A determination of two Michel parameters in purely leptonic tau decays

9 pp. (1993).
Using the ARGUS detector at the e+e− storage ring DORIS II, we have determined the Michel parameters ϱ and ξ of τ → μννandτ → eνν decays. From a data sample with 333 events/pb around s= 10GeV, we select 3230 events ǂe+e−→ τ+τ−→ (μ±μμ)(eǂvv) and determine ϱ from the e and μ momentum spectra and ξ from the correlations between e and μ momenta. For ϱ we obtain ϱτ→e = 0.79 ± 0.08 ± 0.06 and ϱτ→μ = 0.76 ± 0.08 in accordance with the V - A structure of the decays. The momentum correlations are only sensitive to the product ξτ→eξτ→μ. Setting ξτ→e = ξτ→μ, we obtain |ξτ| = 0.90 ± 0.15 ± 0.10 also in accordance with V - A. The updated decay fractions are B(τ →evv) = (17.5 ± 0.3 ± 0.5)% and B(τ → μvv) = (17.4 ± 0.3 ± 0.5)%.