Evidence for a Soft Nuclear Equation-of-State from Kaon Production in Heavy-Ion Collisions

The production of pions and kaons has been measured in ${}^{197}\mathrm{Au}{+}^{197}\mathrm{Au}$ collisions at beam energies from 0.6 to $1.5A\mathrm{GeV}$ with the kaon spectrometer at SIS/GSI. The $K^{+}$ meson multiplicity per nucleon is enhanced in $\mathrm{Au}+\mathrm{Au}$ collisions by factors up to 6 relative to $\mathrm{C}+\mathrm{C}$ reactions, whereas the corresponding pion ratio is reduced. The ratio of the $K^{+}$ meson excitation functions for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{C}+\mathrm{C}$ collisions increases with decreasing beam energy. This behavior is expected for a soft nuclear equation-of-state.