Gauge-invariant nuclear Compton amplitude manifesting low-energy theorems

The electromagnetic field representation of the nuclear electromagnetic interaction is developed, by extending previous work on the nuclear current (one-photon amplitude) to the nuclear Compton amplitude. The resulting form of the Compton amplitude is gauge invariant and, in transverse gauge, manifests the low-energy theorems for the $0^{+}$→$0^{+}$ case, which is discussed in detail. Deuteron forward photodisintegration is also studied numerically, and it is shown that the generalized Siegert form of the retarded electric dipole interaction efficiently incorporates pion-exchange currents implicitly, while large explicit forms of these currents are needed in the standard representation of the interaction. Gauge invariance and Reiss’s strong-field momentum-translation approximation are also discussed.