The atomic nuclei we have discussed so far all have normal nuclear matter density. But as we learned in the plasma physics discussion, matter - in this case nuclear matter - can exist in a wide range of temperatures and densities. Nuclear matter can be heated and also compressed in collisions of nuclei. In this way, one can explore the phase diagram of nuclear matter.

Let us discuss three phenomena where GSI and university groups together with their international collaboration partners have made substantial contributions to the understanding of the phase diagram of nuclear matter. These are the observation of a liquid-to-gas transition, the measurement of hadron properties in the nuclear medium, and the phase transition to the quark-gluon plasma.

A liquid-to-gas transition is what we observe every day when we boil water. The average distance between water molecules is of the order of 0.1 nm as a result of an attractive interaction at large distances and a strong repulsion at short distances. The underlying potential is shown in as a function of the distance (see Figure).

When we heat water by transferring energy to its molecules the temperature increases until we reach 100 °C. At this point the temperature stays constant although we keep transferring energy. This energy is used to break the bonds between the water molecules; thereby water is converted to vapor. The temperature increases again only after all water has been evaporated.

The force between two nucleons in a nucleus shows a very similar dependence on the distance. The scale is however changed by 5 orders of magnitude leading to a density which is almost 15 orders of magnitude higher than that of normal matter. If one would compress the earth to nuclear matter density it would easily fit into a sphere with a radius of about 200 m; just to give you a feeling what nuclear matter density means.

Nuclei in their ground state thus behave very similar to a liquid and because of that a liquid-to-gas transition of nuclear matter was theoretically predicted already some 25 years ago. Only a few years back such a transition was indeed observed at GSI.

We show the experimental data of the ALADIN collaboration. When one heats nuclear matter in a nuclear collision one first observes an increase of the temperature followed by a plateau-like behavior corresponding to the breaking of bonds between nucleons, i.e. one observes the evaporation of nuclear matter. Only after completion of the evaporation process the temperature rises again.

This is an excellent example how analogies between different areas of physics help to understand the underlying generic process, in this case the breaking of bonds between constituents.