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

Abstract

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→ρν_τ, ττπ(K)ν_τ,$\tau \to e\bar \nu _e \nu _\tau$,$\tau \to \mu \bar \nu _\mu \nu _\tau$ 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=M_Z to be 〈P〉=(−13.0±0.9±0.9)% and theτ polarization forward-backward asymmetry to be A ^FB_pol =(−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 ^lept_eff =0.2334±0.0012 within the context of the Standard Model.