Speaker
Description
The rare second-class decay mode of the $\tau$ into $\eta\pi\nu$ could be
observed for the first time at Belle II. It is important to try to derive
a reliable evaluation of the branching fraction and of the energy
distribution of this mode within the standard-model. Many predictions
exist already in the literature which, unfortunately, can differ by one to
two orders of magnitude. In this talk I discuss an approach based on a
systematic use of the property of analyticity (of form factors and
amplitudes) in QCD. In particular, I will show that the scalar form factor
in the $\tau$ decay can be related to photon-photon scattering and
radiative $\phi$ decay amplitudes for which precise experimental
measurements (by Belle and Kloe) exist.
