Isobars and the medium-energy (γ,p) reaction

Recently, precise experimental measurements have been obtained for the reaction ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$($\gamma$,$p$)${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ for photon energies 50-350 MeV. For photon energies greater than 100 MeV, isobar photoproduction is expected to provide an important part of the ($\gamma$,$p_0$) reaction. We evaluate the isobar contribution to the ($\gamma$,$p$) reaction, and we add this amplitude to a term which corresponds to direct knockout of the proton by the photon. We compare our results in detail with the experimental data from Glasgow and Bates on ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. Our calculation uses coupling constants and nuclear wave functions determined from independent experiments, and we neglect the isobar-nucleus interaction. Agreement with the experimental data is quite reasonable, and isobar photoproduction provides a very large contribution to the nuclear photoeffect in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, for photon energies above about 100 MeV. We consider the effects of various corrections to our formalism, such as the effect of distortion of the outgoing nucleon by the nuclear force, center of mass corrections, and the inclusion of $\rho$ mesons in addition to pions at the isobar decay vertex. None of these corrections alters the qualitative features of the isobar amplitude and its importance in the nuclear photoeffect at medium energies.