Measurement of the τ lepton polarization in Z boson decays in proton-proton collisions at √s=13 TeV

Abstract

The polarization of τ leptons is measured using leptonic and hadronic τ lepton decays in Z → τ^+τ^− events in proton-proton collisions at \sqrt{s} = 13 TeV recorded by CMS at the CERN LHC with an integrated luminosity of 36.3 fb^{−1}. The measured τ− lepton polarization at the Z boson mass pole is P_\tau (Z) = −0.144 ± 0.006 (stat) ± 0.014 (syst) = −0.144 ± 0.015, in good agreement with the measurement of the τ lepton asymmetry parameter of A_τ = 0.1439 ± 0.0043 = -P_\tau (Z) at LEP. The τ lepton polarization depends on the ratio of the vector to axial-vector couplings of the τ leptons in the neutral current expression, and thus on the effective weak mixing angle sin^2 \theta ^{eff}_W, independently of the Z boson production mechanism. The obtained value sin^2 \theta ^{eff}_W = 0.2319 ± 0.0008(stat) ± 0.0018(syst) = 0.2319 ± 0.0019 is in good agreement with measurements at e^+e^− colliders.