Hydrodynamic Excitations from Chiral Kinetic Theory and the Hydrodynamic Frames

TL;DR

This paper explores how Lorentz invariance affects hydrodynamic modes in chiral kinetic theory, revealing a boost transformation linking different hydrodynamic frames and reproducing known holographic results.

Contribution

It introduces a modified momentum current in chiral kinetic theory, derives new hydrodynamic modes, and connects these to the Landau-Lifshitz frame via a magnetic-field-dependent boost.

Findings

Hydrodynamic modes differ from traditional Landau-Lifshitz modes.

A boost transformation relates modes in different frames.

Reproduction of the holographic chiral drag force without holography.

Abstract

In the framework of chiral kinetic theory (CKT), we consider a system of right- and left-handed Weyl fermions out of thermal equilibrium in a homogeneous weak magnetic field. We show that the Lorentz invariance implies a modification in the definition of the momentum current in the phase space, compared to the case in which the system is in global equilibrium. Using this modified momentum current, we derive the linearized conservation equations from the kinetic equation up to second order in the derivative expansion. It turns out that the eigenmodes of these equations, namely the hydrodynamic modes, differ from those obtained from the hydrodynamic in the Landau-Lifshitz (LL) frame at the same order. We show that the modes of the former case may be transformed to the corresponding modes in the latter case by a global boost. The velocity of the boost is proportional to the magnetic field as well as the difference between the right- and left-handed charges susceptibility. We then compute the chiral transport coefficients in a system of non-Abelian chiral fermions in the no-drag frame and by making the above boost, obtain the well-known transport coeffiecients of the system in the LL frame. Finally by using the idea of boost, we reproduce the AdS/CFT result for the chiral drag force exerted on a quark at rest in the rest frame of the fluid, without performing any holographic computations.