Medium Effects in Kaon and Antikaon Production in Nuclear Collisions at Subthreshold Beam Energies

Production cross sections of K and K− mesons have been measured in C+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− mesons are significantly steeper than the ones of the K mesons. The excitation functions for K and K− mesons nearly coincide when correcting for the threshold energy. In contrast, the K yield exceeds the K− yield by a factor of about 100 in proton-proton collisions at beam energies near the respective nucleon-nucleon thresholds.