Strangeness and in-Medium Effects in Heavy Ion Collisions at SIS Energies

Abstract

Heavy ion collisions in the beam energy range 1–2 AGeV offer interesting possibilities to study the bulk properties of hot and dense hadronic matter. Indeed, at these bombarding energies the density of the nuclear system created during the reaction is expected to reach up to three times the normal nuclear density and its temperature is of about 90 AMeV [1,2,3,4]. Under such conditions the fireball consists of a composite system of strongly interacting nucleons energetic enough to give rise to resonance state excitation and meson production [5,6,7]. The abundance and dynamical properties of emerging particles, which have been extensively studied over the two last decades in both experimental data and theories, are recognized as useful tools to provide information about the nuclear equation of state at high densities. Beside probing the global properties of compressed hadronic matter, recent theoretical works predict that the hot and dense nucle ar medium might undergo a phase transition towards partially restored chiral symmetry of QCD, leading to possible dropping hadron masses [8,9]. Such a scenario has attracted much interest recently since it was succesfully used to describe the low mass dilepton enhancement observed by the CERES and HELIOS-3 collaborations at CERN SPS [10,11]. Similar theories suggest that comparable in-medium effects should be observed at lower incident energies and not only on vector mesons but also on pseudo-scalar mesons [8].