QCD phase diagram in a magnetic background for different values of the pion mass

TL;DR

This study uses lattice QCD simulations to analyze how a magnetic background affects the pseudo-critical temperature and magnetic catalysis across different pion masses, revealing that inverse magnetic catalysis diminishes at higher pion masses.

Contribution

It provides the first comprehensive analysis of the magnetic field's impact on the QCD phase diagram across a wide pion mass range, highlighting the secondary nature of inverse magnetic catalysis.

Findings

Decreasing T_c with magnetic field persists across all pion masses studied.

Inverse magnetic catalysis disappears at high pion masses.

The primary effects of magnetic fields relate to gauge field modifications.

Abstract

We investigate the behavior of the pseudo-critical temperature of $N_f = 2+1$ QCD as a function of a static magnetic background field for different values of the pion mass, going up to $m_\pi \simeq 660$ MeV. The study is performed by lattice QCD simulations, adopting a stout staggered discretization of the theory on lattices with $N_t = 6$ slices in the Euclidean temporal direction; for each value of the pion mass the temperature is changed moving along a line of constant physics. We find that the decrease of $T_c$ as a function of $B$, which is observed for physical quark masses, persists in the whole explored mass range, even if the relative variation of $T_c$ appears to be a decreasing function of $m_\pi$, approaching zero in the quenched limit. The location of $T_c$ is based on the renormalized quark condensate and its susceptibility; determinations based on the Polyakov loop lead to compatible results. On the contrary, inverse magnetic catalysis, i.e. the decrease of the quark condensate as a function of $B$ in some temperature range around $T_c$, is not observed when the pion mass is high enough. That supports the idea that inverse magnetic catalysis might be a secondary phenomenon, while the modifications induced by the magnetic background on the gauge field distribution and on the confining properties of the medium could play a primary role in the whole range of pion masses