Beam-induced and cosmic-ray backgrounds observed in the ATLAS detector during the LHC 2012 proton-proton running period

This paper discusses various observations on beam-induced and cosmic-ray backgrounds in the ATLAS detector during the LHC 2012 proton-proton run. Building on published results based on 2011 data, the correlations between background and residual pressure of the beam vacuum are revisited. Ghost charge evolution over 2012 and its role for backgrounds are evaluated. New methods to monitor ghost charge with beam-gas rates are presented and observations of LHC abort gap population by ghost charge are discussed in detail. Fake jets from colliding bunches and from ghost charge are analysed with improved methods, showing that ghost charge in individual radio-frequency buckets of the LHC can be resolved. Some results of two short periods of dedicated cosmic-ray background data-taking are shown; in particular cosmic-ray muon induced fake jet rates are compared to Monte Carlo simulations and to the fake jet rates from beam background. A thorough analysis of a particular LHC fill, where abnormally high background was observed, is presented. Correlations between backgrounds and beam intensity losses in special fills with very high β are studied

Transverse momentum, rapidity, and centrality dependence of inclusive charged-particle production in p + Pb collisions measured by the ATLAS experiment

Measurements of the per-event charged-particle yield as a function of the charged-particle transverse momentum and rapidity are performed using p+Pb collision data collected by the ATLAS experiment at the LHC at a centre-of-mass energy of √sNN = 5.02 TeV. Charged particles are reconstructed over pseudorapidity |η| < 2.3 and transverse momentum between 0.1 GeV and 22 GeV in a dataset corresponding to an integrated luminosity of 1 μb−1. The results are presented in the form of charged-particle nuclear modification factors, where the p+Pb charged-particle multiplicities are compared between central and peripheral p+Pb col- lisions as well as to charged-particle cross sections measured in pp collisions. The p+Pb col- lision centrality is characterized by the total transverse energy measured in −4.9 < η < −3.1, which is in the direction of the outgoing lead beam. Three different estimations of the num- ber of nucleons participating in the p+Pb collision are carried out using the Glauber model and two Glauber–Gribov colour-fluctuation extensions to the Glauber model. The values of the nuclear modification factors are found to vary significantly as a function of rapidity and transverse momentum. A broad peak is observed for all centralities and rapidities in the nuc- lear modification factors for charged-particle transverse momentum values around 3 GeV. The magnitude of the peak increases for more central collisions as well as rapidity ranges closer to the direction of the outgoing lead nucleus

Study of hard double-parton scattering in four-jet events in pp collisions at √s=7 TeV with the ATLAS experiment

Inclusive four-jet events produced in proton–proton collisions at a centre-of-mass energy of√s = 7 TeV are analysed for the presence of hard double-parton scattering using data corres- ponding to an integrated luminosity of 37.3 pb−1, collected with the ATLAS detector at the LHC. The contribution of hard double-parton scattering to the production of four-jet events is extracted using an artificial neural network, assuming that hard double-parton scattering can be approximated by an uncorrelated overlaying of dijet events. For events containing at least four jets with transverse momentum pT ≥ 20 GeV and pseudorapidity |η| ≤ 4.4, and at least one having pT ≥ 42.5 GeV, the contribution of hard double-parton scattering is estim- ated to be fDPS = 0.092 +0.005 −0.011 (stat.) +0.033 −0.037 (syst.). After combining this measurement with those of the inclusive dijet and four-jet cross-sections in the appropriate phase space regions, the effective cross-section, σeff , was determined to be σeff = 14.9 +1.2 −1.0 (stat.) +5.1 −3.8 (syst.) mb. This result is consistent within the quoted uncertainties with previous measurements of σeff , performed at centre-of-mass energies between 63 GeV and 8 TeV using various final states, and it corresponds to 21+7 −6% of the total inelastic cross-section measured at √s = 7 TeV. The distributions of the observables sensitive to the contribution of hard double-parton scattering, corrected for detector effects, are also provided