Abstract
We report the first measurement of the Michel parameter & xi;0 in the & tau;- & RARR; & mu;- & nu; over bar & mu;& nu;& tau; decay with a new method proposed just recently. The measurement is based on the reconstruction of the & tau;- & RARR; & mu;- & nu; over bar & mu;& nu;& tau; events with subsequent muon decay in flight in the Belle central drift chamber.
The analyzed data sample of 988 fb-1 collected by the Belle detector corresponds to approximately 912 x 106 & tau;+& tau;- pairs. We measure & xi;0 = 0.22 & PLUSMN; 0.94(stat) & PLUSMN; 0.42(syst), which is in agreement with the standard model prediction of & xi;0 = 1.
Statistical uncertainty dominates in this study, being a limiting factor, while systematic uncertainty is well under control. Our analysis proved the practicability of this promising method and its prospects for further precise measurement in future experiments.