Measurement of the Tau Polarisation at LEP

Abstract

The polarisation of $τ$'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 ${\cal A}_τ = 0.1451 \pm 0.0059$ which are consistent with the hypothesis of $e$-$τ$ universality. Assuming universality, the value ${\cal A}_{e{-}τ} = 0.1474 \pm 0.0045$ is obtained from which the effective weak mixing angle $\sin^2 {θ_{\mathrm{W}}^{\mathrm{eff}}} =0.23147 \pm 0.00057 $ is derived.