Nucleon-nucleon cross sections in neutron-rich matter and isospin transport in heavy-ion reactions at intermediate energies

Nucleon-nucleon (NN) cross sections are evaluated in neutron-rich matter using a scaling model according to density, momentum, and isospin dependent nucleon effective masses. It is found that the in-medium NN cross sections are not only reduced but also have a different isospin dependence compared with the free-space ones. Because of the neutron-proton effective mass splitting the difference between nn and pp scattering cross sections increases with the increasing isospin asymmetry of the medium. Within the transport model IBUU04, the in-medium NN cross sections are found to influence significantly the isospin transport in heavy-ion reactions. With the in-medium NN cross sections, a symmetry energy of E-sym(rho) approximate to 31.6(rho/rho(0))(0.69) was found most acceptable compared with both the MSU isospin diffusion data and the presently acceptable neutron-skin thickness in Pb-208. The isospin dependent part K-asy(rho(0)) of isobaric nuclear incompressibility was further narrowed down to -500 +/- 50 MeV. The possibility of determining simultaneously the in-medium NN cross sections and the symmetry energy was also studied. The proton transverse flow, or even better the combined transverse flow of neutrons and protons, can be used as a probe of the in-medium NN cross sections without much hindrance from the uncertainties of the symmetry energy.