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 $\mathrm{C}+\mathrm{C}$ 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.