Relation between color-deconfinement and chiral restoration

Quantum Chromodynamics (QCD) is believed to undergo phase transitions from hadronic matter to a quark-gluon plasma (QGP) at high temperature. The QGP phase transitions can consist of two distinct transitions – the deconfinement transition and the chiral phase transition. Several theoretical studies [1] have clarified that confinement would be closely related to chiral dynamics and the confined phase should be accompanied by spontaneous breaking of the chiral symmetry. This means that the deconfinement transition temperature must be lower than the chiral phase transition temperature. Actually, in the presence of massive fundamental quarks, neither the deconfinement transition nor the chiral phase transition has any welldefined order parameter. In the lattice QCD simulations [2], it has been observed that the approximate order parameters for two phase transitions, that is, the Polyakov loop and the chiral condensate both show crossover behavior like the magnetization under an external magnetic field. We cannot define the critical temperature any more. Instead, we can only locate the “critical point” by the peak of susceptibility, namely the pseudocritical temperature. The lattice QCD simulations have revealed that the respective peaks of the Polyakov loop and the chiral susceptibilities are located at the same temperature. It has been a puzzle why and how those pseudo-critical temperatures should be identical.