A unitary isobar model for pion photo- and electroproduction on the protonup to 1 GeV

A new operator for pion photo- and electroproduction has been developed for nuclear applications at photon equivalent energies up to I GeV. The model contains Born terms, vector mesons and nucleon resonances (P-33(1232), P-11 (1440), D-13(1520), S-11(1535), F-15(1680), and D-33(1700)). The resonance contributions are included taking into account unitarity to provide the cor rect phases of the pion photoproduction multipoles. The Q(2) dependence of electromagnetic resonance vertices is described with appropriate form facto rs in the electromagnetic helicity amplitudes. Within this model we have ob tained good agreement with the experimental data for pion photo- and electr oproduction on the nucleon for both differential cross sections and polariz ation observables. The model can be used as a starting point to predict and analyze forthcoming data. (C) 1999 Elsevier Science B.V.