Measurement of the τ lepton polarization in Z boson decays in proton-proton collisions at $$ \sqrt{s} $$ = 13 TeV

A bstract The polarization of τ leptons is measured using leptonic and hadronic τ lepton decays in Z → τ + τ − events in proton-proton collisions at $$ \sqrt{s} $$ 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 $$ {\mathcal{P}}_{\tau}\left(\textrm{Z}\right) $$ 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 = $$ -{\mathcal{P}}_{\tau}\left(\textrm{Z}\right) $$ − 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 $$ {\theta}_{\textrm{W}}^{\textrm{eff}} $$ θ W eff , independently of the Z boson production mechanism. The obtained value sin 2 $$ {\theta}_{\textrm{W}}^{\textrm{eff}} $$ θ W eff = 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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