The QCD phase diagram for external magnetic fields

TL;DR

This study investigates how external magnetic fields influence the finite temperature transition in QCD, finding that the transition temperature decreases with increasing magnetic field strength, contrary to some model predictions.

Contribution

The paper provides the first lattice QCD results showing the magnetic field dependence of the transition temperature using physical quark masses and continuum extrapolation.

Findings

Transition temperature decreases with magnetic field

Crossover persists up to high magnetic fields

Transition strength increases mildly with field

Abstract

The effect of an external (electro)magnetic field on the finite temperature transition of QCD is studied. We generate configurations at various values of the quantized magnetic flux with $N_f=2+1$ flavors of stout smeared staggered quarks, with physical masses. Thermodynamic observables including the chiral condensate and susceptibility, and the strange quark number susceptibility are measured as functions of the field strength. We perform the renormalization of the studied observables and extrapolate the results to the continuum limit using $N_t=6,8$ and 10 lattices. We also check for finite volume effects using various lattice volumes. We find from all of our observables that the transition temperature $T_c$ significantly decreases with increasing magnetic field. This is in conflict with various model calculations that predict an increasing $T_c(B)$. From a finite volume scaling analysis we find that the analytic crossover that is present at B=0 persists up to our largest magnetic fields $eB \approx 1 \textmd{GeV}^2$, and that the transition strength increases mildly up to this $eB\approx1 \textmd{GeV}^2$.