Importance of the direct knockout mechanism in relativistic calculations for (gamma, p) reactions

Results of relativistic calculations of the direct knockout (DKO) mechanism for the photon induced removal of a proton from a target nucleus over a wide range of energies and nuclei are presented. Spectroscopic factors used in the calculations are fixed from consistent analyses of the quasifree electron scattering process (e,e'p). The results indicate that within the uncertainties of the model, the knockout contributions are generally close to the experimental data for missing momenta below approximately 500 MeV/c. This is in disagreement with nonrelativistic analyses which often find that the direct knockout contribution can be quite small compared to the data and that meson exchange corrections can be important. The present study suggests that meson exchange current contributions may not be as large when treated in a relativistic framework. We also point out some difficulties we encountered in analyzing the data for a 12C target at photon energies below 80 MeV.