Improved tau polarisation measurement

Using 22 pb-1 of data collected at LEP in 1992 on the peak of the Z resonance, the ALEPH collaboration has measured the polarisation of the tau leptons decaying into {Mathematical expression}, πν, ρν and a1 ν from their individual decay product distributions. The measurement of the tau polarisation as a function of the production polar angle yields the two parameters N τ and N e, where, in terms of the axial and vector couplings g Al and g Vl, N l=2 g Vl gAl/(g Vl 2 +g Al 2 ). This analysis follows to a large extent the methods devised for the 1990 and 1991 data but with improvements which bring a better understanding of the systematic uncertainties. Combining the 1992 measurements with our previously published results yields N τ=0.136±0.012±0.009 and N e=0.129±0.016±0.005. Assuming e-τ universality, the measurements imply an effective weak mixing angle of sin 2 θ W eff =0.2332±0.0014.