The topic "Heavy Ion Reactions from Nuclear to Quark Matter" aims at two topics, which are interrelated: The properties of hot and dense nuclear matter and the medium dependence of hadron properties in this environment. Secondly it concentrates on studies which investigate the phase transition from hadronic matter (nuclear matter) to the quark gluon plasma (quark matter).
In normal nuclei, nuclear matter with different ratios of the proton and the neutron densities can be studied at zero temperature and at densities ranging from small densities at the surface of nuclei up to the saturation density of about 0.17 fm-3 nucleons in the center of heavy nuclei. Heavy ion collisions allow to heat up and to compress nuclear matter for short times of less than 10-22 seconds. The aim of heavy ion collisions of several hundred MeV per nucleon or even several GeV per nucleon is to heat up and to compress nuclear matter and by that probe its equation of state. For this one needs a thermal equilibrium and information about the temperature and the density of nuclear matter. In peripheral collsions one does not obtain nuclear matter in a very dense and hot phase. Thus one is forced to select central collisions and to find observables, which give information about the temperature and the density distribution during the collision. Information about density and temperature can be obtained by observing the emitted particles if one has reliable descriptions of the heavy ion reaction. The distribution of these emitted particles give also information about modification of the mass and perhaps also other properties of particles in dense and hot nuclear matter. The nuclear potential for example for negative charged kaons is getting more attractive with the density, while the potential for positive charged kaons is getting more and more repulsive with increasing density. This corresponds to a decreasing mass for negative charged kaons with increasing density and of an increasing mass of positive charged pions with an increasing density. This medium dependence of the kaon mass effects the production probability of kaons. Thus the number of kaons produced in a heavy ion collision can give information about the medium dependence of the kaon mass. In addition one observes a very short mean free path for negative charged kaons and a long mean free path for positive charged kaons, due to the different quark content of this particles. The mean free path of pions, which are produced in large numbers, is very short and thus pions can give information about the shadowing of these particles due to nuclear matter. The energy and momentum distribution of pions allows to get information about the dynamics of the heavy ion collision.
Experimental investigations about these questions are performed for example at the GSI in Darmstadt, at SPIRAL in Caen, at the Superconducting Cyclotron facility at MSU in East Lansing and in the future also more and more at the heavy ion accelerator facility in the RIKEN Laboratory in Wako near Tokyo in Japan. The GSI is focusing in near future to measure dilepton production in hadron-nucleus and nucleus-nucleus collisions with the HADES detector which is currently in the commissioning phase. It is based on a ring-imaging Cherenkov detector, which can track the particles in a toroidal field. In addition HADES contains shower and time-of-flight detectors which allow to distinguish electrons and hadrons.
Heavy ion collisions at higher energies at CERN, at RHIC and in the future also at LHC aim to study the phase transition from hadronic matter to the quark-gluon plasma. A central relativistic collision of two heavy ions creates a fireball, which is nearly in the state of chemical equilibrium. The relative intensity of the production of different particles allows to extract for different mean energies the "freeze-out" temperature. If an ultrarelativistic collision is producing the quark-gluon plasma, the condensation goes directly into the hadronic equilibrium state at freeze-out. This assumption is supported by the fact that simple statistical equilibrium considerations yield the correct particle ratios as a function of temperature and baryonic chemical potential. A comparison with theoretical consideration shows that one expects already at SPS and then also at RHIC energies the phase transition into the quark-gluon plasma. The freeze-out temperatures are there around 180 MeV while the freeze-out tempertures at AGS and at SIS energies are appreciably lower and the baryonic chemical potential is higher. One assumes that that in nuclear collisions at SPS and at RHIC energies the deconfined quark-gluon plasma phase is created and then condensed in a low density hadron gas. The interpretation of this data with the help of a phase transition to the quark-gluon plasma is supported by an anomalous reduction of the J/psi meson production rate and by an increase of low mass dilepton pairs. The low energy dilepton pairs reflect probably the reduction of the rho-meson mass near the QCD phase boundary from hadronic to quark matter. The new data from RHIC indicate with increasing beam energy a softening of the equation of state. The energy step from RHIC to the future ALICE detector at the LHC is even larger than the step from CERN to RHIC. The ALICE program at the LHC will mainly study quark-gluon matter with an equal number of quarks and antiquarks at an extreme high temperature with a very small value of the baryonic chemical potential. Theoretical considerations indicate that the energy density from RHIC will increase by a factor 20 going to LHC. One hopes at LHC to produce so high color field strength that one can study the phase transition to a predicted "Colored glass" condensate. This phase of the quark-gluon plasma would have a totally different parton momentum distribution which should be observable. If one wants to study the hadron-quark phase transition at a high baryonic chemical potential, one needs heavy ion energies between 20 and 50 GeV per nucleon. This is the aim of the future plans of the GSI. The planned Compressed Baryon Matter (CBM) detector at the future GSI facility will study in the QCD phase diagram the region of the high baryonic chemical potential.
The quark-gluon plasma was also probably formed in the micro second region after the big bang. So ultrarelativistic heavy ion collisions are able to recreate a mini big bang and give information about this cosmological aera.
The Erice School in September 2003 on "Heavy Ion Reactions from Nuclear to Quark Matter" will bring together specialists in the experimental and theoretical field of heavy ion reactions, studying the equation of state of nuclear matter, the medium dependence of the properties of hadrons, the phase transition from hadronic matter to the quark-gluon plasma and the recondensation of quark matter to hadrons. In the lectures, seminars and afternoon workshops a survey of the present status of these problems is given by leading scientists in this field. Participants including PhD students have the possibility to speak in workshops about their own contributions to these topics.