquark production. The fiducial cross-section is measured with a precision of 5.8 % (top quark) and 7.8 % (top antiquark), respectively. The total cross-sections are measured to be$\\sigma_{\\mathrm{tot}}(tq) = 56.7^{+4.3}_{-3.8}\\;$ pb for top-quark production and$\\sigma_{\\mathrm{tot}}(\\bar{t}q) = 32.9^{+3.0}_{-2.7}\\;$ pb for top-antiquark production. In addition, the ratio of top-quark to top-antiquark production cross-sections is determined to be$R_t=1.72 \\pm 0.09$ , with an improved relative precision of 4.9 % since several systematic uncertainties cancel in the ratio. The differential cross-sections as a function of the transverse momentum and rapidity of both the top quark and the top antiquark are measured at both the parton and particle levels. The transverse momentum and rapidity differential cross-sections of the accompanying jet from the$t$ -channel scattering are measured at particle level. All measurements are compared to various Monte Carlo predictions as well as to fixed-order QCD calculations where available.

A measurement of the splitting scales occuring in thek_(\\mathrm{t}{}{j})et-clustering algorithm is presented for final states containing aZboson. The measurement is done using 20.2 fb ⁻¹of proton-proton collision data collected at a centre-of-mass energy of√s̅ = 8TeV by the ATLAS experiment at the LHC in 2012. The measurement is based on charged-particle track information, which is measured with excellent precision in thep_(\\mathrm{T}{}{r})egion relevant for the transition between the perturbative and the non-perturbative regimes. The data distributions are corrected for detector effects, and are found to deviate from state-of-the-art predictions in various regions of the observables.

We present charged-particle distributions sensitive to the underlying event, measured by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV, in low-luminosity Large Hadron Collider fills corresponding to an integrated luminosity of 1.6 nb ⁻¹ . The distributions were constructed using charged particles with absolute pseudorapidity less than 2.5 and with transverse momentum greater than 500 MeV, in events with at least one such charged particle with transverse momentum above 1 GeV. These distributions characterise the angular distribution of energy and particle flows with respect to the charged particle with highest transverse momentum, as a function of both that momentum and of charged-particle multiplicity. The results have been corrected for detector effects and are compared to the predictions of various Monte Carlo event generators, experimentally establishing the level of underlying-event activity at LHC Run 2 energies and providing inputs for the development of event generator modelling. The current models in use for UE modelling typically describe this data to 5% accuracy, compared with data uncertainties of less than 1%.

Same- and opposite-sign charge asymmetries are measured in lepton+jetstt̄events in which ab -hadron decays semileptonically to a soft muon, using data corresponding to an integrated luminosity of 20.3 fb ⁻¹from proton–proton collisions at a centre-of-mass energy of√s̅=8TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. The charge asymmetries are based on the charge of the lepton from the top-quark decay and the charge of the soft muon from the semileptonic decay of ab -hadron and are measured in a fiducial region corresponding to the experimental acceptance. Four CP asymmetries (one mixing and three direct) are measured and are found to be compatible with zero and consistent with the Standard Model.

High-precision measurements by the ATLAS Collaboration are presented of inclusiveW⁺→ℓ⁺ν ,W⁻→ℓ⁻ν̄andZ/γ^(*)→ℓℓ( ℓ=e,μ ) Drell-Yan production cross sections at the LHC. The data were collected in proton-proton collisions at√s̅ = 7TeV with an integrated luminosity of 4.6 fb ⁻¹ . DifferentialW⁺andW⁻cross sections are measured in a lepton pseudorapidity range|η_(ℓ)| = 2.5 . DifferentialZ/γ^(*)cross sections are measured as a function of the absolute dilepton rapidity, for|y_(ℓℓ)| < 3.6 , for three intervals of dilepton mass,m_(ℓℓ) , extending from 46 to 150 GeV. The integrated and differential electron- and muon-channel cross sections are combined and compared to theoretical predictions using recent sets of parton distribution functions. The data, together with the final inclusivee^(±)pscattering cross-section data from H1 and ZEUS, are interpreted in a next-to-next-to-leading-order QCD analysis, and a new set of parton distribution functions, ATLAS-epWZ16, is obtained. The ratio of strange-to-light sea-quark densities in the proton is determined more accurately than in previous determinations based on collider data only, and is established to be close to unity in the sensitivity range of the data. A new measurement of the CKM matrix element|V_(cs)|is also provided.

