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Excitation function of strangeness in A+A reactions from SIS to RHIC energies

The properties of $K^+$ and $K^-$ mesons are studied in nuclear reactions from SIS to RHIC energies within the covariant transport approach HSD in comparison to the experimental data whenever available. Whereas kaon abundancies and spectra indicate little repulsive or vanishing selfenergies in the medium, antikaons 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 again well described by the HSD approach based on quark, diquark, string and hadronic degress of freedom. At RHIC energies hadronic rescatterings are found to enhance the strangeness yield from the partonic phase quite substantially.