Measurement of τ polarisation at LEP

Using the data collected with the L3 detector at LEP between 1990 and 1995, corresponding to an integrated luminosity of 149 pb−1, the τ longitudinal polarisation has been measured as a function of the production polar angle using the τ decays τ−→h−ντ (h=π,ρ,a1) and τ−→ℓ−ν̄ℓντ (ℓ=e,μ). From this measurement the quantities Ae and Aτ, which depend on the couplings of the electron and the τ to the Z, are determined to be Ae=0.1678±0.0127±0.0030 and Aτ=0.1476±0.0088±0.0062, consistent with the hypothesis of e–τ universality. Under this assumption a value of Aℓ=0.1540±0.0074±0.0044 is obtained, yielding the value of the effective weak mixing angle sin2θW=0.2306±0.0011.