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.