Kaons in dense matter: results from KaoS

The production of kaons and antikaons in nucleus-nucleus collisions has been studied with the kaon spectrometer at SIS/GSI. The K^-/K⁺ ration was measured in C+C, Ni+Ni, Au+Au collisions at 1.5 AGeV and was found to be almost independent of the system size. In Ni+Ni collisions at 1.93 AGeV we measured the phase-space distributions of K^- and K⁺ mesons. Within the framework of transport models, the measured K⁺ and K^- yields can only be explained if in-medium modifications of the K meson masses are assumed.
In Au-Au collisions at 1 AGeV, we observe non-vanishing elliptic flow near mid-rapidity and in-plane flow at target rapidity for K⁺ mesons. This effect can be explained by transport models when a repulsive in-medium K⁺ N potential is taken into account. The ratio of the K⁺ meson excitation functions for Au+Au and C+C collisions increases with decreasing beam energies. This effect is expected for a soft nuclear equation of state.