Quasifree Eta Photoproduction from Nuclei

Quasifree $\eta$ photoproduction from nuclei is studied in the Distorted Wave Impulse Approximation (DWIA). The elementary eta production operator contains Born terms, vector meson and nucleon resonance contributions and provides an excellent description of the recent low energy Mainz measurements on the nucleon. The resonance sector includes the $S_{11}(1535)$, $P_{11}(1440)$ and $D_{13}(1520)$ states whose couplings are fixed by independent electromagnetic and hadronic data. Different models for the $\eta N$ t-matrix are used to construct a simple $\eta A$ optical potential based on a $t \rho$-approximation. We find that the exclusive $A(\gamma,\eta N)B$ process can be used to study medium modifications of the $N^*$ resonances, particularly if the photon asymmetry can be measured. The inclusive $A(\gamma, \eta)X$ reaction is compared to new data obtained on $^{12}C$, $^{40}Ca$, and is found to provide a clear distinction between different models for the $\eta N$ t-matrix.Comment: 30 pages in RevTeX including 14 embedded PS figures; Replaced with revised version. Added more discussion about the imaginary part of the eta optical potentia