Chiral spin symmetry and hot/dense QCD

Abstract

Above the chiral symmetry restoration crossover around $T_{ch}\sim 155$ MeV a new regime arises in QCD, a stringy fluid, which is characterized by an approximate chiral spin symmetry of the thermal partition function. This symmetry is not a symmetry of the Dirac Lagrangian and is a symmetry of the electric part of the QCD Lagrangian. In this regime the medium consists of the chirally symmetric and approximately chiral spin symmetric hadrons that are made of the chirally symmetric quarks connected into the color singlet compounds by a confining chromoelectric field. This regime is evidenced by the approximate chiral spin symmetry of the spatial and temporal correlators and by the breakdown of the thermal perturbation theory at the crossover between the partonic (the quark–gluon plasma) and the stringy fluid regimes at $\sim 3T_{ch}$. The chiral spin symmetry smoothly disappears above $\sim 3T_{ch}$ which means that the chromoelectric confining interaction gets screened. A direct evidence that the stringy fluid medium consists of densely packed hadrons is the pion spectral function that shows a distinct pion state and its first radial excitation above $T_{ch}$. Another direct evidence of the hadron degrees of freedom in the stringy fluid is the bottomonium spectrum with the 1S, 2S, 3S and 1P, 2P radial and orbital excitations that become broad with temperature. The hadrons between $T_{ch}$ and $\sim 3T_{ch}$ in the stringy fluid interact strongly which makes the stringy fluid more a liquid rather than a gas. We discuss how this chiral spin symmetric regime extends into the finite chemical potentials domain and present a qualitative sketch of the QCD phase diagram.