Low-Energy Theorems and the Forward Proton Production inD(γ,p)n

We investigated the electric transition operator in view of existing meson-exchange charge densities. We show that even in the long-wavelength limit the electric dipole operator has important two-body contributions which are not covered by the conventional Siegert operator. These two-body parts are of mixed isospin and lead to a breakdown of the selection rules. The generalization of the Bethe-Levinger sum rule is given. As a numerical application we calculated the forward proton production in $\mathrm{D}(\gamma ,p)n$ where we show the importance of the discussed terms.