First Measurement of the Michel Parameter?0 in the t- ? μ- ?^-μ?t Decay at Belle

We report the first measurement of the Michel parameter ζ′ in the τ-→μ-ν¯μντ decay with a new method proposed just recently. The measurement is based on the reconstruction of the τ-→μ-ν¯μντ 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 912×106 τ+τ- pairs. We measure ζ′=0.22±0.94(stat)±0.42(syst), which is in agreement with the standard model prediction of ζ′=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.