Isospin Violation in Chiral Perturbation Theory and the Decays $\eta \text{ra}\pi \mathrm{\ell }\nu$ and $\tau \text{ra}\eta \pi \nu$

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 $K_{\mathrm{\ell }3}$ amplitudes are reported. Using the one–loop formulae for the ${\eta }_{\mathrm{\ell }3}$ form factors, rather precise predictions for the decay rates of $\eta \rightarrow \pi \ell \nu$ can be obtained. Finally, I present an estimate of the $\tau \text{ra}\eta \pi \nu$ branching ratio derived from the dominant meson resonance contributions to this decay.