Subthreshold Production of Kaons and Antikaons in Nucleus-Nucleus Collisions at Equivalent Beam Energies

Kaon production has been studied in Ni+Ni collisions at beam energies of 0.8--1.8 GeV/nucleon with the kaon spectrometer at GSI. The K⁺ production cross section increases as Ebeam^5.3±0.2. Both K⁺ and K^- mesons are predominantly produced in central collisions. The K^-/K⁺ ratio measured at equivalent beam energies below the respective particle production threshold is considerably larger for Ni+Ni collisions than for nucleon-nucleon collisions near threshold. This is evidence for an enhanced K^- production in the nuclear medium.