Meson Production and Meson Properties at Finite Nuclear Density

The properties of $\pi, \eta, K^+$ and $K^-$ mesons are studied in nuclear reactions from SIS to SPS energies within the covariant transport approach HSD in comparison to the experimental data. Whereas the pion, $\eta$ and kaon abundancies and spectra indicate little or vanishing selfe-nergies for these mesons in the medium, antikaons (as well as antiprotons) are found to experience strong attractive potentials in nucleus-nucleus collisions at SIS energies. However, even when including these potentials the $K^+$ and $K^-$ spectra at AGS energies are noticeably underestimated showing an experimental excess of strangeness that points towards a nonhadronic phase in these reactions. On the other hand the $K^+, K^-$ production at SPS energies is well described by the hadronic approach without incorporating any parton degrees of freedom.