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

TL;DR

This work analyzes the chiral magnetic effect within the strongly coupled Sakai-Sugimoto model, clarifying how to implement the QED axial anomaly and define chiral currents through consistent and covariant anomalies. It shows that Bardeen's finite counterterm is required to obtain the covariant anomaly, which in equilibrium drives the vector CME current to zero, while the axial current matches topological QCD expectations in the chirally symmetric phase and is reduced but nonvanishing in the broken phase. The results highlight a tension between consistent-anomaly predictions (nonzero CME) and covariant-anomaly predictions (no CME) in holographic QCD, and they compare favorably with lattice and weak-coupling insights in certain limits. The findings imply that, at strong coupling, the CME may be absent, raising questions about Landau-level pictures and the role of boundary terms and non-equilibrium dynamics in realistic heavy-ion contexts.

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.