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

Abstract

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 ${E}_{\mathrm{beam}}^{5.3\ifmmode\pm\else\textpm\fi{}0.2}$. Both ${K}^{+}$ and ${K}^{\ensuremath{-}}$ mesons are predominantly produced in central collisions. The ${K}^{\ensuremath{-}}/{K}^{+}$ 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 ${K}^{\ensuremath{-}}$ production in the nuclear medium.