Measurement of ? polarisation in Z/? ? ? ? ? decays in proton–proton collisions at ?s = 8 TeV with the ATLAS detector

Abstract

This paper presents a measurement of the polarisation of ? leptons produced in Z/????? decays which is performed with a dataset of proton—proton collisions at s?=8 TeV, corresponding to an integrated luminosity of 20.2 fb?1 recorded with the ATLAS detector at the LHC in 2012. The Z/????? decays are reconstructed from a hadronically decaying ? lepton with a single charged particle in the final state, accompanied by a ? lepton that decays leptonically. The ? polarisation is inferred from the relative fraction of energy carried by charged and neutral hadrons in the hadronic ? decays. The polarisation is measured in a fiducial region that corresponds to the kinematic region accessible to this analysis. The ? polarisation extracted over the full phase space within the Z/?? mass range of 66 <mZ/??< 116 GeV is found to be P?=?0.14±0.02(stat)±0.04(syst). It is in agreement with the Standard Model prediction of P?=?0.1517±0.0019, which is obtained from the ALPGEN event generator interfaced with the PYTHIA 6 parton shower modelling and the TAUOLA ? decay library.