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→ev\(\bar v\), τ→μv\(\bar v\), 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 parametersAτ andA e , whereA l =2g v l g A l /(g v l )2+(g A l )2] The resultsAτ=0.143±0.023 andA e =0.120±0.026 are consistent with the hypothesis of electron-tau universality. Assuming universality yields a measurement of the effective weak mixing angle sin2θ w eff =0.2332±0.0022.