PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS, no. 2, Article no. 023C01
We present a measurement of the Michel parameters of the tau lepton, (eta) over bar and xi kappa, in the radiative leptonic decay tau(-) -> l(-)nu(tau)(nu) over bar (l)gamma using 711 fb(-1) of collision data collected with the Belle detector at the KEKB e(+)e(-) collider. The Michel parameters are measured in an unbinned maximum likelihood fit to the kinematic distribution of e(+)e(-) -> tau(+)tau(-) -> (pi(+)pi(0)(nu) over bar (tau))(l(-)nu(tau)(nu) over bar (l)gamma) (l = e or mu). The measured values of the Michel parameters are (eta) over bar = - 1.3 +/- 1.5 +/- 0.8 and xi kappa = 0.5 +/- 0.4 +/- 0.2, where the first error is statistical and the second is systematic. This is the first measurement of these parameters. These results are consistent with the Standard Model predictions within their uncertainties, and constrain the coupling constants of the generalized weak interaction.