Fusion of strings vs. percolation and the transition to the quark gluon plasma

In most of the models of hadronic collisions the number of exchanged colour strings grows with energy and atomic numbers of the projectile and target. At high string densities interaction between them becomes important, which should melt them into the quark-gluon plasma state in the end. It is shown that under certain reasonable assumptions about the the string interaction, a phase transition to the quark gluon plasma indeed takes place in the system of many colour strings. It may be of the first or second order, depending on the particular mechanism of the interaction. The critical string density is about unity in both cases. In the latter case the percolation of strings occurs above the critical density. The critical density may have been already reached in central Pb-Pb collisions at 158 A GeV.