Production cross sections of K 1 and K 2 mesons have been measured in C 1 C collisions at beam energies per nucleon below and near the nucleon-nucleon threshold. At a given beam energy, the spectral slopes of the K 2 mesons are significantly steeper than the ones of the K 1 mesons. The excitation functions for K 1 and K 2 mesons nearly coincide when correcting for the threshold energy. In contrast, the K 1 yield exceeds the K 2 yield by a factor of about 100 in proton-proton collisions at beam energies near the respective nucleon-nucleon thresholds. [S0031-9007 (99)08547-6] PACS numbers: 25.75.DwThe properties of strange mesons in a medium of finite baryon density are essential for our understanding of the strong interaction. According to various theoretical approaches, antikaons feel strong attractive forces in the nuclear medium, whereas the in-medium kaon-nucleon potential is expected to be slightly repulsive [1][2][3][4][5].Predictions have been made that the effective mass of the K 2 meson decreases with increasing nuclear density leading to K 2 condensation in neutron stars above 3 times saturation density r 0 . This effect is expected to influence significantly the evolution of supernova explosions: the K 2 condensate softens the nuclear equation of state and thus causes a core with 1.5-2 solar masses to collapse into a black hole rather than to form a neutron star [6,7].Experimental evidence for the attractive in-medium K 2 N potential was found in K 2 nucleus scattering [8] and kaonic atoms [9]. Strong effects are expected in relativistic nucleus-nucleus collisions where baryonic densities of several times the saturation density r o can be reached. Under these conditions the K 2 effective mass will be reduced and thus the kinematical threshold for the process NN ! K 2 1 K 1 1 NN (which in free space corresponds to a kinetic beam energy of 2.5 GeV) will be lowered. As a consequence, the K 2 yield in A 1 A collisions at bombarding energies below the NN threshold will be enhanced significantly as compared to the case without in-medium mass reduction. In contrast, the yield of K 1 mesons is predicted to be decreased as the K 1 effective mass and thus the in-medium K 1 production threshold is slightly increased [3,10,11]. The in-medium KN potentials are also expected to affect the propagation of kaons and antikaons, hence modifying their emission pattern in nucleus-nucleus collisions.According to these considerations, the K 2 ͞K 1 ratio observed in nucleus-nucleus collisions at beam energies below the NN threshold is sensitive to the in-medium properties of kaons and antikaons. Moreover, relativistic transport calculations find distinct differences in the K 1 and K 2 spectral slopes, again due to medium effects. In particular, these calculations predict that the K 2 spectra fall off steeper than the K 1 spectra due to the decrease of the K 2 effective mass in the nuclear medium [12].Recent experimental studies of kaon and antikaon production in Ni 1 Ni collisions found a large K 2 yield at 1.8A GeV [13,14], a...

The production of pions and kaons has been measured in 197 Au+ 197 Au collisions at beam energies from 0.6 to 1.5 A·GeV with the Kaon Spectrometer at SIS/GSI. The K + meson multiplicity per nucleon is enhanced in Au+Au collisions by factors up to 6 relative to C+C reactions whereas the corresponding π ratio is reduced. The ratio of the K + meson excitation functions for Au+Au and C+C collisions increases with decreasing beam energy. This behavior is expected for a soft nuclear equation-of-state.

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Abstract. The production of K + and of K − mesons in heavy-ion collisions at beam energies of 1 to 2 AGeV has systematically been investigated with the Kaon Spectrometer KaoS. The ratio of the K + production excitation function for Au+Au and for C+C reactions increases with decreasing beam energy, which is expected for a soft nuclear equation-of-state. A comprehensive study of the K + and of the K − emission as a function of the size of the collision system, of the collision centrality, of the kaon energy, and of the polar emission angle has been performed. The K − /K + ratio is found to be nearly constant as a function of the collision centrality and can be explained by the dominance of strangeness exchange. On the other hand the spectral slopes and the polar emission patterns are different for K − and for K + . Furthermore the azimuthal distribution of the particle emission has been investigated. K + mesons and pions are emitted preferentially perpendicular to the reaction plane as well in Au+Au as in Ni+Ni collisions. In contrast for K − mesons in Ni+Ni reactions an in-plane flow was observed for the first time at these incident enegies.

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