Leptonic pion decay
= Leptonic pion decay
{title2=$\pi^+\to\ell^+\nu_\ell$}
The <pion decay constant> gives $\langle0|J_A^\mu|\pi(p)\rangle\propto f_\pi p^\mu$. Contracting the momentum with the leptonic <weak charged current> and applying the external Dirac equations makes the amplitude proportional to the charged-lepton mass. This is an explicit example of <helicity suppression>.