Smallness of the imaginary part of the eta-nucleus scattering length

Abstract

In two recent analyses of eta meson production a very small imaginary part was obtained for the eta-He3 scattering length, which is in contradiction with most theoretical predictions. In this report a plausible explanation is given to this unexpectedly weak absorption by suppression of the two main inelasticity channels. It is stressed that the real and imaginary parts of the elementary scattering length do not necessarily always have the same isospin algebraic and spatial properties in a nuclear environment and caution should be exercised in their use in multiple scattering calculations and even constructing simple optical potentials.