GAUGE-INVARIANT NUCLEAR PION-PHOTOPRODUCTION AMPLITUDE WITH BINDING-INDUCED CONTRIBUTIONS

The amplitude for pion photo- and electroproduction from finite nuclei is constructed in a model that ignores rescattering of the pion. A co nsistent expansion in inverse powers of the nucleon mass is performed, which generates effective one-body and two-body nuclear operators. Th e former agree with the Blomqvist-Laget effective operators, while the latter are new and arise from binding-induced and binding-modified em ission. Current conservation is applied to this set of operators and t hey are cast separately into manifestly gauge-invariant forms. A simpl e approximation to the complicated two-body terms is constructed, whic h shares many of the features of the latter.