The nuclear equation of state probed by K+ production in heavy ion collisions

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 (Au+Au) over light (C+C) systems when going far below threshold which has been observed by the KaoS Collaboration strongly favours a soft equation of state. This observation holds despite of the influence of an in-medium kaon potential predicted by effective chiral models which is necessary to reproduce the experimental K+ yields. Phase space effects are discussed with respect to the K+ excitation function.