Hot QCD matter around the chiral crossover: a lattice study with $O(a)$-improved Wilson fermions

TL;DR

This lattice QCD study investigates the properties of hot QCD matter around the chiral crossover, revealing how pion quasiparticles and spectral functions evolve with temperature, and providing insights into chiral symmetry restoration.

Contribution

The paper presents the first lattice QCD analysis of thermal modifications of vector and axial-vector spectral functions near the chiral crossover using $O(a)$-improved Wilson fermions with physical quark masses.

Findings

Pion quasiparticle becomes lighter with increasing temperature.

Enhancement of low-energy spectral functions at finite temperature.

Suppression of the chiral order parameter near the crossover.

Abstract

Using lattice QCD simulations with $O(a)$-improved Wilson quarks and physical up, down and strange quark masses, we investigate the properties of thermal QCD matter at the temperatures $T=\{128,154,192\}\mathrm{MeV}$ with a fixed lattice spacing $a=0.064\mathrm{fm}$ and volume $V=(6.1\text{fm})^3$. We find that the pion quasiparticle, defined as the low-energy pole in the two-point function of the axial charge, becomes lighter as the temperature increases and give an argument based on hydrodynamics as to why the pole becomes purely diffusive above the chiral crossover. We study the thermal modification of the isovector vector spectral function using the Backus-Gilbert method, finding an enhancement at low energies and a depletion at energies around $1\mathrm{GeV}$.The analogous study of the axial-vector channel reveals a larger enhancement at energies below $1\mathrm{GeV}$, and we show that these findings are consistent with rigorous spectral sum rules. The difference between vector and axial-vector correlators, an order parameter for chiral symmetry, turns out to be overall suppressed by more than an order of magnitude at the crossover.