Pion and kaon production as a probe for hot and dense nuclear matter

Abstract

The study of particle production in heavy ion reactions represents a valuable tool to extract information on the properties of hot and dense nuclear matter. Pions, kaons and protons were detected in mass-symmetric heavy ion reactions from C+C to Au+Au at incident energies between 0.6 and 2.0 A·GeV with the magnetic spectrometer KaoS installed at SIS, GIS. The study of K + mesons is considered to represent an ideal tool to extract information on the nuclear equation of state (EOS). First results indicate a soft EOS. The yield of K − in Ni+Ni collisions is higher than expected from NN collisions. A possible interpretation of this observation is an in-medium mass modification. The center-of-mass pion spectra deviate from a Boltzmann distribution. The results indicate that high-energy pions are emitted at an early stage of the collision while low-energy pions can be emitted also rather late. This is evidenced using (i) the centrality dependence of the pion yield, (ii) a comparison of oppositely charged pion spectra and (iii) the shielding by spectator matter in peripheral collisions.