Measurement of $\tau $ polarisation in $Z/\gamma ^{*}\rightarrow \tau \tau $ decays in proton–proton collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

This paper presents a measurement of the polarisation of $\tau $ leptons produced in $Z/\gamma ^{*}\rightarrow \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 ^{*}\rightarrow \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 $Z/\gamma ^{*}$ mass range of 66 $< m_{Z/\gamma ^{*}} < $ 116 GeV is found to be $P_{\tau } =-0.14 \pm 0.02 (\text {stat}) \pm 0.04 (\text {syst})$ . 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.