Global analysis of isospin dependent microscopic nucleon-nucleus optical potentials in a Dirac-Brueckner-Hartree-Fock approach

Background: For the study of exotic nuclei it is important to have an optical model potential that is reliable not only for stable nuclei but can also be extrapolated to nuclear systems with exotic numbers of protons and neutrons. An efficient way to obtain such a potential is to develop a microscopic optical potential (MOP) based on a fundamental theory with a minimal number of free parameters, which are adjusted to describe stable nuclei all over the nuclide chart. Purpose: The choice adopted in the present work is to develop the MOP within a relativistic scheme which provides a natural and consistent relation between the spin-orbit part and the central part of the potential. The Dirac-Brueckner-Hartree-Fock (DBHF) approach provides such a microscopic relativistic scheme, which is based on a realistic nucleon-nucleon interaction and reproduces the saturation properties of symmetric nuclear matter without any adjustable parameter. Its solution using the projection technique within the subtracted T-matrix representation provides a reliable extension to asymmetric nuclear matter, which is important to describe the features of isospin asymmetric nuclei. The present work performs a global analysis of the isospin dependent nucleon-nucleus MOP based on the DBHF calculation in symmetric and asymmetric nuclear matter. Methods: The DBHF approach is used to evaluate the relativistic structure of the nucleon self-energies in nuclear matter at various densities and asymmetries. The Schrodinger equivalent potentials of finite nuclei are derived from these Dirac components by a local density approximation (LDA). The density distributions of finite nuclei are taken from the Hartree-Fock-Bogoliubov approach with Gogny D1S force. An improved LDA approach (ILDA) is employed to get a better prediction of the scattering observables. A chi(2) assessment system based on the global simulated annealing algorithm is developed to optimize the very few free components in this study. Results: The nucleon-nucleus scattering calculations are carried out for a broad spectrum of n and p scattering experiments below 200 MeV with targets ranging from C-12 to Pb-208. The scattering observables including the neutron total cross section, proton reaction cross section, elastic scattering angular distribution, analyzing power, and spin rotation are evaluated and compared with the experimental data, as well as with results derived from the widely used phenomenological Koning-Delaroche global potential. Conclusions: Results with the present relativistic MOP reproduce the n, p + A scattering observables with good accuracy over a broad range of targets and a large region of energies fitting only the free-range factor t in ILDA and minor adjustments of the scalar and vector potentials in the low-density region.