 Spin-zero two-body helicity selection

A spin-zero parent at rest decaying to two particles requires their helicities to be equal. Choosing particle 1 along $+z$ and particle 2 along $-z$ gives total projection $J_z=h_1-h_2=0$. In negative-pion decay, the massless antineutrino has positive helicity, so the negative charged lepton must also have positive helicity. This is the suppressed component for a left-chiral weak charged current.