Importance of initial-final state correlations for the formation of fragments in heavy ion collisions

Using quantum molecular dynamics simulations, we investigate the formation of fragments in symmetric reactions between beam energies of $E=30A\mathrm{MeV}$ and $600A\mathrm{MeV}.$ After a comparison with existing data we investigate some observables relevant to tackle equilibration: $d\sigma {/dE}_{\mathrm{rat}},$ the double differential cross section $d^2\sigma {/p}_t{\mathrm{dp}}_z{\mathrm{dp}}_t,\dots .$ Apart maybe from very energetic $(E>~400A\mathrm{MeV})$ and very central reactions, none of our simulations gives evidence that the system passes through a state of equilibrium. Later, we address the production mechanisms and find that, whatever the energy, nucleons finally entrained in a fragment exhibit strong initial-final state correlations, in coordinate as well as in momentum space. At high energy those correlations resemble the ones obtained in the participant-spectator model. At low energy the correlations are equally strong, but more complicated; they are a consequence of the Pauli blocking of the nucleon-nucleon collisions, the geometry, and the excitation energy. Studying a second set of time-dependent variables (radii, densities, etc.), we investigate in detail how those correlations survive the reaction, especially in central reactions where the nucleons have to pass through the whole system. It appears that some fragments are made of nucleons which were initially correlated, whereas others are formed by nucleons scattered during the reaction into the vicinity of a group of previously correlated nucleons.