Measurement of the Michel parameters and the average τ neutrino helicity from t decays in e⁺e⁻ → τ⁺τ⁻

Abstract

The Michel parameters ϱ, η, ξ, and ξδ, the chirality parameter ξh and the Τ polarization P_Τ are measured using 32012 Τ pair decays. Their values are extracted from the energy spectra of leptons and hadrons in Τ̅→l̅ν̅₁ν_τ and Τ̅ → π̅ν_Τ decays, the energy and decay angular distributions in Τ- ̅→ϱ̅ν_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, ξh = −0.970±0.053±0.011, and P_Τ = −0.154±0.018±0.012. The measurement is in agreement with the V-A hypothesis for the weak charged current.