A search for singly produced vector-likeQquarks, whereQcan be either aTquark with charge+2/3or aYquark with charge-4/3 , is performed in proton-proton collisions recorded with the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 20.3 fb ⁻¹and was produced with a centre-of-mass energy of√s̅=8TeV. This analysis targetsQ → Wbdecays where theWboson decays leptonically. A veto on massive large-radius jets is used to reject the dominanttt̄background. The reconstructedQ -candidate mass, ranging from 0.4 to 1.2 TeV, is used in the search to discriminate signal from background processes. No significant deviation from the Standard Model expectation is observed, and limits are set on theQ → Wbcross-section times branching ratio. The results are also interpreted as limits on theQWbcoupling and the mixing with the Standard Model sector for a singletTquark or aYquark from a doublet.Tquarks with masses below 0.95 TeV are excluded at 95% confidence level, assuming a unit coupling and a BR (T→ Wb) = 0.5 , whereas the expected limit is 1.10 TeV.

Measurements of the electroweak production of aWboson in association with two jets at high dijet invariant mass are performed using√s̅ = 7and8TeV proton-proton collision data produced by the Large Hadron Collider, corresponding respectively to 4.7 and 20.2 fb ⁻¹of integrated luminosity collected by the ATLAS detector. The measurements are sensitive to the production of aWboson via a triple-gauge-boson vertex and include both the fiducial and differential cross sections of the electroweak process.

Measurements of top quark spin observables intt̄events are presented based on 20.2 fb ⁻¹of√s̅ = 8TeV proton-proton collisions recorded with the ATLAS detector at the LHC. The analysis is performed in the dilepton final state, characterised by the presence of two isolated leptons (electrons or muons). There are 15 observables, each sensitive to a different coefficient of the spin density matrix oftt̄production, which are measured independently. Ten of these observables are measured for the first time. All of them are corrected for detector resolution and acceptance effects back to the parton and stable-particle levels. The measured values of the observables at parton level are compared to Standard Model predictions at next-to-leading order in QCD. The corrected distributions at stable-particle level are presented and the means of the distributions are compared to Monte Carlo predictions. No significant deviation from the Standard Model is observed for any observable.

Ratios of top-quark pair toZ -boson cross sections measured from proton–proton collisions at the LHC centre-of-mass energies of√s̅=13 TeV, 8TeV, and 7TeV are presented by the ATLAS Collaboration. Single ratios, at a given√s̅for the two processes and at different√s̅for each process, as well as double ratios of the two processes at different√s̅ , are evaluated. The ratios are constructed using previously published ATLAS measurements of thett̅andZ -boson production cross sections, corrected to a common phase space where required, and a new analysis ofZ → ℓ⁺ ℓ⁻whereℓ=e,μat√s̅=13 TeV performed with data collected in 2015 with an integrated luminosity of3.2 fb ⁻¹ . Correlations of systematic uncertainties are taken into account when evaluating the uncertainties in the ratios. The correlation model is also used to evaluate the combined cross section of theZ→ e⁺e⁻and theZ→ μ⁺ μ⁻channels for each√s̅value. The results are compared to calculations performed at next-to-next-to-leading-order accuracy using recent sets of parton distribution functions. The data demonstrate significant power to constrain the gluon distribution function for the Bjorken- xvalues near 0.1 and the light-quark sea forx<0.02 .