Inverse magnetic catalysis and the Polyakov loop

Abstract

We study the physical mechanism of how an external magnetic field influences the QCD quark condensate. Two competing mechanisms are identified, both relying on the interaction between the magnetic field and the low quark modes. While the coupling to valence quarks enhances the condensate, the interaction with sea quarks suppresses it in the transition region. The latter `sea effect' acts by ordering the Polyakov loop and, thereby, reduces the number of small Dirac eigenmodes and the condensate. It is most effective around the transition temperature, where the Polyakov loop effective potential is flat and a small correction to it by the magnetic field can have a significant effect. Around the critical temperature, the sea suppression overwhelms the valence enhancement, resulting in a net suppression of the condensate, named inverse magnetic catalysis. We support this physical picture by lattice simulations including continuum extrapolated results on the Polyakov loop as a function of temperature and magnetic field. We argue that taking into account the increase in the Polyakov loop and its interaction with the low-lying modes is essential to obtain the full physical picture, and should be incorporated in effective models for the description of QCD in magnetic fields in the transition region.Comment: 19 pages, 8 figures, new reference added, version accepted for publicatio

Screening masses in strong external magnetic fields

“…As regards the effects more directly related to color interactions, various studies have considered the possible influence of an external magnetic field on the static quark-antiquark potential [14][15][16][17][18][19], which has been clarified by recent lattice results [20,21], and might have consequences relevant to the spectrum of heavy quark bound states [22][23][24][25][26][27][28][29][30][31][32]. At zero temperature, the potential becomes anisotropic and the string tension σ is larger (smaller) in the direction orthogonal (parallel) to the magnetic field B [20,21]; at finite T , in particular in the region right below the pseudocritical temperature T c , the magnetic field induces a general suppression of σ [21], leading to an early onset of deconfinement, in agreement with the observed dependence of T c on B [33][34][35] In this paper we extend the study to the region of temperatures above T c , in order to investigate the effects of a magnetic background on the interactions between heavy quarks in the Quark-Gluon Plasma. In this phase, the effective interaction is no longer confining and can instead be described by a screened Coulomb form, with two different screening lengths/masses characterizing the (color)electric and the (color)magnetic sectors.…”

Chiral and deconfinement transitions in a magnetic background using the functional renormalization group with the Polyakov loop

“…In the recent papers [6,24], the authors suggest a resolution of the discrepancy between the model calculations and the lattice simulations. The chiral condensate can be written as …”

“…[10][11][12][13][14][15][16][17][18][19][20]. Possible resolutions to the disagreement have been suggested [21][22][23][24][25][26][27][28] and we will discuss these at the end of the paper.…”

Phase diagram for charged scalars in a magnetic field at finite temperature

“…Latter results, obtained by considering smaller lattice spacing and physical quark masses, found an opposite behavior [23,24]. The most recent results, show that such a decrease should be associated to a back reaction of the Polyakov loop, which indirectly feels the magnetic field and drives down the critical temperature for the chiral transition [25].…”