Approximations to Brueckner theory for nucleon-nucleus collisions

The concept of quasiparticles has been very fruitful for the development of theories of many-body systems of strongly interacting particles. A central problem for a quantitative treatment is the interaction between the quasiparticles (the effective interaction between particles). The Brueckner reaction matrix theory and the Jastrow variational approach have been used extensivley, especially for nuclei. Our present concern is the collisions between nuclei. An exact calculation of the effective interaction between the nucleus is then extremely complicated. Only the simplest extreme, the nucleon-nucleus optical potential, can be treated fully. Approximate treatments are called for. The present paper investigates some approximations to the Brueckner theory. We are primarily interested in the imaginary part of the one-body (one-nucleon) mean field, as it relates to the damping of collective motion. Of special interest is the increase at low density. Comparisons with Skyrme forces show that they exaggregate this density dependence. (orig.)