Existence of a second phase transition in A − A collisions at energies available at the CERN Large Hadron Collider: A physical interpretation

The observed fluid behavior of the matter produced in heavy ion and p p collisions implies that the strings stretched between the constituents of the projectile and target should have a hard core giving rise to a repulsion between them. It is shown that in these collisions there are two critical string densities, one corresponding to the well-studied percolation of strings and another to the percolation of the hard core of the strings. These two critical densities are related to two critical temperatures, one T ≈ 160 MeV would correspond to the restoration of chiral symmetry and partial deconfinement and a second transition T ≈ 220 MeV form the fluid behavior of quantum chromodynamics matter strongly interacting to a quasi free gas of quarks and gluons. This argument is supported by the recently published results on the sudden increase in the degrees of freedom above the deconfinement temperature in p p and A − A collisions [Mishra , ].