A study of the decays of tau leptons produced on theZ resonance at LEP

Abstract

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⁻)v_τ)=11.9±0.7±0.7%, BR (τ⁻ → ρ⁻ v_τ)= 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, α_s (m_τ) = 0.26 ^+0.09_−0.12 . Extrapolating the α_s value fromm _τ tom _Z yields α_s (m_Z) = 0.109 ^+0.012_−0.028 .

The average polarizationP _τ of taus produced in Z → τ^s τ^s decays has also been measured using the above decay modes. The weighted mean of the polarizations obtained from the four decay modes isP _τ=−0.24±0.07. This value ofP _τ gives, in the improved Born approximation, a ratio between the axial and vector coupling constants of the tau of υ_τ/a_τ = 0.12 ± 0.04, and hence a value of the effective electroweak mixing parameter sin² θ_W(m ²_Z ).