The τ leptonic current: lifetime, leptonic branching ratios and Michel parameters

A review of results probing the τ leptonic current is given. No evidence is found for a departure from a V-A current at a precision level about one order of magnitude worse than for the muon. Precise measurements of the τ lifetime are presented by LEP experiments, indicating a smaller value compared to older experiments. The world average of the lifetime is 291.6 ±1.6 (fs). LEP experiments also provide accurate values for the leptonic branching ratios. The new world average are B _e = 17.793 ± 0.086(%) and B _μ = 17.333 ± 0.087(%). No deviation of lepton universality in the charged current is observed with an accuracy of 3‰ for e — μ and μ — τ separately.