COHERENT ETA-PHOTOPRODUCTION FROM NUCLEI IN A RELATIVISTIC IMPULSE APPROXIMATION APPROACH

We study coherent eta photoproduction from nuclei in a relativistic im pulse approximation approach. For the elementary production amplitude we use a standard relativistic parametrization based on a set of four Lorentz- and gauge-invariant amplitudes. The photonuclear amplitude is evaluated without recourse to a nonrelativistic reduction; the full r elativistic structure of the amplitude is maintained. On general argum ents we show that the coherent process is sensitive to only one of the elementary amplitudes. Moreover, we show that the nuclear structure i nformation is fully contained in the ground-state tensor density. The tensor density is evaluated in a mean-field approximation to the Walec ka model and it is shown to be sensitive to relativistic effects. Dist ortion effects are incorporated through an eta-nucleus optical potenti al that is computed in a simple ''t rho'' approximation.