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

Kaon production has been studied in $\mathrm{Ni}+\mathrm{Ni}$ collisions at beam energies of 0.8–1.8 GeV/nucleon with the kaon spectrometer at GSI. The $K^{+}$ production cross section increases as $E_{\mathrm{beam}}^{5.3\pm 0.2}$. 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 $\mathrm{Ni}+\mathrm{Ni}$ collisions than for nucleon-nucleon collisions near threshold. This is evidence for an enhanced $K^{-}$ production in the nuclear medium.