We investigate the origin of subthreshold K ϩ production in heavy ion collisions at intermediate energies. In particular we study the influence of the pion induced K ϩ 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.

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The formalism of the relativistic (or Dirac-) Brueckner approach in infinite nuclear matter is described. As nucleon-nucleon interaction the one-boson exchange potentials Bonn A,B,C and for comparison the Walecka model are used. The T-matrix is determined from the Thompson equation and is projected onto five covariant amplitudes. By the restriction to positive energy states an ambiguity arises in the relativistic Brueckner approach which is discussed here in terms of the pseudo-scalar and the pseudo-vector projection. The influence of the coupling of the nucleon via the T-matrix as an effective two-nucleon interaction to the nuclear medium is expressed by the self-energy. In particular we investigate the scalar and vector components of the self-energy for the different one-boson exchange potentials and discuss their density and momentum dependence. We estimate the uncertainty of the self-energy due to the pseudo-scalar and the pseudo-vector choice. Usually the momentum dependence of the self-energy is thought to be weak, however, we find that this depends on the one-boson exchange potentials. For the Bonn potentials, in contrast to the $\sigma\omega$-potential, the momentum dependence is strikingly strong above as well as below the Fermi surface. We compare with the results of other groups and study the effects on the equation of state and the nucleon optical potential.Comment: 23 pages Latex, using Revtex style, 10 PS-figures, revised version (only minor changes) accepted for publication in Phys. Rev.

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The properties of the high energy pions observed in heavy ion collisions, in particular in the system Au on Au at 1 GeV/nucleon are investigated. The reaction dynamics is described within the Quantum Molecular Dynamics (QMD) approach. It is shown that high energy pions freeze out early and originate from the hot, compressed matter. N * -resonances are found to give an importnat contribution toward the high energy tail of the pion. Further the role of in-medium effects in the description of charged pion yield and spectra is investigated using a microscopic potential derived from the Brueckner G-matrix which is obtained with the Reid soft-core potential. It is seen that the high energy part of the spectra is relatively more suppressed due to in-medium effects as compared to the low energy part. A comparision to experiments further demonstrates that the present calculations describe reasonably well the neutral (TAPS) and charged (FOPI) pion spectra. The observed energy dependence of the π − /π + ratio, i.e. deviations from the isobar model prediction, is due to Coulomb effects and again indicate that high energy pions probe the hot and dense phase of the reaction. These findings are confirmed independently by a simple phase space analysis.