Kaon production in heavy ion collisions and the nuclear equation of state

Double differential cross sections for protons, π+ and K+ are measured in Au + Au collisions at 1.0 GeV/u and Θlab=44°±4° with a new magnetic spectrometer at SIS. The pion spectra show an enhancement at low momenta with respect to a thermal distribution and little dependence on the collision centrality. The high-energetic pions measured in semi-central collisions at approximately midrapidity exhibit an anisotropic azimuthal angular distribution, peaking at 90° and 270° with respect to the reaction plane. The same feature is observed for protons. The kaon to proton ratio increases with the number of participating nucleons, i.e. with the centrality of the collision, whereas the pion to proton ratio stays constant. The first data on subthreshold K+ production in a heavy mass collision system are discussed with respect to nuclear matter properties by a comparison to microscopic transport calculations.