The picture above shows theoretical calculations within the Isospin Quantum Molecular Dynamic (IQMD) model, for the case of Au + Au collisions at a beam energy of 2 AGeV [ see, S.A. Bass et al., Phys. Lett. B335, 289 (1994)].

The time-space evolution of such a collision process is illustrated in three time steps of 10 fm/c (3x10-23 s) each, as viewed in the center-of-mass system of the two colliding nuclei: the approach phase, due to relativistic effects, the nuclei are shortened in the direction of motion (left); the high-density formation phase (middle); and the expansion phase (right). It can be seen that nucleon resonances (green) are excited in the high-density phase, before decaying via emission of mesons (red). In the final phase of the collision, all particles fly out in all directions like an explosion (fireball).

As a result of the described collision one obtains about 200 charged particles (protons and pions) and about 25 photons from the 0 decay. Vector mesons such the and  meson,  produced during the collision, may decay within the transient hot and compressed collision zone formed in such collisions. Vector mesons can decay into e+e- pairs which leave the interaction zone undisturbed, thus allowing a determination of their invariant mass. A second-generation dilepton spectrometer, called HADES , is currently under construction at GSI, in order to study in-medium modifications of the vector mesons (, , ) produced in relativistic heavy-ion collisions at the SIS accelerator facility of GSI.

( for more details see, the International Experiment HADES at GSI).