THE INFLUENCE OF THE NUCLEAR MEDIUM ON THE BARYON RESONANCE EXCITATION

Abstract

We have analyzed the influence of the nuclear medium on the photoexcitation of nucleon resonances, keeping separate the kinematical from the dynamical effects. We have updated the fit to the proton and neutron photoabsorption cross-sections and then evaluated the nuclear cross-section by means of a folding formula, showing that the Fermi motion alone is not able to reproduce the behavior observed in the recent Genova-Frascati experiments. The discrepancy with the data can be substantially eliminated by introducing both a quenching of the excitation strength and a broadening of the resonance widths. According to some recent theoretical works, which are also briefly discussed, the broadening can be due to a rescattering of the nucleon resonances in nuclei, while the damping of the resonances can be attributed to mechanisms involving the internal structure of the nucleon.