Correlation measurements in Z→τ+τ− and the τ neutrino helicity

Using data collected with the ALEPH detector at LEP correlations between the decay products of the τ+ and τ− produced in the decay of the Z have been measured. The measurements performed in the decays τ→πν and τ→ϱν place limits on deviations from the Standard Model. These are given by the parameter ζ with ζ = −1 for the Standard Model. The measured values are ζπ = −0.95 ± 0.11 ± 0.05 and ζϱ = −1.03 ± 0.11 ± 0.05. For models which predict ζπ = ζϱ = ζ — interpreted as the average τ neutrino helicity — the measurement is ζ = −0.99 ± 0.07 ± 0.04.