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 root s(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 jet energy scale (JES) and its systematic uncertainty are determined for jets measured with the ATLAS detector using proton-proton collision data with a centre-of-mass energy of [Formula: see text] TeV corresponding to an integrated luminosity of [Formula: see text][Formula: see text]. Jets are reconstructed from energy deposits forming topological clusters of calorimeter cells using the anti-[Formula: see text] algorithm with distance parameters [Formula: see text] or [Formula: see text], and are calibrated using MC simulations. A residual JES correction is applied to account for differences between data and MC simulations. This correction and its systematic uncertainty are estimated using a combination of in situ techniques exploiting the transverse momentum balance between a jet and a reference object such as a photon or a [Formula: see text] boson, for [Formula: see text] and pseudorapidities [Formula: see text]. The effect of multiple proton-proton interactions is corrected for, and an uncertainty is evaluated using in situ techniques. The smallest JES uncertainty of less than 1 % is found in the central calorimeter region ([Formula: see text]) for jets with [Formula: see text]. For central jets at lower [Formula: see text], the uncertainty is about 3 %. A consistent JES estimate is found using measurements of the calorimeter response of single hadrons in proton-proton collisions and test-beam data, which also provide the estimate for [Formula: see text] TeV. The calibration of forward jets is derived from dijet [Formula: see text] balance measurements. The resulting uncertainty reaches its largest value of 6 % for low-[Formula: see text] jets at [Formula: see text]. Additional JES uncertainties due to specific event topologies, such as close-by jets or selections of event samples with an enhanced content of jets originating from light quarks or gluons, are also discussed. The magnitude of these uncertainties depends on the event sample used in a given physics analysis, but typically amounts to 0.5-3 %.

The integrated elliptic flow of charged particles produced in Pb+Pb collisions at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}sNN=2.76 TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}v2, was measured in the pseudorapidity range \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}|η|≤2.5 with the event-plane method. In order to include tracks with very low transverse momentum \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}pT, thus reducing the uncertainty in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek …

A search is presented for direct top squark pair production using events with at least two leptons including a same-flavour opposite-sign pair with invariant mass consistent with the [Formula: see text] boson mass, jets tagged as originating from [Formula: see text]-quarks and missing transverse momentum. The analysis is performed with proton-proton collision data at [Formula: see text] collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of 20.3 fb [Formula: see text]. No excess beyond the Standard Model expectation is observed. Interpretations of the results are provided in models based on the direct pair production of the heavier top squark state ([Formula: see text]) followed by the decay to the lighter top squark state ([Formula: see text]) via [Formula: see text], and for [Formula: see text] pair production in natural gauge-mediated supersymmetry breaking scenarios where …

The process pp --> W± J/ψ provides a powerful probe of the production mechanism of charmonium in hadronic collisions, and is also sensitive to multiple parton interactions in the colliding protons. Using the 2011 ATLAS dataset of 4.5 fb−1 of √ s[overbar]= 7 TeV pp collisions at the LHC, the first observation is made of the production of W ± + prompt J/ψ events in hadronic collisions, using W± --> μνμ and J/ψ --> μ+ μ−. A yield of 27.4 [+7.5 over -6.5] W± + prompt J/ψ events is observed, with a statistical significance of 5.1σ. The production rate as a ratio to the inclusive W ± boson production rate is measured, and the double parton scattering contribution to the cross section is estimated.

This paper presents a study of the performance of the muon reconstruction in the analysis of proton–proton collisions at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}s=7 TeV at the LHC, recorded by the ATLAS detector in 2010. This performance is described in terms of reconstruction and isolation efficiencies and momentum resolutions for different classes of reconstructed muons. The results are obtained from an analysis of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}J/ψ meson and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}Z boson decays to dimuons, reconstructed from a data sample corresponding to an integrated luminosity of 40 pb\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}-1. The measured performance is compared to Monte Carlo predictions …

A measurement of the parity-violating decay asymmetry parameter, α b, and the helicity amplitudes for the decay Λ b 0→ J/ψ (μ+ μ−) Λ 0 (p π−) is reported. The analysis is based on 1400 Λ b 0 and Λ b 0 baryons selected in 4.6 fb− 1 of proton–proton collision data with a center-of-mass energy of 7 TeV recorded by the ATLAS experiment at the LHC. By combining the Λ b 0 and Λ b 0 samples under the assumption of C P conservation, the value of α b is measured to be 0.30±0.16 (stat)±0.06 (syst). This measurement provides a test of theoretical models based on perturbative QCD or heavy-quark effective theory.

