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To probe the$Wtb$vertex structure, top-quark and$W$ -boson polarisation observables are measured from$t$ -channel single-top-quark events produced in proton--proton collisions at a centre-of-mass energy of 8 TeV. The dataset corresponds to an integrated luminosity of 20.2 fb $^{-1}$ , recorded with the ATLAS detector at the LHC. Selected events contain one isolated electron or muon, large missing transverse momentum and exactly two jets, with one of them identified as likely to contain a$b$ -hadron. Stringent selection requirements are applied to discriminate$t$ -channel single-top-quark events from background. The polarisation observables are extracted from asymmetries in angular distributions measured with respect to spin quantisation axes appropriately chosen for the top quark and the$W$boson. The asymmetry measurements are performed at parton level by correcting the observed angular distributions for detector effects and hadronisation after subtracting the background contributions. The measured top-quark and$W$ -boson polarisation values are in agreement with the Standard Model predictions. Limits on the imaginary part of the anomalous coupling$g_{\\mathrm{R}}$are also set from model-independent measurements.

The distributions of transverse momentum and longitudinal momentum fraction of charged particles in jets are measured in Pb+Pb and pp collisions with the ATLAS detector at the LHC. The distributions are measured as a function of jet transverse momentum and rapidity. The analysis utilises an integrated luminosity of 0.14 nb $^{-1}$of Pb+Pb data and 4.0 pb $^{-1}$of pp data collected in 2011 and 2013, respectively, at the same centre-of-mass energy of 2.76 TeV per colliding nucleon pair. The distributions measured in pp collisions are used as a reference for those measured in Pb+Pb collisions in order to evaluate the impact on the internal structure of jets from the jet energy loss of fast partons propagating through the hot, dense medium created in heavy-ion collisions. Modest but significant centrality-dependent modifications of fragmentation functions in Pb+Pb collisions with respect to those in pp collisions are seen. No significant dependence of modifications on jet$p_{\\mathrm{T}}$and rapidity selections is observed except for the fragments with the highest transverse momenta for which some reduction of yields is observed for more forward jets.

A search with the ATLAS detector is presented for the Standard Model Higgs boson produced by vector-boson fusion and decaying to a pair of bottom quarks, using 20.2fb⁻¹of LHC proton–proton collision data at√s̅= 8 TeV. The signal is searched for as a resonance in the invariant mass distribution of a pair of jets containingb -hadrons in vector-boson-fusion candidate events. The yield is measured to be-0.8 ± 2.3times the Standard Model cross-section for a Higgs boson mass of 125 GeV. The upper limit on the cross-section times the branching ratio is found to be 4.4 times the Standard Model cross-section at the 95% confidence level, consistent with the expected limit value of 5.4 (5.7) in the background-only (Standard Model production) hypothesis.

The centrality dependence of the mean charged-particle multiplicity as a function of pseudorapidity is measured in approximately 1$\\mu$ b $^{-1}$of proton--lead collisions at a nucleon--nucleon centre-of-mass energy of$\\sqrt{s_{_{\\rm{NN}}} = 5.02$TeV using the ATLAS detector at the Large Hadron Collider. Charged particles with absolute pseudorapidity less than 2.7 are reconstructed using the ATLAS pixel detector. The$p$ +Pb collision centrality is characterised by the total transverse energy measured in the Pb-going direction of the forward calorimeter. The charged-particle pseudorapidity distributions are found to vary strongly with centrality, with an increasing asymmetry between the proton-going and Pb-going directions as the collisions become more central. Three different estimations of the number of nucleons participating in the$p$ +Pb collision have been carried out using the Glauber model as well as two Glauber--Gribov inspired extensions to the Glauber model. Charged-particle multiplicities per participant pair are found to vary differently for these three models, highlighting the importance of including colour fluctuations in nucleon--nucleon collisions in the modelling of the initial state of$p$ +Pb collisions.

The decays B+c→J/ψD+s and B+c→J/ψD∗+s are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 and 20.6 fb−1 of pp collisions collected at centre-of-mass energies s√=7 TeV and 8 TeV, respectively. Signal candidates are identified through J/ψ→μ+μ− and D(∗)+s→ϕπ+(γ/π0) decays. With a two-dimensional likelihood fit involving the B+c reconstructed invariant mass and an angle between the μ+ and D+s candidate momenta in the muon pair rest frame, the yields of B+c→J/ψD+s and B+c→J/ψD∗+s, and the transverse polarisation fraction in B+c→J/ψD∗+s decay are measured. The transverse polarisation fraction is determined to be Γ±±(B+c→J/ψD∗+s)/Γ(B+c→J/ψD∗+s)=0.38±0.23±0.07, and the derived ratio of the branching fractions of the two modes is BB+c→J/ψD∗+s/BB+c→J/ψD+s=2.8+1.2−0.8±0.3, where the first error is statistical and the second is systematic. Finally, a sample of B+c→J/ψπ+ decays is used to derive the ratios of branching fractions BB+c→J/ψD+s/BB+c→J/ψπ+=3.8±1.1±0.4±0.2 and BB+c→J/ψD∗+s/BB+c→J/ψπ+=10.4±3.1±1.5±0.6, where the third error corresponds to the uncertainty of the branching fraction of D+s→ϕ(K+K−)π+ decay. The available theoretical predictions are generally consistent with the measurement.

