Measurement of the Michel parameters and the chirality parameter in τ decays from e +e − → τ+τ−

Abstract

The Michel parameters ρ, η, ξ and ξδ , the chirality parameter ξ h , and the τ polarization P τ are measured using 32012 Z → τ + τ − decays recorded by the L3 dectector at LEP. Their values are extracted from the energy spectra of leptons and hadrons in τ → l ¯ v¯ l v τ and τ − → π − ν τ decays, the energy and decay angular distributions in τ − → ρ − ν τ decays, and the correlations in the energy spectra and angular distributions of the decay products. Assuming universality in leptonic and semileptonic τ decays, the results are ρ = 0.794 ± 0.039 ± 0.031, η = 0.25 ± 0.17 ± 0.11, ξ = 0.94 ± 0.21 ± 0.07, ξδ = 0.81 ± 0.14 ± 0.06, ξ h = −0.970 ± 0.053 ± 0.011, and P τ = −0.154 ± 0.018 ± 0.012.