Hadron polarization in heavy-lepton decays

In the decay of the $\tau$ 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 $V-A$ current. The laboratory momentum distribution of pions in the decay chain $\tau \to \rho \nu$, $\rho \to \pi \pi$ reflects the polarization of the $\rho$ meson and provides an experimental check of the nature of the decay current. This method requires the identification of the decay mode $\rho \nu$ but does not depend on finding the $\tau$ laboratory direction.