Phase diagram of QCD in a magnetic background

Massimo D’Elia,Francesco Sanfilippo,Alfredo Stanzione,Lorenzo Maio

doi:10.1103/physrevd.105.034511

Abstract

We provide numerical evidence that the thermal QCD crossover turns into a first order transition in the presence of large enough magnetic background fields. The critical endpoint is found to be located between $eB = 4$ GeV$^2$ (where the pseudocritical temperature is $T_c = (98 \pm 3)$ MeV) and $eB = 9$ GeV$^2$ (where the critical temperature is $T_c = (63 \pm 5)$ MeV). Results are based on the analysis of quark condensates and number susceptibilities, determined by lattice simulations of $N_f = 2+1$ QCD at the physical point, discretized with three different lattice spacings, $a = 0.114, 0.086$ and $0.057$ fm, via rooted stout staggered fermions and a Symanzik tree level improved pure gauge action. We also present preliminary results regarding the confining properties of the thermal theory, suggesting that they could change drastically going across the phase transition

Highlights

The investigation of QCD properties in a magnetic background field has been the subject of various studies in the last few years, see, e.g., Refs. [1,2,3] for recent reviews
Part of the interest is directly related to phenomenology: strong background fields are expected in noncentral heavy ion collisions [4,5,6,7,8,9], in astrophysical objects like magnetars [10], and might have been produced during the cosmological electroweak phase transition [11,12], influencing the subsequent evolution of the Universe, including the cosmological QCD transition
One of the most relevant aspects regards the influence of the magnetic field on the QCD phase diagram

Summary

Introduction

The investigation of QCD properties in a magnetic background field has been the subject of various studies in the last few years, see, e.g., Refs. [1,2,3] for recent reviews. The investigation of QCD properties in a magnetic background field has been the subject of various studies in the last few years, see, e.g., Refs. Lattice studies of Nf 1⁄4 2 QCD, adopting standard staggered fermions and heavier-than-physical quark masses, showed a slightly increasing behavior of the crossover temperature Tc as a function of the magnetic field B [13,14]. That was not confirmed by an investigation of Nf 1⁄4 2 þ 1 QCD at the physical point discretized via improved staggered fermion, showing instead an appreciable decrease of Tc, of the order of 10–20%, for magnetic fields going up to eB ∼ 1 GeV2 [15], a behavior confirmed by later lattice studies [16]. The reason for the discrepancy of early results has been clarified by later studies: it should be ascribed to lattice artifacts [17], while the decreasing behavior of Tc as a function of B is observed for larger than physical pion masses [18,19]

Objectives

Methods

Results

Conclusion