Using a relativistic transport model for the expansion stage of S+Au collisions at 200 GeV/nucleon, we show that the recently observed enhancement of lowmass dileptons by the CERES collaboration can be explained by the decrease of vector meson masses in hot and dense hadronic matter.

The change of hadron properties in dense matter based on various theoretical approaches are reviewed. Incorporating these medium effects in the relativistic transport model, which treats consistently the change of hadron masses and energies in dense matter via the scalar and vector fields, heavy-ion collisions at energies available from SIS/GSI, AGS/BNL, and SPS/CERN are studied. This model is seen to provide satisfactory explanations for the observed enhancement of kaon, antikaon, and antiproton yields as well as soft pions in the transverse direction from the SIS experiments. In the AGS heavy-ion experiments, it can account for the enhanced K + /π + ratio, the difference in the slope parameters of the K + and K − transverse kinetic energy spectra, and the lower apparent temperature of antiprotons than that of protons. This model also provides possible explanations for the observed enhancement of low-mass dileptons, phi mesons, and antilambdas in heavy-ion collisions at SPS energies. Furthermore, the change of hadron properties in hot dense matter leads to new signatures of the quark-gluon plasma to hadronic matter transition in future ultrarelativistic heavy-ion collisions at RHIC/BNL.

The flow of kaons, i.e., the average kaon transverse momentum as a function of rapidity, and the associated flow parameter in heavy-ion collisions at SIS/GSI energies is investigated in the relativistic transport model. It is found that the pattern of kaon flow at the final stage is sensitive to the kaon potential used in the model and is thus a useful probe of the kaon potential in a nuclear medium.

We determine an 'empirical' kaon dispersion relation by analysing and fitting recent experimental data on kaon production in heavy-ion collisions. We then investigate its effects on hadronic equation of state at high densities and on neutron star properties. We find that the maximum mass of neutron stars can be lowered by about 0.4M⊙, once kaon condensation as constrained by our empirical dispersion relation is introduced. We emphasize the growing interplay between hadron physics, relativistic heavy-ion physics and the physics of compact objects in astrophysics. 25.75.Dw, 97.60.Jd, 26.60.+c, 24.10.Lx There is currently growing interplay between the physics of hadrons (especially the properties of hadrons in dense matter which might reflect spontaneous chiral symmetry breaking and its restoration), the physics of relativistic heavy-ion collisions (from which one might extract hadron properties in dense matter), and the physics of compact objects in astrophysics (which needs as inputs the information gained from the first two fields). A notable example is the kaon (K andK), which, being a Goldstone boson with strangeness, plays a special role in all of the three fields mentioned.Ever since the pioneering work of Kaplan and Nelson [1] on the possibility of kaon condensation in nuclear matter, much theoretical effort has been devoted to the study of kaon properties in dense matter. Brown et al.[2] have carried out a detailed study of free-space and in-medium kaon-nucleon scattering using chiral perturbation theory. Yuba et al. studied kaon in-medium properties based on phenomenological off-shell meson-nucleon interactions [3]. Weise and collaborators investigated this problem using the Nambu−Jona-Lasinio model, treating the kaon as a quark-antiquark excitation [4]. Recently they have extended the chiral perturbation calculation to include the coupled-channel effects which are important for the K − meson [5]. Another type of study, which is based on the extension of the Walecka meanfield model from SU(2) to SU(3), was pursued by Schaffner et al. [6] and Knorren et al. [7]. Although quantitatively results from these different models are not identical, qualitatively, a consistent picture has emerged; namely, in nuclear matter the K + feels a weak repulsive potential, whereas the K − feels a strong attractive potential. Measurements of kaon spectra and flow have been carried out in heavy-ion collisions at SIS (1-2 AGeV), AGS (10 AGeV), and SPS (200 AGeV) energies [8]. By comparing transport model predictions with experimental data, one can learn not only the global reaction dynamics, but more importantly, the kaon properties in dense matter. Of special interest is kaon production in heavy-ion collisions at SIS energies, as it has been shown that particle production at subthreshold energies is sensitive to its properties in dense matter [9]. Recently, high quality data concerning K + and K − production in heavy-ion collisions at SIS energies have been published by the KaoS collaboration at GSI [10]. The KaoS data show...