Nuclear phase transformations: new states of dense matter

Heavy ion collisions with velocities nearing the quarter of light speed allow us to study the behaviour of nuclear matter under extreme temperature and density conditions. Experimental studies show that the decay of hot nuclei can take the form of the emission of smaller nuclei (nuclear fragmentation) or of the emission of only light particles such as neutrons, hydrogen, or helium nuclei (vaporization). The second process has similarities with liquid-gas phase transformation. Recently 2 new experimental facts have supported the phase transformation interpretation: energy fluctuations and correlations with abnormal amplitudes. Energy fluctuations imply negative values of the specific heat and a negative specific heat is involved in the liquid-gas transformation of any finite system. Abnormal amplitude correlations show the possibility of a spinodal decomposition that, till now, has only been seen in liquid-gas transformation. (A.C.)