NUCLEAR-DEPENDENCE OF THE COHERENT ETA-PHOTOPRODUCTION REACTION IN A RELATIVISTIC APPROACH

We study the nuclear (or A) dependence of the coherent eta photoproduc tion reaction in a relativistic impulse approximation approach. We use a standard relativistic parametrization of the elementary amplitude, based on a set of four Lorentz-and gauge-invariant amplitudes, to calc ulate the coherent production cross section from He-4, C-12, and Ca-40 . In contrast to nonrelativistic treatments, our approach maintains th e full relativistic structure of the process. The nuclear structure af fects the process through the ground-state tensor density. This densit y is sensitive to relativistic effects and depends on A in a different manner than the vector density used in nonrelativistic approaches. Th is peculiar dependence results in He-4 having a cross section signific antly smaller than that of C-12-in contrast to existent nonrelativisti c calculations. Distortion effects are incorporated through an eta-nuc leus optical potential that is computed in a simple ''t rho'' approxim ation.