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

TL;DR

This paper measures the polarization of tau leptons in Z boson decays at the LHC, providing results consistent with previous LEP measurements and offering insights into the weak mixing angle using proton-proton collision data.

Contribution

First measurement of tau lepton polarization in Z decays at the LHC, extracting the effective weak mixing angle from proton-proton collision data.

Findings

Tau polarization measured as -0.144 ± 0.015.

Effective weak mixing angle determined as 0.2319 ± 0.0019.

Results agree with previous LEP measurements.

Abstract

The polarization of $\tau$ leptons is measured using leptonic and hadronic $\tau$ lepton decays in Z $\to$ $\tau^+\tau^-$ 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 $\tau^-$ polarization at the Z boson mass pole is $\mathcal{P}_{\tau}$(Z) = $-$0.144 $\pm$ 0.006 (stat) $\pm$ 0.014 (syst) = $-$0.144 $\pm$ 0.015, in good agreement with the measurement of the $\tau$ lepton asymmetry parameter of $A_{\tau}$ = 0.1439 $\pm$ 0.0043 = $-\mathcal{P}_{\tau}$(Z) at LEP. The $\tau$ polarization depends on the ratio of the vector to axial-vector couplings of the $\tau$ leptons in the neutral current expression, and thus on the effective weak mixing angle $\sin^{2}\theta_\mathrm{W}^{\text{eff}}$, independently of the Z boson production mechanism. The obtained value $\sin^{2}\theta_\mathrm{W}^{\text{eff}}$ = 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.