A measurement of τ polarization at LEP

Abstract

With a data sample of 86 000 Z → τ⁺τ⁻(γ) events collected in 1990 through 1993 we have measured the polarization of τ-leptons as a function of the production polar angle using the following 1-prong τ decay modes: τ⁻ → e^-ν̅ν_eν_τ, τ⁻→π⁻(K⁻)ν_τ, τ⁻ → ϱ⁻ν_τ and τ⁻ →a₁⁻ν_τ. We obtain for the ratio vector to axial-vector weak neutral couplings for electrons g_Ve/g_Ae=0.0791±0.0099(stat)±0.0025 (syst) and taus g_Vτ/g_Aτ=0.0752±0.0063 (stat)±0.0045 (syst) consistent with the hypothesis of e - τ universality. Assuming universality of the e - τ neutral current we determine the effective electroweak mixing angle to be sin² θ^eff_w = 0.2309 ± 0.0016.