We perform a systematic study of the fragmentation path of excited nuclear
matter in central heavy ion collisions at the intermediate energy of 0.4AGeV. The theoretical calculations are based on a Relativistic
Boltzmann-Uehling-Uhlenbeck (RBUU) transport equation including stochastic
effects. A Relativistic Mean Field (RMF) approach is used, based on a
non-linear Lagrangian, with coupling constants tuned to reproduce the high
density results of calculations with correlations.
At variance with the case at Fermi energies, a new fast clusterization
mechanism is revealed in the early compression stage of the reaction dynamics.
Fragments appear directly produced from phase-space fluctuations due to
two-body correlations. In-medium effects of the elastic nucleon-nucleon cross
sections on the fragmentation dynamics are particularly discussed. The
subsequent evolution of the primordial clusters is treated using a simple
phenomenological phase space coalescence algorithm.
The reliability of the approach, formation and recognition, is investigated
in detail by comparing fragment momentum space distributions {\it and
simultaneously} their yields with recent experimental data of the FOPI
collaboration by varying the system size of the colliding system, i.e. its
compressional energy (pressure, radial flow). We find an excellent agreement
between theory and experiment in almost all the cases and, on the other hand,
some limitations of the simple coalescence model. Furthermore, the temporal
evolution of the fragment structure is explored with a clear evidence of an
earlier formation of the heavier clusters, that will appear as interesting
relics of the high density phase of the nuclear Equation of State (EoS).Comment: 21 pages, 8 figures, Latex Elsart Style, minor corrections in p.7,
two refs. added, Nucl.Phys.A, accepte