Measurement of $τ$ polarisation in $Z/γ^{*}\rightarrowττ$ decays in proton-proton collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

Abstract

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