We study systematically the production of strangeness in nuclear reactions from SIS to SPS energies within the covariant hadronic transport approach HSD. Whereas the proton and pion rapidity distributions as well as pion transverse momentum spectra are well described in the hadronic transport model from 2-200 A·GeV, the K + and K − spectra are noticeably underestimated at AGS energies while the K + spectra match well at SIS and SPS energies with the experimental data. We conclude that the failure of the hadronic model at AGS energies points towards a nonhadronic phase during the collision of heavy systems around 10 A·GeV. * Supported by BMBF and GSI Darmstadt † Part of the PhD thesis of J. Geiss

We study the production of cc pairs in nuclear reactions at SPS energies within the covariant transport approach HSD. The production of cc is treated perturbatively employing experimental cross sections while the interactions of cc pairs with baryons are included by conventional cascade-type two-body collisions. Adopting 6 mb for the cc-baryon cross sections the data on J/Ψ suppression in p + A reactions are reproduced in line with calculations based on the Glauber model. Additionally the dissociation of the cc pairs by strings is included in a purely geometrical way. We find good agreement with experimental data from the NA38 and NA50 collaboration with an estimate for the string radius of R s ≈ 0.2 − 0.3 f m. * Supported by BMBF and GSI Darmstadt

Excited states of the nucleon are described as RPA configurations on a mean-field ground state taken from the MIT bag model. A residual interaction of a structure as in the Nambu-Jona-Lasinio model is used. The particle-hole states are coupled to good total angular momentum and isospin. Valence excitations of particle-hole type and quark-antiquark (qq) states from the Dirac-sea are included. The dependence of the baryon spectrum and multipole response functions on the coupling constant G is studied. At critical values of G the 3qground state becomes degenerate with strongly collective qq modes. The model is used to calculate the electric polarizability of the neutron α N . Without residual interaction α N = 7 · 10 −4 f m 3 is found. With residual interaction the value increases to α N = (7 − 11) · 10 −4 f m 3 . The measured value of α N is reproduced within experimental error bars. † Supported by GSI and BMBF 1

The tensor-RPA approach developed previously in part I is applied to the Nambu-Jona-Lasinio (NJL) model. As a first step we investigate the structure of Dirac-Hartree-Fock solutions for a rotationally and isospin invariant ground-state density. Whereas vacuum properties can be reproduced, no solitonic configuration for a system with unit baryon number is found. We then solve the tensor-RPA equation employing simple models of the nucleon ground state. In general the ph interaction effects a decrease of the excited states to lower energies. Due to an enhanced level density at low energies the obtained spectra cannot be matched with the experimental data when a standard MIT-bag configuration is used. However, when the size of the nucleon quark core is reduced to ≈ 0.3f m a fair description of the baryon spectrum in the positive-parity channel is achieved. For this purpose the residual interaction turns out to be crucial and leads to a significant improvement compared with the mean-field spectra.

In this paper we develop a theoretical framework which allows us to study excitations of the nucleon. Assuming an effective two-body interaction as a model for low-energy QCD, we derive a relativistic TDHF equation for a many-body system of quarks. To render the Dirac-sea contribution to the mean field finite, we introduce a symmetry conserving regularization scheme. In the small amplitude limit we derive an RPA equation. The structure of the ph interaction and modifications due to the regularization scheme are discussed. We give a prescription to obtain a nucleon state with good angular momentum (J) and isospin (T ) quantum numbers on mean-field level. To study excitations, we develop a tensor-RPA approach, which is an extension of the conventional RPA techniques to systems with a nonscalar ground state. This allows us to construct excited states with good (J/T ) quantum numbers. We discuss a method to reduce the overcomplete ph-space and compute the tensor-RPA interaction matrix elements. Finally we extend our scheme to include ( 3 2 + , 32 )-states.