Simulations Of Heavy-ion Collisions Research Articles

Braaten-Pisarski method at finite chemical potential

The effective perturbation theory developed by Braaten and Pisarski for gauge theories at finite temperature is extended to finite chemical potential. As a first application the collisional energy loss of a heavy quark propagating through a quark-gluon plasma with non-vanishing quark chemical potential is considered. Assuming μ T ⋍ 1 , motivated by numerical simulations of heavy ion collisions at RHIC energies, we find that the effect of the quark chemical potential is rather small, unless the energy density instead of the temperature is fixed.

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

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

Dynamical treatment of Fermi motion in a microscopic description of heavy ion collisions.

A quasiclassical Pauli potential is used to simulate the Fermi motion of nucleons in a molecular dynamical simulation of heavy ion collisions. The thermostatic properties of a Fermi gas with and without interactions are presented. The inclusion of this Pauli potential into the quantum molecular dynamics (QMD) approach yields a model with well defined fermionic ground states, which is therefore also able to give the excitation energies of the emitted fragments. The deexcitation mechanisms (particle evaporation and multifragmentation) of the new model are investigated. The dynamics of the QMD with Pauli potential is tested by a wide range of comparisons of calculated and experimental double-differential cross sections for inclusive p-induced reactions at incident energies of 80 to 160 MeV. Results at 256 and 800 MeV incident proton energy are presented as predictions for completed experiments which are as yet unpublished.

Refinements to the treatment of scattering in Vlasov-Boltzmann calculations.

A new treatment of scattering, which conserves the total angular momentum and accounts for differences in {ital n}-{ital n}, {ital n}-{ital p}, and {ital p}-{ital p} cross sections, is proposed for use in the Vlasov-Nordheim (or Boltzmann-Uhling-Uhlenbeck) simulations of heavy-ion collisions. This treatment is used in the classical Vlasov-Boltzmann simulations of the collisions between two equal mass ({ital A}=50) nuclei at {ital E}{sub lab}{approx gt}400 MeV/nucleon. By comparison with the exact results for such collisions, it is shown to be significantly more accurate than the commonly used method in which the angular momentum is not exactly conserved.

Single-particle potential in dense nuclear matter.

The single-particle potential in nuclear matter is calculated microscopically for several Hamiltonians for densities ranging from 0.1 to 0.5 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}3}$. These Hamiltonians include nucleon-nucleon potentials fit to scattering data and three-nucleon potentials fit to the binding energies of few-body nuclei and saturation properties of nuclear matter. The single-particle potential is a key ingredient in Boltzmann-Uehling-Uhlenbeck simulations of heavy-ion collisions. Parametrizations of the density and momentum dependence of the single-particle potential that may be useful in such simulations are discussed and compared to phenomenological prescriptions currently in use.