Past exam of the gauge field theory course of the University of Cambridge / 2013 / Paper 1 / 3 / d / Solution

The operator $A^{\mu }\bar{\psi }{\gamma }_{\mu }\psi$ has mass dimension $1+3=4$, so $[e]=0$. It is the renormalizable electromagnetic current coupling when completed as quantum electrodynamics: a massless Abelian gauge field couples minimally to the conserved Dirac current, with consistent gauge fixing and Ward identities. Indeed, replacing ${\partial }_{\mu }$ by a gauge covariant derivative in the Dirac equation produces this interaction. The gauge structure, rather than dimension counting alone, ensures the necessary cancellations.^[ref]