A study of the decays of tau leptons produced on theZ 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$$\tau ^ - \to e^ - \bar v_e v_\tau ,\tau ^ - \to \mu ^ - \bar v_\mu v_\tau ,\tau ^ - \to \pi ^ - (K^ - )v_\tau ,\tau ^ - \to \rho {}^ - v_\tau $$ and their charge conjugates have been studied. The following branching ratios have been measured;$$BR(\tau ^ - \to e^ - \bar v_e v_\tau ) = 18.6 \pm 0.8(stat.) \pm 0.6(sys.)\% ,$$,$$BR\left( {\tau ^ - \to \mu ^ - \bar v_\mu v_\tau } \right) = 17.4 \pm 0.7 \pm 0.6\% ,$$, Br(τ− → π− (K−)vτ)=11.9±0.7±0.7%, BR (τ− → ρ− vτ)= 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, αs (mτ) = 0.26−0.12+0.09. Extrapolating the αs value frommτ tomZ yields αs (mZ) = 0.109−0.028+0.012.The average polarizationPτ of taus produced in Z → τs τs decays has also been measured using the above decay modes. The weighted mean of the polarizations obtained from the four decay modes isPτ=−0.24±0.07. This value ofPτ gives, in the improved Born approximation, a ratio between the axial and vector coupling constants of the tau of υτ/aτ = 0.12 ± 0.04, and hence a value of the effective electroweak mixing parameter sin2 θW(mZ2).