Momentum transport in strongly coupled anisotropic plasmas in the presence of strong magnetic fields

TL;DR

This paper uses holographic models to analyze momentum transport and shear viscosity in strongly coupled anisotropic plasmas under strong magnetic fields, revealing anisotropic effects on energy loss and fluid behavior.

Contribution

It provides the first detailed holographic calculations of anisotropic shear viscosities and momentum diffusion coefficients in magnetic fields, including a phenomenologically relevant Einstein-Maxwell-dilaton model.

Findings

Magnetic fields enhance energy loss and momentum diffusion in transverse directions.

Shear viscosity is smaller along the magnetic field, indicating closer to perfect fluid behavior.

The EMD model predicts entropy density and critical temperature in unexplored phase diagram regions.

Abstract

We present a holographic perspective on momentum transport in strongly coupled, anisotropic non-Abelian plasmas in the presence of strong magnetic fields. We compute the anisotropic heavy quark drag forces and Langevin diffusion coefficients and also the anisotropic shear viscosities for two different holographic models, namely, a top-down deformation of strongly coupled $\mathcal{N} = 4$ Super-Yang-Mills (SYM) theory triggered by an external Abelian magnetic field, and a bottom-up Einstein-Maxwell-dilaton (EMD) model which is able to provide a quantitative description of lattice QCD thermodynamics with $(2+1)$-flavors at both zero and nonzero magnetic fields. We find that, in general, energy loss and momentum diffusion through strongly coupled anisotropic plasmas are enhanced by a magnetic field being larger in transverse directions than in the direction parallel to the magnetic field. Moreover, the anisotropic shear viscosity coefficient is smaller in the direction of the magnetic field than in the plane perpendicular to the field, which indicates that strongly coupled anisotropic plasmas become closer to the perfect fluid limit along the magnetic field. We also present, in the context of the EMD model, holographic predictions for the entropy density and the crossover critical temperature in a wider region of the $(T,B)$ phase diagram that has not yet been covered by lattice simulations. Our results for the transport coefficients in the phenomenologically realistic magnetic EMD model could be readily used as inputs in numerical codes for magnetohydrodynamics.