Hard Photons and Subthreshold Mesons from Nucleus-Nucleus Collisions

The study of nucleus-nucleus collisions at an energy high enough to allow for an appreciable nuclear overlap is the only experimental means to obtain information about the properties of nuclear matter at a density clearly above the saturation density of heavy nuclei ρ o = 0.16 fm-3. Considering the lack of success of all attempts to reproduce ρ o and the nuclear matter binding energy per nucleon ɛ = -16 MeV in (non relativistic) self consistent calculations on the basis of realistic nucleon-nucleon potentials1), experimental information about the ρ-dependence of ɛ, i.e. the equation of state (eos), may help to solve this long-standing problem of nuclear physics. Additionally, such information is extremely valuable for astrophysics, as the stability of neutron stars as well as the dynamics of a supernova of type II are strongly depending2) on the nuclear eos. The density variations in nuclear ground states are very small and do not yield relevant information; but the EO-giant resonance (breathing mode) energies can be related to the compressibility Ko of nuclear matter which determines the eos near the minimum ρ o. A recent analysis3) of a series of new EO-energy determinations results in Ko = 300 MeV, a value which is larger than the 140 MeV used predominantly2) in astrophysical calculations.