Hadron polarization in heavy-lepton decays

In the decay of the $\ensuremath{\tau}$ lepton into a spin-one meson and a neutrino, the meson state with helicity opposite its charge is forbidden by angular momentum conservation if the decay is mediated by the conventional $V\ensuremath{-}A$ current. The laboratory momentum distribution of pions in the decay chain $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\rho}\ensuremath{\nu}$, $\ensuremath{\rho}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}$ reflects the polarization of the $\ensuremath{\rho}$ meson and provides an experimental check of the nature of the decay current. This method requires the identification of the decay mode $\ensuremath{\rho}\ensuremath{\nu}$ but does not depend on finding the $\ensuremath{\tau}$ laboratory direction.