On the interplay between magnetic field and anisotropy in holographic QCD

TL;DR

This paper explores how anisotropy and magnetic fields influence strongly interacting gauge theories, revealing complex phase structures, chiral transition behaviors, and diverse observable responses in holographic QCD models.

Contribution

It provides a detailed holographic analysis of the interplay between anisotropy and magnetic fields, uncovering new phase geometries and their effects on physical observables.

Findings

Intermediate geometries depend on dominant effects of anisotropy or magnetic field.

Chiral transition temperature varies with anisotropy and magnetic field, showing inverse catalysis.

Butterfly velocity exhibits complex behavior, surpassing conformal limits in some regimes.

Abstract

We investigate the combined effects of anisotropy and a magnetic field in strongly interacting gauge theories by the gauge/gravity correspondence. Our main motivation is the quark-gluon plasma produced in off-central heavy-ion collisions which exhibits large anisotropy in pressure gradients as well as large external magnetic fields. We explore two different configurations, with the anisotropy either parallel or perpendicular to the magnetic field, focusing on the competition and interplay between the two. A detailed study of the RG flow in the ground state reveals a rich structure where depending on which of the two, anisotropy or magnetic field, is stronger, intermediate geometries with approximate AdS$_4\times \mathbb{R}$ and AdS$_3\times \mathbb{R}^2$ factors arise. This competition is also manifest in the phase structure at finite temperature, specifically in the dependence of the chiral transition temperature on anisotropy and magnetic field, from which we infer the presence of inverse magnetic and anisotropic catalyses of the chiral condensate. Finally, we consider other salient observables in the theory, including the quark-antiquark potential, shear viscosity, entanglement entropy and the butterfly velocity. We demonstrate that they serve as good probes of the theory, in particular, distinguishing between the effects of the magnetic field and anisotropy in the ground and plasma states. We also find that the butterfly velocity, which codifies how fast information propagates in the plasma, exhibits a rich structure as a function of temperature, anisotropy and magnetic field, exceeding the conformal value in certain regimes.