A determination of two Michel parameters in purely leptonic tau decays

Using the ARGUS detector at the e⁺e^- storage ring DORIS II, we have determined the Michel parameters ρ{variant} and ξ of τ → μνν and τ → eνν decays. From a data sample with 333 events/pb around s = 10 GeV, we select 3230 events e⁺e^- → τ⁺τ^- → (μ^±μμ)(e^{{double barred pipe}vv)} and determine ρ{variant} from the e and μ momentum spectra and ξ from the correlations between e and μ momenta. For ρ{variant} we obtain ρ{variant}_τ→e = 0.79 ± 0.08 ± 0.06 and ρ{variant}_τ→μ = 0.76 ± 0.08 in accordance with the V - A structure of the decays. The momentum correlations are only sensitive to the product ξ_τ→eξ_τ→μ. Setting ξ_τ→e = ξ_τ→μ, we obtain |ξ_{τ| = 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)%.