Helicity suppression
= Helicity suppression
A spin-zero parent decaying through a chiral <weak charged current> to a charged lepton and a massless <neutrino> cannot supply the helicities selected by a massless interaction. A charged-lepton mass insertion makes the amplitude proportional to $m_\ell$ and the <decay width> proportional to $m_\ell^2$. A spin-one <W boson> does not suffer this suppression.