Precise Measurements of the Leptonic Branching Ratios of the Tau Lepton

Abstract

Precise measurements are reported for the leptonic branching ratios of the $\ensuremath{\tau}$ lepton from the reaction ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ observed with the MAC detector operating at the SLAC storage ring PEP. Results from samples in which one $\ensuremath{\tau}$ decays into three charged particles and the other to $e$ or $\ensuremath{\mu}$ are ${B}_{e}=0.180\ifmmode\pm\else\textpm\fi{}0.009\ifmmode\pm\else\textpm\fi{}0.006$ and ${B}_{\ensuremath{\mu}}=0.183\ifmmode\pm\else\textpm\fi{}0.009\ifmmode\pm\else\textpm\fi{}0.005$. A sample of $e\ensuremath{\mu}$ events gives ${B}_{e}{B}_{\ensuremath{\mu}}=0.0288\ifmmode\pm\else\textpm\fi{}0.0017\ifmmode\pm\else\textpm\fi{}0.0019$. With the assumption of $e\ensuremath{-}\ensuremath{\mu}$ universality, these results are combined to give ${B}_{e}=0.178\ifmmode\pm\else\textpm\fi{}0.005$, which implies a $\ensuremath{\tau}$ lifetime of (2.85\ifmmode\pm\else\textpm\fi{}0.09)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ sec.