On chiral spin symmetry and the QCD phase diagram

TL;DR

This paper investigates the emergence and disappearance of an approximate chiral spin symmetry in QCD at high temperatures, providing evidence from screening masses and spectral functions, and discusses its implications for the QCD phase diagram.

Contribution

It offers new non-perturbative evidence for a chiral spin symmetric phase in QCD and explores its continuation into the phase diagram based on symmetry and screening mass analysis.

Findings

Chiral spin symmetry appears above the chiral crossover temperature.

Resonance peaks in pion spectral functions diminish with increasing temperature.

The symmetry disappears around three times the crossover temperature, indicating a change in dominant interactions.

Abstract

Recently, an approximate $SU(4)$ chiral spin-flavour symmetry was observed in multiplet patterns of QCD meson correlation functions, in a temperature range above the chiral crossover. This symmetry is larger than the chiral symmetry of massless QCD, and can only arise effectively when colour-electric quark-gluon interactions dynamically dominate the quantum effective action. At temperatures about three times the crossover temperature, these patterns disappear again, indicating the screening of colour-electric interactions, and the expected chiral symmetry is recovered. In this contribution we collect independent evidence for such an intermediate temperature range, based on screening masses and the pion spectral function. Both kinds of observables behave non-perturbatively in this window, with resonance-like peaks for the pion and its first excitation disappearing gradually with temperature. Using symmetry arguments and the known behaviour of screening masses at small densities, we discuss how this chiral spin symmetric band continues into the QCD phase diagram.