Precise Measurements of the Leptonic Branching Ratios of the Tau Lepton

Precise measurements are reported for the leptonic branching ratios of the T lepton from the reaction e+e+ T+Tobserved with the MAC detector operating at PEP. Results from samples in which one T decays into three charged particles and the other to e or /,L are B, = 0.180 f 0.009 f 0.006 and B, = 0.183f0.009f0.005. A sample of ep events gives Be-B, = 0.0288f0.0017f0.0019. With the assumption of e-p universality these results are combined to give B, = 0.178 f 0.005 which implies a T lifetime of (2.85 f 0.09) x lo-l3 sec. PACS numbers: 14.6O.Jj, 13.35.+s