Nuclear dependence of the coherent eta photoproduction reaction in a relativistic approach

We study the nuclear (or A) dependence of the coherent eta photoproduction reaction in a relativistic impulse approximation approach. We use a standard relativistic parameterization of the elementary amplitude, based on a set of four Lorentz- and gauge-invariant amplitudes, to calculate the coherent production cross section from He-4, C-12, and Ca-40. In contrast to nonrelativistic treatments, our approach maintains the full relativistic structure of the process. The nuclear structure affects the process through the ground-state tensor density. This density is sensitive to relativistic effects and depends on A in a different manner than the vector density used in nonrelativistic approaches. This peculiar dependence results in He-4 having a cross section significantly smaller than that of C-12---in contrast to existent nonrelativistic calculations. Distortion effects are incorporated through an eta-nucleus optical potential that is computed in a simple ``t rho'' approximation.Comment: 4 pages, 3 postscript figures. The replace is due to a misspelling in the Authors' lis

The η\eta-3N problem with separable interactions

The $\eta$-3N-interaction is studied within the four-body Faddeev-Yakubovsky theory adopting purely separable forms for the two- and three-body subamplitudes, limiting the basic two-body interactions to s-waves only. The corresponding separable approximation for the integral kernels is obtained by using the Hilbert-Schmidt procedure. Results are presented for the $\eta$-$^3$H scattering amplitude and for the total elastic cross section for energies below the triton break-up threshold.Comment: revised version accepted for Phys. Rev. C, 16 pages revtex including 6 eps-figures, formal part shortene