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

Philip James Clark, Sinead Farrington, Michele Faucci Giannelli, Yanyan Gao, Ahmed Hasib, Christos Leonidopoulos, Victoria Jane Martin, Liza Mijović, Corrinne Mills, Benjamin Wynne, Atlas Collaboration

Research output: Contribution to journalArticlepeer-review

## Abstract

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.
Original language English 163 The European Physical Journal C (EPJ C) C78 2 https://doi.org/10.1140/epjc/s10052-018-5619-1 Published - 24 Feb 2018

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