Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 301 / 2 / ii / Solution

In four spacetime dimensions $[\psi ]=3/2$, $[\varphi ]=1$, and $[{\partial }_{\mu }]=1$. HenceThe coupling is an irrelevant coupling by power counting in quantum field theory, so it is a nonrenormalizable interaction that would ordinarily define only an effective theory with a cutoff. Here it is also a redundant operator: integration by parts gives $\lambda \varphi {\partial }_{\mu }{j}^{\mu }$ up to a boundary term, and the Dirac current $j^{\mu }=\bar{\psi }{\gamma }^{\mu }\psi$ is conserved. This derivative coupling to a conserved current is removed exactly by the local phase redefinition $\psi =e^{-i\lambda \varphi }\chi$.