Compression and Expansion in Central Collisions

Dynamics of central collisions of heavy nuclei in the energy range from few tens of MeV/nucleon to a couple of GeV/nucleon is discussed. As the beam energy increases and/or the impact parameter decreases, the maximum compression increases. It is argued that the hydrodynamic behavior of matter sets in the vicinity of balance energy. At higher energies shock fronts are observed to form within head-on reaction simulations, perpendicular to beam axis and separating hot compressed matter from cold. In the semicentral reactions a weak tangential discontinuity develops in-between these fronts. The hot compressed matter exposed to the vacuum in directions parallel to the shock fronts begins to expand collectively into these directions. The expansion affects particle angular distributions and mean energy components and further shapes of spectra and mean energies of particles emitted into any one direction. The variation of slopes and the relative yields measured within the FOPI collaboration are in a general agreement with the results of simulations. As to the FOPI data on stopping, they are consistent with the preference for transverse over the longitudinal motion in the head-on Au + Au collisions. Unfortunately, though, the data cannot be used to decide directly on that preference due to acceptance cuts. Tied to the spatial and temporal changes in the reactions are changes in the entropy per nucleon.