Michel parameters and τ neutrino helicity from decay correlations in Z → τ+τ-

The Michel parameters and the average τ-neutrino helicity are measured using correlations between the decays of the τ+ and τ- produced on the Z resonance and observed in the ALEPH detector at LEP. The Michel parameters, ρ{variant}l, ηl, ξl, (δξ)l, are determined from τ → lνlντ with l = (e, μ), and the average τ neutrino helicity, 〈h(ντ)〉, from τ → hν with h = (π, ρ{variant}, a1). The results obtained with e-μ universality are: ρ{variant}l = 0.751±0.039±0.022, ηl = -0.04±0.15±0.11, ξl = 1.18±0.15±0.06, (δξ)l = 0.88±0.11±0.07, and the average τ neutrino helicity 〈h(ντ)〉 = -1.006±0.032±0.019. No significant deviation from the Standard Model V-A prediction is observed.