Measurement of the Michel parameters (η‾, ξκ) in the radiative leptonic decay of τ

We present the first measurement of the Michel parameters η‾ and ξκ in the radiative leptonic decay of the τ lepton using 703 fb-1 of data collected with the Belle detector at the KEKB e+e− collider. The Michel parameters are measured by an unbinned maximum likelihood fit to the kinematic information of e+e−→τ+τ−→(π+π0ν‾)(l−νν‾γ)(l=e or μ). The preliminary values of the measured Michel parameters are η‾=−2.0±1.5±0.8 and ξκ=0.6±0.4±0.2, where the first error is statistical and the second is systematic.