A consistent dynamical model for pion-photoproduction

Pion-photoproduction amplitude is calculated within a consistent isobar model already used to fix the Delta(++)(1232 MeV) parameters in pion-nucleon scattering and bremsstrahlung. This amplitude is expressed in terms of (physical) on-shell quantities and off-shell contributions coming from final state interactions. These latter are isolated in principal value integrals on the non-polar part of the pion-nucleon K-matrix, being the main effect of them to dress the bare form factors present in the gamma N -> Delta vertex. First, we hide the dressing into effective form factors getting at k(gamma)(2) = 0, G(M) = 2.97 +/- 0.08 and G(E) = 0.055 +/- 0.010 in full consistence with recent chiral effective field theory calculations. Then, different models to regularize the mentioned integrals are analyzed. We choose those better describing the non-resonant multipoles, in order to get model independent results for the resonant ones. Finally, we try to predict the bare form factors getting G(M)(0) = 1.69 +/- 0.02 and G(E)(0) = 0.028 +/- 0.008, which are consistent with recent lattice quark calculations and other more sophisticated dynamical models. (c) 2007 Elsevier B.V. All rights reserved.