Chiral spin symmetry and hot/dense QCD

TL;DR

This paper explores a new high-temperature regime in QCD characterized by approximate chiral spin symmetry, where hadrons form a stringy fluid with strong interactions, bridging the hadronic and quark-gluon plasma phases.

Contribution

It introduces the concept of a stringy fluid regime in QCD with approximate chiral spin symmetry above the chiral crossover temperature, supported by spectral and correlator evidence.

Findings

Identification of a stringy fluid regime with chiral spin symmetry above T_{ch}

Evidence from pion and bottomonium spectra showing hadronic states persist above T_{ch}

Smooth disappearance of chiral spin symmetry above ~3 T_{ch} indicating screening of confining interactions

Abstract

Above the chiral symmetry restoration crossover around T_{ch} ~ 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 stringy fluid regimes at ~ 3 T_{ch}. The chiral spin symmetry smoothly disappears above ~ 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 ~ 3 T_{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.