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

Abstract

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