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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...