On the origin of the Nc¹ scaling in the confined but chirally symmetric phase at high T

There is lattice evidence that the QCD matter above the chiral restoration temperature Tch T ch and below the deconfinement temperature Td T d , called stringy fluid, is characterized by approximate chiral spin symmetry, which is a symmetry of confinement in QCD with light quarks. The energy density, pressure and entropy density in the stringy fluid scale as Nc¹ N c 1 , which is in contrast to the Nc⁰ N c 0 scaling in the hadron gas and to the Nc² N c 2 scaling in the quark-gluon plasma. Here we clarify the origin of the Nc¹ N c 1 scaling. We employ a solvable field-theoretical large Nc N c chirally symmetric and confining model. In vacuum the confining potential induces a spontaneous breaking of chiral symmetry. The mesons are spatially localized states of quarks and antiquarks. Still in the confining regime the system undergoes the chiral restoration phase transition at Tch T ch because of Paili blocking of the quark levels required for the existence of the quark condensate, by the thermal excitation of quarks and antiquarks. The same Paili blocking leads to a delocalization of the color singlet low-spin meson-like states that become infinitely large in the chiral limit. Consequently the stringy fluid represents a very dense medium of the overlapping huge color-singlet low-spin quark-antiquark systems. The Bethe–Salpeter equation that determines the rest-frame excitation energies of the color-singlet quark-antiquark system is Nc N c -independent both in vacuum and in the medium in the confining regime. The excitation energy of the quark-antiquark color-singlet systems scales as Nc⁰ N c 0 , i.e. as meson mass in vacuum. The Nc¹ N c 1 scaling of the energy density in the stringy fluid is provided by the fluctuations of the color-singlet quark-antiquark systems.