COHERENT ETA-PHOTOPRODUCTION ON HE-4 AND C-12 IN THE NEAR-THRESHOLD REGION

Coherent eta meson photoproduction on He-4 and C-12 is considered in t he near-threshold region. The elementary eta photoproduction operator includes contributions from the S-11(1535) and D-13(1520) resonances a s well as t-channel vector meson exchange and the nucleon pole terms. Due to the suppression of the dominant S-11(1535) resonance for spin a nd isospin saturated nuclei, the reaction is mainly governed by omega exchange. Furthermore, the influence of Fermi motion and of different prescriptions for the choice of the invariant reaction energy W-gamma N in the elementary amplitude is studied. (C) 1997 Elsevier Science B. V.