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arXiv · 2309.12408

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

CMS Collaboration
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Abstract

The polarization of $τ$ leptons is measured using leptonic and hadronic $τ$ lepton decays in Z $\to$ $τ^+τ^-$ 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 $τ^-$ polarization at the Z boson mass pole is $\mathcal{P}_τ$(Z) = $-$0.144 $\pm$ 0.006 (stat) $\pm$ 0.014 (syst) = $-$0.144 $\pm$ 0.015, in good agreement with the measurement of the $τ$ lepton asymmetry parameter of $A_τ$ = 0.1439 $\pm$ 0.0043 = $-\mathcal{P}_τ$(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 $\sin^{2}θ_\mathrm{W}^{\text{eff}}$, independently of the Z boson production mechanism. The obtained value $\sin^{2}θ_\mathrm{W}^{\text{eff}}$ = 0.2319 $\pm$ 0.0008 (stat) $\pm$ 0.0018 (syst) = 0.2319 $\pm$ 0.0019 is in good agreement with measurements at e$^+$e$^-$ colliders.

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CMS Collaboration. 2024-01-23. Measurement of the $τ$ lepton polarization in Z boson decays in proton-proton collisions at $\sqrt{s}$ = 13 TeV. https://doi.org/10.1007/jhep01(2024)101

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