Measurement of the $\mathit{\tau }$ Neutrino Helicity and Michel Parameters in Polarized ${\mathit{e}}^{+}{\mathit{e}}^{-}$ Collisions

We present a new measurement of the $\tau$ neutrino helicity $h_{{\nu }_{\tau }}$ and the $\tau$ Michel parameters $\rho$, $\eta$, $\xi$, and the product $\delta \xi$. The analysis exploits the highly polarized SLC electron beam to extract these quantities directly from a measurement of the $\tau$ decay spectra, using the 1993–1995 SLD data sample of 4328 $e^{+}{e}^{-}\to Z^0\to {\tau }^{+}{\tau }^{-}$ events. From the decays $\tau \to \pi {\nu }_{\tau }$ and $\tau \to \rho {\nu }_{\tau }$ we obtain a combined value $h_{{\nu }_{\tau }}=-0.93\mathrm{}\pm 0.10\pm 0.04$. The leptonic decay channels yield combined values of $\rho =0.72\mathrm{}\pm 0.09\pm 0.03$, $\xi =1.05\mathrm{}\pm 0.35\pm 0.04$, and $\delta \xi =0.88\mathrm{}\pm 0.27\pm 0.04$.