Measurement of the TAU polarisation at LEP

The polarisation of Tau 's produced in Z decays is measured using 160 pb^-2 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 Ao=0,1504+-0,0068 and A tau= 0,1451+-0,0059 which are consistent with the hypothesis of e-tau universality. Assuming universality, the value Ae-tau=0,1474+-0,0045 is obtained from which the effective weak mixing angle sin ^2(Theta)W ^eff=0,23147+-0,00057 is derived.