Medium Effects in K⁺ and K^- Poduction in Nuclear Collisions at Subthreshold Beam Energies

We have conducted systematic experimental studies of K⁺ and K^- production and propagation in nucleus-nucleus collisions at beam energies close to and below the production threshold for free nucleon-nucleon (NN) collisions. In noncentral Au+Au collision, the K⁺ mesons are preferentially emitted perpendicular to the reaction plane and the excitation functions for K⁺ and K^- mesons nearly coincide when correcting for the threshold energy. In contrast, for NN collisions the K⁺ yield exceeds the K^- yield by two orders of magnitude near the respective NN threshold. Both effects are considered to be experimental signatures of a modification of kaon properties in the nuclear medium.