Measurement of Michel Parameters (η̅, ξκ) in the radiative leptonic decay of tau at Belle

We present the first measurement of the Michel parameters η̅ 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^+π^0 ν̅)(l^-νν̅γ) (l=e or μ). The preliminary values of the measured Michel parameters are η̅ = -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.