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

Abstract 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 l , η l , ξ l , ( δξ ) l , are determined from τ → l ν l ν τ with l = ( e , μ ), and the average τ neutrino helicity, 〈 h ( ν τ )〉, from τ → hν with h = ( π , ϱ , a 1 ). The results obtained with e − μ universality are: ϱ l = 0.751±0.039±0.022, η l = −0.04±0.15±0.11, ξ l = 1.18±0.15±0.06, ( δξ ) l = 0.88±0.11±0.07, and the average τ neutrino helicity 〈 h ( ν τ )〉 = −1.006±0.032±0.019. No significant deviation from the Standard Model V-A prediction is observed.