L. Sehn

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

Abstract 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.

Effective interactions and the equation of state in heavy ion collisions

Abstract We point out that the effective fields in a heavy ion collision are not only momentum dependent, as e.g. the optical model, but also momentum space configuratio n dependent, leading to a double self consistency. The momentum configurations vary strongly in the evolution of a heavy ion collision and are very different from those of the ground state of nuclear matter. We discuss, this dependence in field theoretical models for thermalized and interpenetrating nuclear matter. We estimate the repulsive effects on the equation of state in these limiting cases and for heavy ion collisions. It is found that the effective equation of state in a heavy ion collision acquires considerable stiffness. We discuss ways to include this effect in realistic calculations of heavy ion collisions.

Realistic forces in heavy-ion collisions at intermediate energies

We study the influence of realistic forces based on (Dirac - )Brueckner theory on the dynamics of heavy-ion collisions in the framework of two models; in the relativistic Boltzmann - Uehling - Uhlenbeck approach and in quantum molecular dynamics. These forces are compared to phenomenological forces, either a Skyrme interaction or the nonlinear Walecka model. Analysing the rapidity distribution and flow observables for semicentral Ca + Ca collisions at 400 A MeV, both models show a strong enhancement of the flow with realistic forces relative to phenomenological forces.

Mean fields in colliding nuclear matter

Abstract The momentum-space configuration of the participant nucleons in the early stage of heavy-ion collisions can be described by a colliding nuclear matter configuration, i.e. by two Lorentz elongated Fermi ellipsoids. We introduce this configuration in the framework of the Hartree approximation of the σω-model. The configuration and the mean fields are constructed self-consistently, respecting the Pauli principle for interpenetrating ellipsoids at low relative velocity in a covariant and density conserving manner. In a second step we approximately construct mean fields for these configurations in the context of Brueckner theory. To do this we parametrize the real part of the self-energy of relativistic Brueckner calculations for equilibrated nuclear matter (one-Fermi sphere) in a Hartree scheme to obtain momentum- and density-dependent coupling parameters. With these we calculate the scalar and vector self-energy of a nucleon in colliding nuclear matter consistently with the configuration. The mean self-energy components show strong correlation and exchange effects. The results can be used to improve relativistic transport calculations for heavy-ion collisions.

Relativistic features of the effective interaction in heavy-ion collisions

Abstract The self-energy of a nucleon in a heavy-ion collision depends on the relative velocity and the density of the colliding nuclei. We estimate the scalar and vector self-energies by parametrizing a non-relativistic Brueckner-HF calculation for a system of two colliding symmetric nuclear-matter streams in terms of a relativistic mean field approximation for this system. We therefore evaluate the effective interaction in colliding nuclear matter by calculating the Brueckner G -matrix for a realistic NN interaction and we discuss the resulting single-particle potential. The single-particle energies are then analyzed in terms of the relativistic mean field approximation thereby determining the scalar and vector self-energy. From these we calculate effective coupling parameters by dividing through the corresponding densities. These coupling parameters, which then effectively contain the momentum dependence, correlations and exchange, are discussed as functions of relative momentum and density. They could be used to improve kinetic calculations of relativistic heavy-ion collisions.

Relativistic kinetic calculations of heavy ion collisions with phase space dependent fields

Abstract In transport calculations of heavy ion collisions the effective interactions - mean fields and NN cross sections - have to be determined consistently with the phase space configurations, which is a difficult problem in general. We approximate a solution by constructing self energies for the limiting configurations of colliding and thermalized nuclear matter by extrapolating ground state Dirac-Brueckner results in a consistent way. These self energies are parametrized in the form of effective coupling coefficients. These are then used in relativistic kinetic calculations in a local phase space configuration approximation (LCA). For the transport calculation we use a relativistic extension of the Landau-Vlasov method with Gaussian test particles. We present first results of such calculations which show the sensitivity of various observables to the configuration effects.

Pauli operator for colliding nuclear matter

We give an analytical solution for the Pauli operator and its angle-averaged value for the momentum-space configuration of colliding nuclear matter. This configuration is covariantly described by two interacting Fermi ellipsoids. The Pauli operator specifies the relativistic Thompson equation in colliding nuclear matter. It is therefore required for a solution of this problem within the relativistic Brueckner approach.

Influence of the pion - nucleon interaction on the collective pion flow in heavy ion reactions

We investigate the influence of the real part of the in-medium pion optical potential on the pion dynamics in intermediate energy heavy ion reactions at 1 GeV/A. For different models, i.e. a phenomenological model and the - hole model, a pionic potential is extracted from the dispersion relation and used in quantum molecular dynamics calculations. In addition to the inelastic scattering processes we thus take care of both real and imaginary parts of the pion optical potential. A strong influence of the real pionic potential on the pion in-plane flow is observed. In general, such a potential has the tendency to reduce the anticorrelation of pion and nucleon flow in non-central collisions.

Origin of subthresholdK+production in heavy ion collisions

We investigate the origin of subthreshold K+ production in heavy ion collisions at intermediate energies. In particular we study the influence of the pion induced K+ creation processes. We find that this channel shows a strong dependence on the size of the system, i.e., the number of participating nucleons as well as on the incident energy of the reaction. In an energy region between 1 and 2 GeV/nucleon the pion induced processes essentially contribute to the total yield and can even become dominant in reactions with a large number of participating nucleons. Thus we are able to reproduce recent measurements of the KaoS Collaboration for 1 GeV/nucleon Au on Au reactions adopting a realistic momentum dependent nuclear mean field.

Origin of subthreshold K{sup +} production in heavy ion collisions

We investigate the origin of subthreshold K+ production in heavy ion collisions at intermediate energies. In particular we study the influence of the pion induced K+ creation processes. We find that this channel shows a strong dependence on the size of the system, i.e., the number of participating nucleons as well as on the incident energy of the reaction. In an energy region between 1 and 2 GeV/nucleon the pion induced processes essentially contribute to the total yield and can even become dominant in reactions with a large number of participating nucleons. Thus we are able to reproduce recent measurements of the KaoS Collaboration for 1 GeV/nucleon Au on Au reactions adopting a realistic momentum dependent nuclear mean field.