Isospin Violation in Chiral Perturbation Theory and the Decays $η\ra π\ell ν$ and $τ\ra ηπν$

Abstract

I discuss isospin breaking effects within the standard model. Chiral perturbation theory presents the appropriate theoretical framework for such an investigation in the low--energy range. Recent results on the electromagnetic contributions to the masses of the pseudoscalar mesons and the $K_{\ell 3}$ amplitudes are reported. Using the one--loop formulae for the $η_{\ell 3}$ form factors, rather precise predictions for the decay rates of $η\rightarrow π\ell ν$ can be obtained. Finally, I present an estimate of the $τ\ra ηπν$ branching ratio derived from the dominant meson resonance contributions to this decay.