The dependence of $K^{+}$ production on the nuclear equation of state is investigated in heavy-ion collisions. An increase of the excitation function of $K^{+}$ multiplicities obtained in heavy $(\mathrm{Au}+\mathrm{Au})$ over light $(\mathrm{C}+\mathrm{C})$ systems when going far below threshold which has been observed by the KaoS Collaboration strongly favors a soft equation of state. This observation holds despite the influence of an in-medium kaon potential predicted by effective chiral models which is necessary to reproduce the experimental $K^{+}$ yields.

• Received 10 July 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.1974