Quasi-free η photoproduction from nuclei

Quasi-free η photoproduction from nuclei is studied in the distorted-wave impulse approximation (DWIA). The elementary η 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 S11(1535), P11(1440) and D13(1520) states whose couplings are fixed by independent electromagnetic and hadronic data. Different models for the ηN t-matrix are used to construct a simple ηA optical potential based on a t ρ approximation. We find that the exclusive A(γ,ηN)B process can be used to study medium modifications of the N* resonances, particularly if the photon asymmetry can be measured. The inclusive A(γ, η)X reaction is compared to new data obtained on 12C, 40Ca, and is found to provide a clear distinction between different models for the ηN t-matrix, and we find that the imaginary part of the ηN scattering length is only about 0.12 fm. (orig.)