QUASI-FREE ETA-PHOTOPRODUCTION FROM NUCLEI

Quasi-free eta photoproduction from nuclei is studied in the Distorted -Wave Impulse Approximation (DWIA), The elementary eta production oper ator contains Born terms, vector meson and nucleon resonance contribut ions and provides an excellent description of the recent low-energy Ma inz measurements on the nucleon. The resonance sector includes the S-1 1(1535), P-11(1440) and D-13(1520) states whose couplings are fixed by independent electromagnetic and hadronic data. Different models for t he eta N t-matrix are used to construct a simple eta A optical potenti al 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 re sonances, particularly if the photon asymmetry can be measured. The in clusive A(gamma,eta)X reaction is compared to new data obtained on C-1 2, Ca-40, and is found to provide a clear distinction between differen t models for the eta N t-matrix, and we find that the imaginary part o f the eta N scattering length is only about 0.12 fm.