Chiralspin symmetry and its implications for QCD

TL;DR

This paper explores the extended symmetries of massless QCD, particularly the SU(2N_F) and SU(2)_{CS} chiralspin symmetries, and discusses their implications for the behavior of quarks and the nature of the QCD medium at high temperatures.

Contribution

It introduces the concept of larger fermion charge symmetries in gauge theories and analyzes their effects on QCD's electric and magnetic interactions, especially at high temperatures.

Findings

QCD exhibits approximate SU(2)_{CS} and SU(2N_F) symmetries above the chiral restoration temperature.

Chromo-magnetic interactions affect near-zero Dirac modes, while chromo-electric interactions influence all modes.

In the high-temperature regime, QCD behaves as a stringy fluid with quarks bound by chromo-electric fields.

Abstract

In a local gauge-invariant theory with massless Dirac fermions a symmetry of the Lorentz-invariant fermion charge is larger than a symmetry of the Lagrangian as a whole. While the Dirac Lagrangian exhibits only a chiral symmetry, the fermion charge operator is invariant under a larger symmetry group, SU(2N_F), that includes chiral transformations as well as SU(2)_{CS} chiralspin transformations that mix the right- and left-handed components of fermions. Consequently a symmetry of the electric interaction, that is driven by the charge density, is larger than a symmetry of the magnetic interaction and of the kinetic term. This allows to separate in some situations electric and magnetic contributions. In particutar, in QCD the chromo-magnetic interaction contributes only to the near-zero modes of the Dirac operator, while confining chromo-electric interaction contributes to all modes. At high temperatures, above the chiral restoration crossover, QCD exhibits approximate SU(2)_{CS} and SU(2N_F) symmetries that are incompatible with free deconfined quarks. Consequently elementary objects in QCD in this regime are quarks with a definite chirality bound by the chromo-electric field, without the chromo-magnetic effects. In this regime QCD can be described as a stringy fluid.