Measurement of the tau polarisation at the Z resonance

Using 18.8 pb-1 of data collected in 1990 and 1991, ALEPH has measured the tau polarisation in the decay modes τ→eν anti ν, τ→μν anti ν, τ→πν, τ→ρν and τ→a1ν, using both the individual tau decay kinematics and the event acollinearity. The measurement of the tau polarisation as a function of the production polar angle yields the two parameters Aτ and Ae, where Al=2gVlgAl/[(gVl)2+(gAl)2]. The results Aτ=0.143±0.023 and Ae=0.120±0.026 are consistent with the hypothesis of electron-tau universality. Assuming universality yields a measurement of the effective weak mixing angle sin2θWeff=0.2332±0.0022. (orig.)