Measurement of τ polarisation in Z / γ ∗ → τ τ decays in proton-proton collisions at s = 8 TeV with the ATLAS detector

Abstract

This paper presents a measurement of the polarisation of $\tau$ leptons produced in $Z/{\gamma }^{\ast }\to \tau \tau$ 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 $Z/{\gamma }^{\ast }\to \tau \tau$ decays are reconstructed from a hadronically decaying $\tau$ lepton with a single charged particle in the final state, accompanied by a $\tau$ lepton that decays leptonically. The $\tau$ polarisation is inferred from the relative fraction of energy carried by charged and neutral hadrons in the hadronic $\tau$ decays. The polarisation is measured in a fiducial region that corresponds to the kinematic region accessible to this analysis. The $\tau$ polarisation extracted over the full phase space within the $Z/{\gamma }^{\ast }$ mass range of 66 $<m_{Z/{\gamma }^{\ast }}<$ 116 GeV is found to be $P_{\tau }=-0.14\pm 0.02\left(\text{stat}\right)\pm 0.04\left(\text{syst}\right)$ . It is in agreement with the Standard Model prediction of $P_{\tau }=-0.1517\pm 0.0019$ , which is obtained from the ALPGEN event generator interfaced with the PYTHIA 6 parton shower modelling and the TAUOLA $\tau$ decay library.