Measurement of τ polarisation in Z/γ∗→ττ decays in proton–proton collisions at s√=8 TeV with the ATLAS detector

This paper presents a measurement of the polarisation of ττ leptons produced in Z/γ∗→ττZ/γ∗→ττdecays which is performed with a dataset of proton—proton collisions at s√=8s=8 TeV, corresponding to an integrated luminosity of 20.2 fb−1−1 recorded with the ATLAS detector at the LHC in 2012. The Z/γ∗→ττZ/γ∗→ττ decays are reconstructed from a hadronically decaying ττ lepton with a single charged particle in the final state, accompanied by a ττ lepton that decays leptonically. The ττ polarisation is inferred from the relative fraction of energy carried by charged and neutral hadrons in the hadronic ττ decays. The polarisation is measured in a fiducial region that corresponds to the kinematic region accessible to this analysis. The ττpolarisation extracted over the full phase space within the Z/γ∗Z/γ∗ mass range of 66 <mZ/γ∗<<mZ/γ∗<116 GeV is found to be Pτ=−0.14±0.02(stat)±0.04(syst)Pτ=−0.14±0.02(stat)±0.04(syst). It is in agreement with the Standard Model prediction of Pτ=−0.1517±0.0019Pτ=−0.1517±0.0019, which is obtained from the ALPGEN event generator interfaced with the PYTHIA 6 parton shower modelling and the TAUOLA ττdecay library.