A search is presented for dark matter pair production in association with a W or Z boson in p p collisions representing 20.3 fb− 1 of integrated luminosity at s= 8 TeV using data recorded with the ATLAS detector at the Large Hadron Collider. Events with a hadronic jet with the jet mass consistent with a W or Z boson, and with large missing transverse momentum are analyzed. The data are consistent with the standard model expectations. Limits are set on the mass scale in effective field theories that describe the interaction of dark matter and standard model particles, and on the cross section of Higgs production and decay to invisible particles. In addition, cross section limits on the anomalous production of W or Z bosons with large missing transverse momentum are set in two fiducial regions.

A measurement of the B s 0→ J/ψ ϕ decay parameters, updated to include flavor tagging is reported using 4.9 fb− 1 of integrated luminosity collected by the ATLAS detector from s= 7 TeV p p collisions recorded in 2011 at the LHC. The values measured for the physical parameters are ϕ s= 0.12±0.25 (stat)±0.05 (syst) rad Δ Γ s= 0.053±0.021 (stat)±0.010 (syst) ps− 1 Γ s= 0.677±0.007 (stat)±0.004 (syst) ps− 1| A∥(0)| 2= 0.220±0.008 (stat)±0.009 (syst)| A 0 (0)| 2= 0.529±0.006 (stat)±0.012 (syst) δ⊥= 3.89±0.47 (stat)±0.11 (syst) rad where the parameter Δ Γ s is constrained to be positive. The S-wave contribution was measured and found to be compatible with zero. Results for ϕ s and Δ Γ s are also presented as 68% and 95% likelihood contours, which show agreement with the Standard Model expectations.

A measurement of the cross section for the production of isolated prompt photons in p p collisions at a center-of-mass energy s= 7 TeV is presented. The results are based on an integrated luminosity of 4.6 fb− 1 collected with the ATLAS detector at the LHC. The cross section is measured as a function of photon pseudorapidity η γ and transverse energy E T γ in the kinematic range 100≤ E T γ< 1000 GeV and in the regions| η γ|< 1.37 and 1.52≤| η γ|< 2.37. The results are compared to leading-order parton-shower Monte Carlo models and next-to-leading-order perturbative QCD calculations. Next-to-leading-order perturbative QCD calculations agree well with the measured cross sections as a function of E T γ and η γ.

A bstract A search for the direct production of charginos and neutralinos in final states with three leptons and missing transverse momentum is presented. The analysis is based on 20.3 fb− 1 of s= 8 TeV proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations and limits are set in R-parity-conserving phenomenological Minimal Supersymmetric Standard Models and in simplified supersymmetric models, significantly extending previous results. For simplified supersymmetric models of direct chargino \left(χ_1^±\right) and next-to-lightest neutralino \left(χ_2^0\right) production with decays to lightest neutralino \left(χ_1^0\right) via either all three generations of sleptons, staus only, gauge bosons, or Higgs bosons, \left(χ_1^±\right) and \left(χ_2^0\right) masses are excluded up to 700 GeV, 380 GeV, 345 …

Measurements of fiducial cross sections for the electroweak production of two jets in association with a Z -boson are presented. The measurements are performed using 20 . 3 fb −1 of proton-proton collision data collected at a centre-of-mass energy of √(s) = 8 TeV by the ATLAS experiment at the Large Hadron Collider. The electroweak component is extracted by a fit to the dijet invariant mass distribution in a fiducial region chosen to enhance the electroweak contribution over the dominant background in which the jets are produced via the strong interaction. The electroweak cross sections measured in two fiducial regions are in good agreement with the Standard Model expectations and the background-only hypothesis is rejected with significance above the 5 σ level. The electroweak process includes the vector boson fusion production of a Z -boson and the data are used to place limits on anomalous triple gauge boson couplings. In addition, measurements of cross sections and differential distributions for inclusive Z -boson-plus-dijet production are performed in five fiducial regions, each with different sensitivity to the electroweak contribution. The results are corrected for detector effects and compared to predictions from the Sherpa and Powheg event generators.