Vacuum-to-pseudoscalar current selection rule ($⟨0|V^{\mu }|P(p)⟩=0,⟨0|A^{\mu }|P(p)⟩\propto p^{\mu }$)

Lorentz covariance allows only a multiple of $p^{\mu }$ for a vacuum-to-spin-zero one-particle current. In its rest frame only the temporal component can remain. A parity-even temporal vector current between a parity-even vacuum and parity-odd particle changes sign, forcing that component to vanish. A temporal axial current is parity-odd and is allowed. This explains why the pion decay constant enters the axial, rather than vector, part of the weak charged current.