Determination of the Michel Parameters and the tau Neutrino Helicity in tau Decay

Abstract

Using the CLEO II detector at the $e^+e^-$ storage ring CESR, we have determined the Michel parameters $ρ$, $ξ$, and $δ$ in $τ^\mp \to l^\mpν\barν$ decay as well as the tau neutrino helicity parameter $h_{ν_τ}$ in $τ^\mp \to π^\mpπ^0ν$ decay. From a data sample of $3.02\times 10^6$ tau pairs produced at $\sqrt{s}=10.6 GeV$, using events of the topology $e^+e^- \to τ^+τ^- \to (l^\pmν\barν) (π^\mpπ^0ν)$ and $e^+e^- \to τ^+τ^- \to (π^\pmπ^0\barν) (π^\mpπ^0ν)$, and the determined sign of $h_{ν_τ}$, the combined result of the three samples is: $ρ= 0.747\pm 0.010\pm 0.006$, $ξ= 1.007\pm 0.040\pm 0.015$, $ξδ= 0.745\pm 0.026\pm 0.009$, and $h_{ν_τ} = -0.995\pm 0.010\pm 0.003$. The results are in agreement with the Standard Model V-A interaction.