Bernd Schürmann

Non-equilibrium processes in heavy-ion collisions at relativistic energies

Abstract We use transport theory to describe the inclusive cross sections for protons and pions produced in collisions between two identical heavy ions at an energy of 800 MeV per particle. In addition to the nucleonic we take the Δ-degree of freedom into account. Thus we consider a two-component system whose distributions in transverse momentum and rapidity we describe by two coupled Fokker-Planck equations. These transport equations contain the one-nucleon knock-out process as initial condition. In the limit of large interaction times they lead to thermal equilibrium (fireball) distributions. For light nuclei the interaction time is not large enough for equilibrium to be reached. A recent experiment for two colliding carbon nuclei at 800 MeV per nucleon shows evidence of nonequilibrium effects. We compare our calculations with experimental data for 12C on 12C and Ne on NaF at 800 MeV/N.

Subthreshold Kaon Production and Compression Effects

Abstract With regard to the use of subthreshold kaons from relativistic heavy-ion collisions as a tool to gain information on the nuclear matter equation of state, we investigate the role of the elementary kaon production cross section. The uncertainties introduced because of the poor knowledge of the latter near threshold are largely eliminated by considering kaon yield ratios.

The inclusive production of kaons in relativistic nucleus-nucleus collisions based on transport theory

Abstract We assume that in relativistic heavy-ion reactions positive kaons (K + ) are produced in sequences of independent baryon-baryon collisions. The basic inputs into our calculation of the K + inclusive differential cross section thus are the baryon momentum distributions after a number n of collisions. These functions are, to a very good approximation, determined by their first and second moments which in turn are calculated by use of a transport theoretical approach. We compare our results with experimental data obtained from the reaction Ne + NaF at 2.1 GeV nucleon . This comparison is first made under the assumption that the kaons, once produced, escape from the participant region without further interactions. In this case, the observed K + total production cross section is reproduced quite well, but the corresponding differential cross sections are not. The inclusion of the K + elastic scattering on the surrounding baryons leads to a much better agreement with the experimental data. We also calculate the K + total production rate as a function of the bombarding energy per nucleon. At all energies, the number of kaons produced by multiply scattered baryons is seen to exceed by far that obtained from nucleons which scatter for the first time. The K + total production cross section remains sizeable in our approach even far below the threshold energy. For a very heavy system like Pb on Pb we estimate a cross section of 10 mb even 800 MeV below threshold.

Scaling behaviour of observables in heavy ion collisions

Abstract Newly introduced exclusive observables in heavy ion collisions are analyzed in terms of the similarity properties of fluid dynamics. The scaling properties of these variables are illustrated by studying simple analytic models. The analysis of corresponding recent experimental data shows fluid-dynamical scaling behaviour in a wide mass number and energy range. Deviations from perfect scaling in well denned kinematic regions are pointed out. A previous analysis of the scaling properties of nucleon cross sections is extended to pion as well as to gamma production.

ANALYTICAL TREATMENT OF HIGH-ENERGY NUCLEUS NUCLEUS COLLISIONS

Abstract We review an analytical study of high-energy nucleus-nucleus collisions based on Boltzmanns kinetic theory of gases. In the first part of the paper, we do not take effects of the nuclear mean field into account. This turns out to be appropriate for particle inclusive production, as the good agreement of the theory with a large variety of inclusive experimental data demonstrates. As soon as physical observables extracted from measurements on an event per event basis are considered, the nuclear potential plays a decisive role. To account for this fact, we extend the theory by including the nuclear mean field in the Boltzmann equation. The results obtained are in qualitative agreement with experimental exclusive observables such as the sideward kinetic energy flow angle and the average transverse momentum, despite the absence of strong compression of nuclear matter in the theory presented.

Scaling properties in the hydrodynamical description of heavy-ion reactions

Abstract We discuss scaling properties of heavy-ion reactions which are consequences of a simple hydrodynamical model. In certain kinematic regions the experimentally observed inclusive cross sections satisfy this scaling and so do the cross sections numerically calculated using hydrodynamics. The simple thermal model obeys the same scaling laws, but leads to a differential cross section with a characteristically different shape at very high and very low energies of the outgoing nucleons. The specific experimental data analyzed do not show evidence for a hydrodynamical flow.

The “subthreshold” production of antikaons in relativistic nuclear collisions

Abstract By use of simplified multiple collision model we calculate the inclusive differential cross section for antikaons produced in relativistic nucleus collisions at bombarding energies per nucleon below the threshold for production from free nucleon nucleon encounters. The primordial spectrum obtained is in qualitative agreement with experimental data for the reaction Si + Si at 2.1 GeV/nucleon. The inclusion of the rescattering and absorption of the antikaons significantly influences the shape of the primordial spectrum but does not substantially alter the degree of agreement with the experimental data.

Transport theory applied to Kaon production in high-energy nucleus-Nucleus collisions

Abstract The inclusive differential cross section for K + -meson production in the reaction Ne+NaF at 2.1 GeV per nucleon is calculated within a simplified multiple scattering approach based on sequences of independent baryon-baryon collisions. The momentum distributions of the baryons needed for the K + spectra are determined by use of the almost analytical, parameter-free model of transport theory. The scattering of the produced kaons by the surrounding baryons is taken into account and found to be essential for a reasonable agreement with the experimental data.

Direct versus thermal particle emission in high-energy heavy-ion collisions

Abstract Recent measurements of proton inclusive cross sections for heavy-ion collisions at several hundred MeV per nucleon are discussed in a phenomenological model which assumes coexistence of direct and thermal particle emission. The model postulates the existence of an additive twocomponent structure for inclusive spectra. The two components consist of a direct and a thermal one. The direct component is described in an extended impulse approximation and the thermal component in the participant-spectator geometrical model. Their normalizations are fixed by geometrical considerations. With this model we calculate the charged particle and proton inclusive spectra for Ne on U at 250 MeV per nucleon, Ne on U and Ne on Al at 400 MeV per nucleon, and Ne on NaF at 800 MeV per nucleon. A comparison is made with corresponding data. At 250 MeV and 400 MeV per nucleon, we find the postulated two-component structure to be instrumental in reproducing satisfactorily the slopes and the magnitudes of the data in the critical forward angle region. At 800 MeV per nucleon, due to the larger transparency of the nuclei, non-equilibrium statistical components appear to be important.

On the origin of the flow angle in high-energy collisions of heavy nuclei

Abstract We expose a “test” participant nucleon to the moving projectile and target spectator nuclear mean fields. A net attraction of the participant is found which results in a sizeable flow angle in collisions of heavy nuclei at an incident energy of 400 MeV/nucleon.