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

The polarization of τ leptons is measured using leptonic and hadronic τ lepton decays in Z → τ ⁺ τ ⁻ events in proton-proton collisions at (Formula presented.) 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 _τ(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 _τ(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 θWeff , independently of the Z boson production mechanism. The obtained value sin ² θWeff = 0.2319 ± 0.0008(stat) ± 0.0018(syst) = 0.2319 ± 0.0019 is in good agreement with measurements at e ⁺ e ⁻ colliders. [Figure not available: see fulltext.].