Aspects of holographic Langevin diffusion in the presence of anisotropic magnetic field

TL;DR

This paper investigates how anisotropic magnetic fields influence the Langevin diffusion of heavy quarks in strongly coupled plasmas, revealing velocity-dependent directional effects and violations of universal relations.

Contribution

It provides a detailed analysis of anisotropic effects on Langevin diffusion coefficients, highlighting velocity and magnetic field influences and identifying violations of universal relations.

Findings

Transverse diffusion depends more on motion direction at ultra-fast speeds.

Perpendicular motion affects longitudinal coefficients more at any fixed velocity.

Universal relation $fect eq fect$ is broken in anisotropic backgrounds.

Abstract

We study the holographic Langevin diffusion coefficients of a heavy quark, when traveling through a strongly coupled anisotropic plasma in the presence of magnetic field~$\mathcal{B}$. In particular, we clarified how motion velocities shape momentum broadening and its directional dependence. It is observed that the transverse Langevin diffusion coefficients depend more on the direction of motion rather than the directions of momentum diffusion at the ultra-fast limit, while an opposite conclusion is found when the moving speed is sufficiently low. For the Longitudinal Langevin diffusion coefficient, we find that motion perpendicular to~$\mathcal{B}$ affects the Langevin coefficients stronger at any fixed velocity. We should also emphasize that all five Langevin coefficients are becoming larger with increasing velocity. We find that the universal relation~$\kappa^{\parallel}>\kappa^{\perp}$ in the isotropic background, is broken in a different new case that a quark moving paralleled to~$\mathcal{B}$. This is one more particular example where the violation of the universal relation occurs for the anisotropic background. Further, we find the critical velocity of the violation will become larger with increasing~$\mathcal{B}$.