Extraction of electromagnetic transition form factors for nucleon resonances

摘要

We explain the application of a recently developed analytic continuation method to extract the electromagneticntransition form factors for the nucleon resonances (Nn∗) within a dynamical coupled-channel model of mesonbaryonnreactions. Illustrative results of the obtained Nn∗ → γN transition form factors, defined at the resonancenpole positions on the complex energy plane, for the well-isolated P33 and D13 and the complicated P11 resonancesnare presented. A formulawas developed to give a unified representation of the effects due to the first two P11 poles,nwhich are near the πu0003 threshold, but are on different Riemann sheets.We also find that a simple formula, with itsnparameters determined in the Laurent expansions of the πN → πN and γN → πN amplitudes, can reproducento a very large extent the exact solutions of the considered model at energies near the real parts of the extractednresonance positions. We discuss the important differences between our approach, which is consistent with thenearlier formulations of resonances, and the phenomenological approaches using the Breit-Wigner parametrizationnof resonant amplitudes to fit the data