The Density Dependence of the Symmetry Energy in Heavy Ion Collisions

The density dependence of the nuclear symmetry energy is important from very low densities in supernova explosions, to the structure of neutron rich nuclei around saturation density, and to several times saturation density in neutron stars. Heavy ion collisions are the only means to study this density dependence in the laboratory. We discuss the knowledge and uncertainty of the symmetry energy and the parameterizations used in heavy ion collisions. Transport theories are necessary to extract the symmetry energy from heavy ion collisions and we discuss their uncertainties and challenges. We finally study in detail two examples, which relate particularly to the high density symmetry energy, which is of particular interest today. These are the preequilibrium emission of nucleons and light clusters, and the π−/π+ yield ratios. For the last case we point out the inconsistencies in the theoretical analyses of different groups and we discuss the open problems.