Extraction of electromagnetic transition form factors for nucleon resonances within a dynamical coupled-channels model

We explain the application of a recently developed analytic continuation method to extract the electromagnetic transition form factors for the nucleon resonances (N*) within a dynamical coupled-channel model of meson-baryon reactions. Illustrative results of the obtained N*→γN transition form factors, defined at the resonance pole positions on the complex energy plane, for the well-isolated P33 and D13 and the complicated P11 resonances are presented. A formula was developed to give a unified representation of the effects due to the first two P11 poles, which are near the πΔ threshold, but are on different Riemann sheets. We also find that a simple formula, with its parameters determined in the Laurent expansions of the πN→πN and γN→πN amplitudes, can reproduce to a very large extent the exact solutions of the considered model at energies near the real parts of the extracted resonance positions. We discuss the important differences between our approach, which is consistent with the earlier formulations of resonances, and the phenomenological approaches using the Breit-Wigner parametrization of resonant amplitudes to fit the data.