Finite-density QCD transition in magnetic field background

TL;DR

This study uses lattice QCD simulations to explore how magnetic fields influence chiral and deconfinement transitions at finite temperature and density, revealing complex behaviors and estimating the critical endpoint in the phase diagram.

Contribution

It provides new insights into the effects of magnetic fields on QCD phase transitions and identifies the magnetic field strength where chiral and deconfinement crossovers merge.

Findings

Magnetic field weakly affects the curvature of the chiral transition temperature.

The chiral width narrows with increasing density at weak magnetic fields.

The chiral and deconfining crossovers merge at a magnetic field of approximately 0.6 GeV².

Abstract

Using numerical simulations of lattice QCD with physical quark masses, we reveal the influence of magnetic-field background on chiral and deconfinement crossovers in finite-temperature QCD at low baryonic density. In the absence of thermodynamic singularity, we identify these transitions with inflection points of the approximate order parameters: normalized light-quark condensate and renormalized Polyakov loop, respectively. We show that the quadratic curvature of the chiral transition temperature in the ``temperature--chemical potential'' plane depends rather weakly on the strength of the background magnetic field. At weak magnetic fields, the thermal width of the chiral crossover gets narrower as the density of the baryon matter increases, possibly indicating a proximity to a real thermodynamic phase transition. Remarkably, the curvature of the chiral thermal width flips its sign at $eB_{\mathrm{fl}} \simeq 0.6\,\mathrm{GeV}^2$, so that above the flipping point $B > B_{\mathrm{fl}}$, the chiral width gets wider as the baryon density increases. Approximately at the same strength of magnetic field, the chiral and deconfining crossovers merge together at $T \approx 140\,\mathrm{MeV}$. The phase diagram in the parameter space ``temperature-chemical potential-magnetic field'' is outlined, and single-quark entropy and single-quark magnetization are explored. The curvature of the chiral thermal width allows us to estimate an approximate position of the chiral critical endpoint at zero magnetic field: $(T_c^{\text{CEP}}, \mu_B^{\text{CEP}})= (100(25)\, \text{MeV},\ 800(140)\,\text{MeV})$.