Study of Michel parameters in leptonic τ 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 $(\tau^\mp\to\ell^\mp\nu\nu,\tau^\pm\to\pi^\pm\pi^0\nu)$ 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.