Measurement of the Tau Polarisation at LEP

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 ${\mathcal{A}}_e=0.1504\pm 0.0068$ and ${\mathcal{A}}_{\tau }=0.1451\pm 0.0059$ which are consistent with the hypothesis of $e$-$\tau$ universality. Assuming universality, the value ${\mathcal{A}}_{e-\tau }=0.1474\pm 0.0045$ is obtained from which the effective weak mixing angle ${\sin }^2{\theta }_{\mathrm{W}}^{\mathrm{eff}}=0.23147\pm 0.00057$ is derived.

52 pages, 30 figures