Precise Measurements of the Leptonic Branching Ratios of the Tau Lepton

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