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 note 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 ($\tau \rightarrow \text{hadrons + } \nu$) with a single charged particle in the final state accompanied by a $\tau$ lepton that decays leptonically ($\tau \rightarrow e/\mu\text{ + }\nu\bar{\nu}$). 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 in the full phase space in the $Z/\gamma^{*}$ mass range of 66 GeV $ < 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 generator interfaced with Pythia6 and Tauola.