High Energy Heavy-Ion Collisions in a RBUU-Approach with Momentum-Dependent Mean-Fields

Abstract

We introduce momentum-dependent scalar and vector fields into the Lorentz covariant RBUU-approach in line with the empirical proton-nucleus relativistic optical potential. Within this extended RBUU-approach we perform numerical simulations for heavy-ion collisions and calculate the transverse flow of nucleons as well as subthreshold production of K+ mesons. By means of these observables we discuss the particular role of the momentum­ dependent forces and their implications on the nuclear equation of state. We find that only a momentum-dependent parameter-set can explain the experimental data on the transverse flow from 150-1000 MeV /u and the differential K+-production cross sections at 1 GeV /u at the same time. The main aim of the high energy heavy-ion physics is to determine the equation of state (EOS) of nuclear matter under extreme conditions far from the ground state. Any conclusion on the properties of hot and dense matter must rely on the comparison of the experimental data with theoretical predictions based on nonequilibrium models. Among these, the BUU-approach1>'2> is a very successful approach in describing the time-dependent evolution of the complex system. As a genuine feature of transport theories it has two important ingredients: the mean-fields or self-energies for nucleons and an in-medium nucleon-nucleon cross-section that accounts for the elastic and inelastic channels. By varying the mean-fields which reflect a certain EOS and comparing the theoretical calculations with the experimental data, one expects to be able to determine the nuclear EOS. Within the framework of BUD-simulations we have succeeded to predict/repro­ duce particle production data in heavy-ion collisions and to clarify their reaction processes.3> In spite of this success the nuclear EOS has not been determined com­ pletely, yet. The mean fields cannot be uniquely determined by the equation of state alone, and, in addition, it is not always possible to extract the nuclear EOS from the results of the BUU calculations without ambiguities for other model inputs. The most important model inputs besides the nuclear incompressibility are the momentum-dependence4> and the Lorentz covariance5> of the mean-fields in the high energy region. Thus we introduce an explicit momentum-dependence of the mean