A relativistic model for the quasifree photoproduction of $\eta$ mesons from complex nuclei is developed. The interactions between fields are introduced through effective Lagrangians. Contributions from several nucleon resonances as well as nucleon Born terms and vector meson exchange diagrams are included. Nucleon and $\eta$ wave functions are solutions of Dirac and Klein-Gordon equations, respectively. Final state interactions of the outgoing particles are included via optical potentials. The effects of these interactions on the cross sections and photon asymmetries are studied and are found to be large. Calculations indicate that at energies near threshold the exclusive reaction takes place mainly through formation of the $S_{11}$(1535) resonance. Comparisons with the nonrelativistic calculations show differences between the two approaches both for the cross sections and photon asymmetries. We give some detailed predictions for the reaction observables for exclusive photoproduction on ${}^{12}$C, ${}^{16}$O, and ${}^{40}$Ca.

• Received 7 March 1997

DOI:https://doi.org/10.1103/PhysRevC.56.1557