Study of an anomalous tau lepton decay using a chiral Lagrangian with   vector mesons
  The hadronic tau decay $\tau^- \to \nu_\tau \eta \pi^- \pi^0$ occurs through V-A weak current. In this decay mode, the vector current contribution is intrinsic parity violating and the axial current contribution is G parity violating. The latter contribution is suppressed due to tiny isospin breaking. We have computed both vector and axial vector form factors using a chiral Lagrangian with vector mesons including the effect of isospin breaking and intrinsic parity violation. A numerical result of the invariant mass distribution is shown and the structure of $\rho$ resonance can be seen in the distribution with respect to $M_{\pi^- \pi^0}$. 
Daiji Kimura, Takuya Morozumi, Hiroyuki Umeeda
hep-ph