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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.

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 .

A measurement of theZZproduction in theℓ⁻ℓ⁺ℓ^(′ -)ℓ^(′ +)andℓ⁻ℓ⁺νν̄channels(ℓ = e, μ)in proton–proton collisions at√s̅ = 8TeV at the Large Hadron Collider at CERN, using data corresponding to an integrated luminosity of 20.3 fb ⁻¹collected by the ATLAS experiment in 2012 is presented. The fiducial cross sections forZZ→ℓ⁻ℓ⁺ℓ^(′ -)ℓ^(′ +)andZZ→ ℓ⁻ℓ⁺νν̄are measured in selected phase-space regions. The total cross section forZZevents produced with bothZbosons in the mass range 66 to 116 GeV is measured from the combination of the two channels to be7.3±0.4\\textrm{(stat)}{±}{0}.3\\textrm{(syst)}{±}{0}.2\\textrm{(lumi)}{}{p}b, which is consistent with the Standard Model prediction of6.6^(+0.7)_(-0.6)pb. The differential cross sections in bins of various kinematic variables are presented. The differential event yield as a function of the transverse momentum of the leadingZboson is used to set limits on anomalous neutral triple gauge boson couplings inZZproduction.

This paper presents a measurement of the polarisation of$W$bosons from$t\\bar{t}$decays, reconstructed in events with one high- $p_{\\mbox{T}}$lepton and at least four jets. Data from$pp$collisions at the LHC were collected at$\\sqrt{s}$= 8 TeV and correspond to an integrated luminosity of 20.2 fb $^{-1}$ . The angle$\\theta^{*}$between the$b$ -quark from the top quark decay and a direct$W$boson decay product in the$W$boson rest frame is sensitive to the$W$boson polarisation. Two different$W$decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying$W$boson, respectively. The most precise measurement of the$W$boson polarisation via the distribution of$\\cos{\\theta^{*}}$is obtained using the leptonic analyser and events in which at least two of the jets are tagged as$b$ -quark jets. The fitted fractions of longitudinal, left- and right-handed polarisation states are$F_{\\mathrm{0}}= 0.709 \\pm {0.019}$ ,$F_{\\mathrm{L}}= 0.299 \\pm {0.015}$and$F_{\\mathrm{R}}= -0.008 \\pm {0.014}$ , and are the most precisely measured$W$boson polarisation fractions to date. Limits on anomalous couplings of the$Wtb$vertex are set.

Measurements of the jet activity intt̄events produced in proton–proton collisions at√s̅=8TeV are presented, using20.3 fb ⁻¹of data collected by the ATLAS experiment at the Large Hadron Collider. The events were selected in the dileptoneμdecay channel with two identifiedb -jets. The numbers of additional jets for various jet transverse momentum ( p_(T) ) thresholds, and the normalised differential cross-sections as a function ofp_(T)for the five highest- p_(T)additional jets, were measured in the jet pseudorapidity range|η|<4.5 . The gap fraction, the fraction of events which do not contain an additional jet in a central rapidity region, was measured for several rapidity intervals as a function of the minimump_(T)of a single jet or the scalar sum ofp_(T)of all additional jets. These fractions were also measured in different regions of the invariant mass of theeμ bb̄system. All measurements were corrected for detector effects, and found to be mostly well-described by predictions from next-to-leading-order and leading-ordertt̄event generators with appropriate parameter choices. The results can be used to further optimise the parameters used in such generators.

This paper describes a measurement of fiducial and differential cross sections of gluon-fusion Higgs boson production in theH→ WW^(∗)→ eνμνchannel, using 20.3 fb ⁻¹of proton-proton collision data. The data were produced at a centre-of-mass energy of√s̅ = 8TeV at the CERN Large Hadron Collider and recorded by the ATLAS detector in 2012. Cross sections are measured from the observedH→ WW^(∗)→ eνμνsignal yield in categories distinguished by the number of associated jets. The total cross section is measured in a fiducial region defined by the kinematic properties of the charged leptons and neutrinos. Differential cross sections are reported as a function of the number of jets, the Higgs boson transverse momentum, the dilepton rapidity, and the transverse momentum of the leading jet. The jet-veto efficiency, or fraction of events with no jets above a given transverse momentum threshold, is also reported. All measurements are compared to QCD predictions from Monte Carlo generators and fixed-order calculations, and are in agreement with the Standard Model predictions.

Two searches for new phenomena in final states containing a same-flavour opposite-lepton (electron or muon) pair, jets, and large missing transverse momentum are presented. These searches make use of proton–proton collision data, collected during 2015 and 2016 at a centre-of-mass energy√s̅=13TeV by the ATLAS detector at the Large Hadron Collider, which correspond to an integrated luminosity of 14.7 fb ⁻¹ . Both searches target the pair production of supersymmetric particles, squarks or gluinos, which decay to final states containing a same-flavour opposite-sign lepton pair via one of two mechanisms: a leptonically decaying Z boson in the final state, leading to a peak in the dilepton invariant-mass distribution around the Z boson mass; and decays of neutralinos (e.g.χ̃₂⁰ → ℓ⁺ℓ⁻ χ̃₁⁰ ), yielding a kinematic endpoint in the dilepton invariant-mass spectrum. The data are found to be consistent with the Standard Model expectation. Results are interpreted in simplified models of gluino-pair (squark-pair) production, and provide sensitivity to gluinos (squarks) with masses as large as 1.70 TeV (980 GeV).