Measurement of the Branching Ratios forτ→πντandτ→μνμντ

On the basis of a sample of 41 events of the type $e^{+}{e}^{-}\to e^{\pm }{X}^{\mp }\left(X\ne e\right)$ and no observed photons, we have observed a clear signal of the decay ${\tau }^{-}\to {\pi }^{-}{\nu }_{\tau }$. We measure the branching ratio for this decay to be $b_{\pi }=0.080\mathrm{}\pm 0.032\pm 0.013$ and for the decay ${\tau }^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }{\nu }_{\tau }$, $b_{\mu }=0.21\mathrm{}\pm 0.05\pm 0.03$,where the first and second errors are, respectively, statistical and systematic. Both measurements agree with theoretical values derived under the assumption that the $\tau$ decays via the standard weak current.