Influence of transport variables on isospin transport ratios

The symmetry energy in the nuclear equation of state affects many aspects of nuclear astrophysics, nuclear structure, and nuclear reactions. Recent constraints from heavy-ion collisions, including isospin diffusion observables, have started to put constraints on the symmetry energy below nuclear saturation density, but these constraints depend on the employed transport model and input physics other than the symmetry energy. To understand these dependencies, we study the influence of the symmetry energy, isoscalar mean-field compressibility and momentum dependence, in-medium nucleon-nucleon cross sections, and light cluster production on isospin diffusion within the pBUU transport code. In addition to the symmetry energy, several uncertain issues strongly affect isospin diffusion, most notably the cross sections and cluster production. In addition, there is a difference in the calculated isospin transport ratios, depending on whether they are computed using the isospin asymmetry either of the residue or of all forward-moving fragments. Measurements that compare the isospin transport ratios of these two quantities would help place constraints on the input physics, such as the density dependence of the symmetry energy.