Emergent chiral spin symmetry, non-perturbative dynamics and thermoparticles in hot QCD

Abstract

Several non-perturbative results for hot QCD are challenging some aspects of the phase diagram and its associated degrees of freedom which were previously believed to be well understood. With increasing temperature, the chiral crossover is followed by an intermediate region with an approximate chiral spin symmetry larger than chiral symmetry, in which pseudo-scalar mesons continue to exist as hadron-like excitations, before at some higher temperature the expected chiral symmetry is recovered. By testing general formal considerations against lattice data, it can be shown that thermally modified versions of stable vacuum particles, so-called thermoparticles, form the constituents of thermal quantum field theories, with properties quite different from what is expected perturbatively. This ``viewpoint'' aims to raise broader and, in particular, phenomenological interest in these directions.

Figures (5)

• Figure 1: Spatial correlation functions of QCD with $N_f=2$ quarks with physical light-quark mass. Distinct $E_1,E_2,E_3$ multiplets of the approximate $SU(4)$ chiral spin symmetry, are visible at temperatures above the crossover. At large temperatures, these reduce to the multiplets of the ordinary chiral symmetry. From Rohrhofer:2019qwq.
• Figure 2: Screening masses of the lightest $\bar{u}d$-mesons, evaluated using HISQ fermions. From Bazavov:2019www.
• Figure 3: Possible QCD phase diagram with a band of approximate chiral spin symmetry (its extension to $\mu_B/T\mathop{\hbox{$>$$\sim$}} 1$ is not known). A chiral phase transiton line with endpoint may be near its lower boundary. From Glozman:2022lda.
• Figure 4: Left: Pion spectral function reconstructed from the correlators in Rohrhofer:2019qwq. Right: Temporal correlator predicted by a thermoparticle (red) or Breit-Wigner (blue) spectral function, compared to lattice data from Rohrhofer:2019qal. Figures modified from Lowdon:2022xcl.
• Figure 5: Left: Entropy density evaluated on the lattice DallaBrida:2021ddx compared to perturbation theory Kajantie:2002wa, from DallaBrida:2021ddx. Right: Temporal correlator in $\phi^4$-theory of a massive real scalar field, compared to perturbation theory and the thermoparticle contribution extracted from the corresponding spatial correlators. From Lowdon:2022xcl.