The nucleon-nucleon cross section in ground-state and colliding nuclear matter

An in-medium NN cross section in nuclear matter is derived in a relativistic framework in terms of self-energies using an optical theorem. In particular, self-energies from Dirac-Brueckner HF calculations are used to obtain density-dependent cross sections which are compared to the customary Cugnon parametrization. The results are extended to typical non-equilibrium momentum configurations in heavy-ion collisions, namely two Fermi spheres separated by a relative momentum, so-called colliding nuclear matter. We find that the cross section is significantly changed being reduced at lower and increased at higher energies. The relevance for heavy-ion collisions is discussed.