Measurement of the Tau Polarisation at LEP

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 \({\cal A}_{e\mbox{-}\tau} = 0.1474 \pm 0.0045\) is obtained from which the effective weak mixing angle \(\sin^2{\theta_{\mathrm{W}}^{\mathrm{eff}}} =0.23147 \pm 0.00057 \) is derived.