Resonance Chiral Lagrangian analysis of $\tau^- \to \eta^{(\prime)} \pi^- \pi^0 \nu_\tau$ decays

The hadronization structure of $\tau^- \to \eta \pi^- \pi^0 \nu_\tau$ decays is analyzed using Chiral Perturbation Theory with resonances, considering only the contribution of the lightest meson resonances at leading order in the $1/N_C$ expansion. After imposing the asymptotic behavior of vector spectral functions ruled by QCD, unknown effective couplings are determined by fitting the $\tau^- \to \eta \pi^- \pi^0 \nu_\tau$ branching ratio and decay spectrum to recent data. Predictions for the partner decay $\tau^- \to \eta^\prime \pi^- \pi^0 \nu_\tau$ and the low-energy behavior of the cross section $\sigma (e^+e^-\to \eta\pi^+\pi^-)$ are also discussed.