HADRON STRUCTURE AND GAUGE-INVARIANCE IN PHOTOPRODUCTION AND ELECTROPRODUCTION OF PIONS

We give a detailed account of the constraints on the general operator for pion photo- and electroproduction provided by the Ward-Takahashi identities. We investigate the role of off-shell effects in electromagnetic and strong interaction form factors. Special attention is paid to the "internal insertions", the coupling of the photon into the dressed piNN vertex. To be able to do this consistently, we use a simple field-theoretical model where the nucleon gets dressed by a neutral scalar meson. In the framework of this model we compare the exact production amplitude with that obtained from ad hoc prescriptions which have recently been proposed. Finally the discussion is extended to exchange currents in electron scattering.