Derivative coupling to a conserved current
= Derivative coupling to a conserved current
An interaction $-\lambda j^\mu\partial_\mu\phi$ with an exactly <conserved current> is redundant: integration by parts gives $\lambda\phi\,\partial_\mu j^\mu$ up to a boundary term. For the <Dirac current>, the local phase redefinition $\psi=e^{-i\lambda\phi}\chi$ removes the coupling, so its on-shell scattering amplitudes vanish.