A phenomenological description of photo- and electroproduction in nucleon resonance region

The π−Δ++ production on the nucleon by real and virtual photons is discussed as initial step in a simple model approach for the two pion photo- and electroproduction on the nucleon, with emphasis on nucleon resonance excitation which is of interest for new facilities like TJNAF. A calculation for π−Δ++ channel in resonance excitation region is presented and compared to existing experimental data along with a discussion of physical effects that we find to be of relevance. The calculation is proposed as a starting basis for the investigation of N∗ electromagnetic form factors using experimental data about two pion production by real and virtual photons.