This article documents the performance of the ATLAS muon identification and reconstruction using the LHC dataset recorded at [Formula omitted] TeV in 2015. Using a large sample of [Formula omitted] and [Formula omitted] decays from 3.2 fb [Formula omitted] of pp collision data, measurements of the reconstruction efficiency, as well as of the momentum scale and resolution, are presented and compared to Monte Carlo simulations. The reconstruction efficiency is measured to be close to [Formula omitted] over most of the covered phase space ( [Formula omitted] and [Formula omitted] GeV). The isolation efficiency varies between 93 and [Formula omitted] depending on the selection applied and on the momentum of the muon. Both efficiencies are well reproduced in simulation. In the central region of the detector, the momentum resolution is measured to be [Formula omitted] ( [Formula omitted]) for muons from [Formula omitted] ( [Formula omitted]) decays, and the momentum scale is known with an uncertainty of [Formula omitted]. In the region [Formula omitted], the [Formula omitted] resolution for muons from [Formula omitted] decays is [Formula omitted] while the precision of the momentum scale for low- [Formula omitted] muons from [Formula omitted] decays is about [Formula omitted].

The production of a$W$boson decaying to$e\\nu$or$\\mu\\nu$in association with a$W$or$Z$boson decaying to two jets is studied using$4.6~\\mathrm{fb}^{-1}$of proton--proton collision data at$\\sqrt{\\rm{s}} = 7$TeV recorded with the ATLAS detector at the LHC. The combined$WW+WZ$cross section is measured with a significance of 3.4 $\\sigma$and is found to be$68 \\pm 7 \\ \\mathrm{(stat.)} \\pm 19 \\ \\mathrm{(syst.)} \\ pb$ , in agreement with the Standard Model expectation of$61.1 \\pm 2.2 \\ \\mathrm{pb}$ . The distribution of the transverse momentum of the dijet system is used to set limits on anomalous contributions to the triple gauge coupling vertices and on parameters of an effective-field-theory model.

A measurement of$\\textit{W}$boson production in lead-lead collisions at$\\sqrt{s_{\\mathrm{NN}}}=$ 2.76 TeV is presented. It is based on the analysis of data collected with the ATLAS detector at the LHC in 2011 corresponding to an integrated luminosity of 0.14$\\mathrm{nb}^{-1}$and 0.15$\\mathrm{nb}^{-1}$in the muon and electron decay channels, respectively. The differential production cross-sections and lepton charge asymmetry are each measured as a function of the average number of participating nucleons$\\langle N_{\\mathrm{part}} \\rangle$and absolute pseudorapidity of the charged lepton. The results are compared to predictions based on next-to-leading-order QCD calculations. These measurements are, in principle, sensitive to possible nuclear modifications to the parton distribution functions and also provide information on scaling of$\\textit{W}$boson production in multi-nucleon systems.

A search for strongly produced supersymmetric particles is conducted using signatures involving multiple energetic jets and either two isolated leptons ( eorμ ) with the same electric charge, or at least three isolated leptons. The search also utilises jets originating from b-quarks, missing transverse momentum and other observables to extend its sensitivity. The analysis uses a data sample corresponding to a total integrated luminosity of 20.3 fb ⁻¹of√s̅ =8 TeV proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider in 2012. No deviation from the Standard Model expectation is observed. New or significantly improved exclusion limits are set on a wide variety of supersymmetric models in which the lightest squark can be of the first, second or third generations, and in which R-parity can be conserved or violated.

Searches for the electroweak production of charginos, neutralinos and sleptons in final states characterized by the presence of two leptons (electrons and muons) and missing transverse momentum are performed using 20.3 fb-1 of proton-proton collision data at sqrt(s) = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. Limits are set on the masses of the lightest chargino, next-to-lightest neutralino and sleptons for different lightest-neutralino mass hypotheses in simplified models. Results are also interpreted in various scenarios of the phenomenological Minimal Supersymmetric Standard Model.