Relativistic Model for Electroproduction of Nucleon Resonances. II

A relativistic gauge-invariant model for electron excitation of nucleon resonances introduced previously is developed and extended. A coupled-channel calculation is carried out retaining the |πN〉 and |πN*(1236)〉 channels and assuming that one eigenphase shift is resonant. The eigenphase shift is determined from phase-shift analyses of π-N scattering, and the mixing angle from the decay widths of the resonance. The model chosen for the resonant amplitude is a product of two factors: a generalized Feynman amplitude for excitation of the resonant channel, and a final-state enhancement factor which is related to an integral over the eigenphase shift. The model is shown to be an approximate solution to the coupled-channel Omn\`es equations. Further extensions consist of including N*(1236) exchange in the excitation amplitude for the |πN〉 channel and of investigating the effect of different fits to GEn on the inelastic form factors. The predictions of the theory are compared with all existing data---in particular, with the recent results from SLAC.