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 $\rho$, $\xi$, and $\delta$ in ${\tau }^{\mp }\to l^{\mp }\nu \bar{\nu }$ decay as well as the tau neutrino helicity parameter $h_{{\nu }_{\tau }}$ in ${\tau }^{\mp }\to {\pi }^{\mp }{\pi }^0\nu$ decay. From a data sample of $3.02\times 10^6$ tau pairs produced at $\sqrt{s}=10.6GeV$, using events of the topology $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}\to (l^{\pm }\nu \bar{\nu })({\pi }^{\mp }{\pi }^0\nu )$ and $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}\to ({\pi }^{\pm }{\pi }^0\bar{\nu })({\pi }^{\mp }{\pi }^0\nu )$, and the determined sign of $h_{{\nu }_{\tau }}$, the combined result of the three samples is: $\rho =0.747\pm 0.010\pm 0.006$, $\xi =1.007\pm 0.040\pm 0.015$, $\xi \delta =0.745\pm 0.026\pm 0.009$, and $h_{{\nu }_{\tau }}=-0.995\pm 0.010\pm 0.003$. The results are in agreement with the Standard Model V-A interaction.

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