Measurement of the Branching Ratios for τ→πντ and τ→μνμντ

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