A precise measurement of the tau polarization and its forward-backward asymmetry at LEP

A measurement of theτ lepton polarization and its forward-backward asymmetry at the Z0 resonance using the OPAL detector is described. The measurement is based on analyses of τ→ρντ, ττπ(K)ντ,\(\tau \to e\bar \nu _e \nu _\tau \),\(\tau \to \mu \bar \nu _\mu \nu _\tau \) andτ→a1 ν τ decays from a sample of 89075 e+e−→τ + τ − candidates corresponding to an integrated luminosity of 117 pb−1. Assuming that theτ lepton decays according to V-A theory, we measure the averageτ polarization at √s=MZ to be 〈P〉=(−13.0±0.9±0.9)% and theτ polarization forward-backward asymmetry to be A pol FB =(−9.4±1.0±0.4)%, where the first error is statistical and the second systematic. These results are consistent with the hypothesis of lepton universality and, when combined, can be expressed as a measurement of sin2 θ eff lept =0.2334±0.0012 within the context of the Standard Model.