Measurement of the $\tau$ Polarisation in $Z/\gamma^*\rightarrow\tau\tau$ Decays with the ATLAS Detector

Abstract

A measurement of the

$\tau$ polarisation in

$Z/\gamma^*\rightarrow\tau\tau$ decays is presented. The analysis is based on the

$20.2$ fb

$^{-1}$ of proton

$-$ proton collision data collected at a centre-of-mass energy of

$\sqrt{s}=8$ TeV by the ATLAS experiment at the CERN Large Hadron Collider in 2012. Events with one leptonic

$\tau$ decay and one hadronic

$\tau$ decay with a single charged particle in the final state are selected. The

$\tau$ polarisation is measured using the kinematic configuration of the hadronic decay. A polarisation of

$P_\tau~=-0.14\;\pm~0.02\;(\textrm{stat})\;\pm~0.04\;(\textrm{syst})$ is measured in the mass range

$66 < m_{Z/\gamma^*} < 116$ GeV. It agrees with the Standard Model prediction of

$P_\tau~=-0.1517\pm0.0014\;(\textrm{stat})\pm0.0013\;(\textrm{syst})$ .