Anomalies and the chiral magnetic effect in the Sakai-Sugimoto model

TL;DR

This paper investigates the chiral magnetic effect within the Sakai-Sugimoto model, clarifying anomaly implementations and demonstrating conditions under which the effect appears or vanishes, with implications for QCD and astrophysics.

Contribution

It provides a detailed analysis of the QED axial anomaly in the Sakai-Sugimoto model and explores the conditions for the chiral magnetic effect to manifest or be suppressed.

Findings

Correctly implements the consistent anomaly in the model.

Finds the chiral magnetic effect vanishes with an axial chemical potential without a counterterm.

Obtains a nontrivial axial current for large magnetic fields with a quark chemical potential.

Abstract

In the chiral magnetic effect an imbalance in the number of left- and right-handed quarks gives rise to an electromagnetic current parallel to the magnetic field produced in noncentral heavy-ion collisions. The chiral imbalance may be induced by topologically nontrivial gluon configurations via the QCD axial anomaly, while the resulting electromagnetic current itself is a consequence of the QED anomaly. In the Sakai-Sugimoto model, which in a certain limit is dual to large-N_c QCD, we discuss the proper implementation of the QED axial anomaly, the (ambiguous) definition of chiral currents, and the calculation of the chiral magnetic effect. We show that this model correctly contains the so-called consistent anomaly, but requires the introduction of a (holographic) finite counterterm to yield the correct covariant anomaly. Introducing net chirality through an axial chemical potential, we find a nonvanishing vector current only before including this counterterm. This seems to imply the absence of the chiral magnetic effect in this model. On the other hand, for a conventional quark chemical potential and large magnetic field, which is of interest in the physics of compact stars, we obtain a nontrivial result for the axial current that is in agreement with previous calculations and known exact results for QCD.