Measurement of tau polarisation in Z/gamma* -> tau tau decays in proton-proton collisions at root s=8 TeV with the ATLAS detector

Abstract

This paper presents a measurement of the polarisation of τ leptons produced in

Z /

γ

∗

→ τ τ

decays which is performed with a dataset of proton-proton collisions at

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 /

γ

∗

→ τ τ

decays are reconstructed from a hadronically decaying τ lepton with a single charged particle in the final state, accompanied by a τ lepton that decays leptonically. The τ polarisation is inferred from the relative fraction of energy carried by charged and neutral hadrons in the hadronic τ decays. The polarisation is measured in a fiducial region that corresponds to the kinematic region accessible to this analysis. The τ polarisation extracted over the full phase space within the

Z /

γ

∗

mass range of 66

<

m

Z /

γ

∗

<

116 GeV is found to be

P τ

= - 0.14 ± 0.02

( stat )

± 0.04

( syst )

. It is in agreement with the Standard Model prediction of

P τ

= - 0.1517 ± 0.0019

, which is obtained from the ALPGEN event generator interfaced with the PYTHIA 6 parton shower modelling and the TAUOLA τ decay library.