Spin-zero two-body helicity selection
= 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>.