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.
Categories: hep-ex