Measurement of the τ lepton polarization in Z bosondecays 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 s√ = 13 TeV recorded by CMS at the CERN LHC with an integrated luminosity of 36.3 fb−1. The measured τ− polarization at the Z boson mass pole is Pτ(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 Aτ = 0.1439±0.0043 = −Pτ(Z) at LEP. The τ 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 sin2θeffW, independently of the Z boson production mechanism. The obtained value sin2θeffW = 0.2319±0.0008 (stat) ± 0.0018 (syst) = 0.2319±0.0019 is in good agreement with measurements at e+e− colliders.

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ORCID for Alexander Belyaev: orcid.org/0000-0002-1733-4408

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