Measurement of Michel Parameters ($bareta$, $xikappa$) in the radiative leptonic decay of tau at Belle

We present the first measurement of the Michel parameters $bar{eta}$ and $xikappa$ in the radiative leptonic decay of the $tau$ lepton using 703 f$mathrm{b}^{-1}$ 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 $e^+e^-rightarrowtau^+tau^-rightarrow (pi^+pi^0 bar{ u})(l^- ubar{ u}gamma)$ $(l=e$ or $mu)$. The preliminary values of the measured Michel parameters are $bar{eta} = -2.0 pm 1.5 pm 0.8$ and $xikappa = 0.6 pm 0.4 pm 0.2$, where the first error is statistical and the second is systematic.