A DETERMINATION OF 2 MICHEL PARAMETERS IN PURELY LEPTONIC TAU-DECAYS

Using the ARGUS detector at the e+e- storage ring DORIS II, we have de termined the Michel parameters rho and xi of tau --> mununuBAR and tau --> enunuBAR decays. From a data sample with 333 events/pb around squ are-root s = 10 GeV, we select 3230 events e+e- --> tau+tau- --> (mu+/ -nunuBAR)(e-/+nunuBAR) and determine rho from the e and mu momentum sp ectra and xi from the correlations between e and mu momenta. For rho w e obtain rho(tau-->e) = 0.79 +/- 0.08 +/- 0.06 and rho(tau-->mu) = 0.7 6 +/- 0.07 +/- 0.08 in accordance with the V - A structure of the deca ys. The momentum correlations are only sensitive to the product xi(tau -->e)xi(tau-->mu). Setting xi(tau-->e) = xi(tau-->mu), we obtain \xi(t au)\ = 0.90 +/- 0.15 +/- 0.10 also in accordance with V - A. The updat ed decay fractions are B(tau --> enunuBAR) = (17.5 +/- 0.3 +/- 0.5)% a nd B(tau --> mununuBAR) = (17.4 +/- 0.3 +/- 0.5)%.