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

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