Study of Michel parameters in leptonic $\tau$ decays at Belle

We present a study of Michel parameters in leptonic $\tau$ decays using experimental information collected at the Belle detector. Michel parameters are extracted in the unbinned maximum likelihood fit of the $({\tau }^{\mp }\to {\mathrm{\ell }}^{\mp }\nu \nu ,{\tau }^{\pm }\to {\pi }^{\pm }{\pi }^0\nu )$ events in the full nine-dimensional phase space. We exploit the spin-spin correlation of tau leptons to extract ${\xi }_{\rho }\xi$ and ${\xi }_{\rho }\xi \delta$ in addition to the $\rho$ and $\eta$ parameters.