MEASUREMENT OF BRANCHING RATIOS AND TAU-POLARIZATION FROM TAU-]E-NU-NU-BAR,TAU-]MU-NU-NU-BAR, AND TAU-]PI(K)NU DECAYS AT LEP

From a sample of 3308 e+e- --> tau+tau- events with an estimated background of 1.9%, we find 964 tau --> e-nu-nuBAR, 903 tau --> mu-nu-nuBAR, and 309 tau --> pi (K)nu candidates. Efficiency and background estimates determined from both Monte Carlo and control sample studies yield the following branching ratios: B(tau --> e-nu-nuBAR) = 17.4 +/- 0.5 (stat) +/- 0.4 (sys)%, B(tau --> mu-nu-nuBAR) = 16.8 +/- 0.5 +/- 0.4%, and B(tau --> pi(K)nu) = 12.1 +/- 0.7 +/- 0.5%. These values are in good agreement with previous measurements. The measured lepton branching ratios, when combined with the world-average measured value for the tau-lifetime, yield a ratio of the tau-Fermi coupling constant to that of the lighter leptons given by G-tau/G(e,mu) = 0.92 +/- 0.04, where it is assumed G(e) = G(e,mu). The average tau-polarization at the Z0 resonance is measured to be -0.01 +/- 0.09 from an analysis of the momentum spectra of the electron, muon, and pion candidates, implying that the ratio of vector to axial vector couplings of the tau to the Z0 is upsilon-tau/a-tau = 0.01 +/- 0.04. The measurements of the average polarizations in the forward and backward hemispheres lead to the efficiency-corrected, forward-backward polarization asymmetry A(pol)FB = -0.22 +/- 0.10, implying for the electron couplings to the Z0 the ratio upsilon-e/a(e) = 0.15 +/- 0.07. Since these values for the tau and electron couplings are consistent with one another, we assume lepton universality to derive upsilon/a = 0.05 +/- 0.04 and a value for the weak mixing angle of sin2theta-WBAR = 0.237 +/- 0.009, with no ambiguity introduced by the relative signs of upsilon and a.