Precise Measurements of the Leptonic Branching Ratios of the Tau Lepton

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