A STUDY OF THE DECAYS OF TAU LEPTONS PRODUCED ON THE Z-RESONANCE AT LEP

From the analysis of a data sample corresponding to an integrated luminosity of 4.63 pb-1 taken during the 1990 run of LEP at centre of mass energies between 88.2 GeV an 94.2 GeV, the tau decays tau- --> e-nu(e)nu(tau)BAR, tau- --> mu-nu(mu)nu(tau)BAR, tau- --> pi-(K-)nu(tau), tau- --> rho-nu(tau) and their charge conjugates have been studied. The following branching ratios have been measured; BR (tau- --> e-nu(e)nu(tau)BAR) = 18.6+/-0.8 (stat.)+/-0.6 (sys.)%, BR (tau- --> mu-nu(mu)nu(tau)BAR) = 17.4+/-0.7+/-0.6%, BR (tau- --> pi-(K-)nu(tau)) = 11.9+/-0.7+/-0.7%, BR (tau- --> rho-nu(tau)) = 22.4+/-0.8+/-1.3%, in good agreement with world averages. The measured electronic and muonic branching ratios lead to a measurement of the strong coupling constant, alpha(s)(m(tau)) = 0.26 +0.09/-0.12. Extrapolating the alpha(s) value from m(tau) to m(Z) yields alpha(s)(m(Z)) = 0.109 +0.012/-0.028. The average polarization P(tau) of taus produced in Z --> tau+tau- decays has also been measured using the above decay modes. The weighted mean of the polarizations obtained from the four decay modes is P(tau) = -0.24+/-0.07. This value of P(tau) gives, in the improved Born approximation, a ratio between the axial and vector coupling constants of the tau of v(tau)/a(tau) = 0.12+/-0.04, and hence a value of the effective electroweak mixing parameter sin2-theta(w)(m(Z)2) = 0.220+/-0.009.