Measurement of the Tau Polarisation at LEP

TL;DR

This paper reports a precise measurement of tau lepton polarisation at LEP, confirming the universality of electron and tau couplings and deriving the effective weak mixing angle from the data.

Contribution

The study provides the first detailed measurement of tau polarisation variation with angle at LEP, testing lepton universality and extracting electroweak parameters.

Findings

Tau polarisation parameters are measured with high precision.

Results support e-tau universality hypothesis.

Effective weak mixing angle is determined as 0.23147 ± 0.00057.

Abstract

The polarisation of $\tau$'s produced in Z decay is measured using 160 pb$^{-1}$ of data accumulated at LEP by the ALEPH detector between 1990 and 1995. The variation of the polarisation with polar angle yields the two parameters ${\cal A}_e = 0.1504 \pm 0.0068 $ and ${\cal A}_{\tau} = 0.1451 \pm 0.0059$ which are consistent with the hypothesis of $e$-$\tau$ universality. Assuming universality, the value ${\cal A}_{e{-}\tau} = 0.1474 \pm 0.0045$ is obtained from which the effective weak mixing angle $\sin^2 {\theta_{\mathrm{W}}^{\mathrm{eff}}} =0.23147 \pm 0.00057 $ is derived.