Meson production and meson properties at finite nuclear density

The properties of π, η, 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, η and kaon abundances and spectra indicate little or vanishing self energies 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. (author)