ATLAS

Abstract:

This Letter presents measurements of the polarization of the top quark in top-antitop quark pair events, using 4:7 fb–1of proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at √s = 7 TeV. Final states containing one or two isolated leptons (electrons or muons) and jets are considered. Two measurements of αℓP, the product of the leptonic spin-analyzing power and the top quark polarization, are performed assuming that the polarization is introduced by either a CP conserving or a maximally CP violating production process. The measurements obtained, αℓPCPC=–0.035 ± 0.014(stat) ± 0:037(syst) and αℓPCPV= 0.020 ± 0:016(stat)+0.013–0.017(syst), are in good agreement with the standard model prediction of negligible top quark polarization.

Abstract:

Characteristics of multi-particle production in proton-proton collisions at √s = 7 TeV are studied as a function of the charged-particle multiplicity, N ch. The produced particles are separated into two classes: those belonging to jets and those belonging to the underlying event. Charged particles are measured with pseudorapidity {pipe}η{pipe}<2.4 and transverse momentum p T>0.25 GeV/c. Jets are reconstructed from charged-particles only and required to have p T>5 GeV/c. The distributions of jet p T, average p T of charged particles belonging to the underlying event or to jets, jet rates, and jet shapes are presented as functions of N ch and compared to the predictions of the pythia and herwig event generators. Predictions without multi-parton interactions fail completely to describe the N ch-dependence observed in the data. For increasing N ch, pythia systematically predicts higher jet rates and harder p T spectra than seen in the data, whereas herwig shows the opposite trends. At the highest multiplicity, the data-model agreement is worse for most observables, indicating the need for further tuning and/or new model ingredients. © 2013 CERN for the benefit of the CMS collaboration.