A Microscopic Calculation of Photoabsorption Cross Sections on Protons and Nuclei

A recently developed model for ρ-meson propagation in dense hadronic matter is applied to total photoabsorption cross sections in γ-proton and γ-nucleus reactions. Within the vector dominance model the photon coupling to the virtual pion cloud of the nucleon, two-body meson-exchange currents, as well as γ-nucleon resonances are included. Whereas the γp reaction is determined by the low-density limit of the model, higher orders in the nuclear density are important to correctly account for the experimental spectra observed on both light and heavy nuclei over a wide range of photon energies, including the region below the pion threshold. In connection with soft dilepton spectra in high-energy heavy-ion collisions we emphasize the importance of photoab-sorption to further constrain the parameters of the model.