DETERMINATION OF THE MICHEL PARAMETER-XI AND PARAMETER-DELTA IN LEPTONIC TAU-DECAYS

Using the ARGUS detector at the e(+)e(-) storage ring DORIS II, we have determined the Michel parameters xi and delta of tau(-/+) --> l(-/+)nu<(nu)over bar> decays. With an integrated luminosity of 445 pb(-1) around root s = 10 GeV, we have produced approximately 415000 tau-pairs. From this data sample, 3262 events were selected with e(+)e(-) --> tau(+)tau(-) --> (l(+/-)nu<(nu)over bar>)(pi(-/+)pi(+)pi(-)nu). The semihadronic decay was used as analyser of the tau-spin and made possible - owing to the spin correlations - the determination of the parity-violating Michel parameters xi and delta in the decay tau(-/+) --> l(-/+) nu<(nu)over bar>. Simultaneously to the determination of the Michel parameters, a measurement of the tau-neutrino helicity h(nu tau), in the decay tau(-/+) --> pi(-/+)pi(+)pi(-)nu was obtained. We observed h(nu tau) = -0.85(-0.17)(+0.15) +/- 0.05, rho = 0.721 +/- 0.040 +/- 0.021, xi = 1.26(-0.26)(+0.30) +/- 0.09, and xi delta = 0.77(-0.16)(+0.18) +/- 0.05. In addition, the combined ARGUS result on h(nu tau), rho, xi, and xi delta are reported using this work and previous measurements.