Evolution of the symmetry energy of hot neutron-rich matter formed in heavy-ion reactions

Abstract

It is shown that the experimentally observed decrease of the nuclear symmetry energy with the increasing centrality or the excitation energy in isotopic scaling analyses of heavy-ion reactions can be well understood analytically within a degenerate Fermi gas model. The evolution of the symmetry energy is found to be mainly due to the variation in the freeze-out density rather than temperature. The isoscaling analyses are useful for probing the interaction part of the nuclear symmetry energy, provided that both the freeze-out temperature and density of the fragments can be inferred simultaneously from the experiments.

6 pages, 2 figures. Title changed little, comments on extracting the symmetry energy from the isoscaling phenomenon are added. Revised version to appear in PRC