[Paper Review] Chiral Magnetic Effect in High-Energy Nuclear Collisions — A Status Report

Dmitri E. Kharzeev, Jinfeng Liao+2 more|arXiv (Cornell University)|Nov 12, 2015

High-Energy Particle Collisions Research12 citations

TL;DR

This review explores the Chiral Magnetic Effect (CME) in high-energy nuclear collisions, where quantum anomalies generate a topologically protected electric current along an external magnetic field due to chirality imbalance in the quark-gluon plasma. Based on experimental data from STAR and ALICE collaborations, the paper evaluates current evidence, identifies key uncertainties, and outlines future theoretical and experimental directions to confirm the CME as a genuine quantum phenomenon in strong interactions.

ABSTRACT

The interplay of quantum anomalies with magnetic field and vorticity results in a variety of novel non-dissipative transport phenomena in systems with chiral fermions, including the quarkgluon plasma. Among them is the Chiral Magnetic Effect (CME) – the generation of electric current along an external magnetic field induced by chirality imbalance. Because the chirality imbalance is related to the global topology of gauge fields, the CME current is topologically protected and hence non-dissipative even in the presence of strong interactions. As a result, the CME and related quantum phenomena affect the hydrodynamical and transport behavior of strongly coupled quark-gluon plasma, and can be studied in relativistic heavy ion collisions where strong magnetic fields are created by the colliding ions. Evidence for the CME and related phenomena has been reported by the STAR Collaboration at Relativistic Heavy Ion Collider at BNL, and by the ALICE Collaboration at the Large Hadron Collider at CERN. The goal of the present review is to provide an elementary introduction into the physics of anomalous chiral effects, to describe the current status of experimental studies in heavy ion physics, and to outline the future work, both in experiment and theory, needed to eliminate the existing uncertainties in the interpretation of the data. 1 ar X iv :1 51 1. 04 05 0v 1 [ he pph ] 1 2 N ov 2 01 5

Motivation & Objective

• To provide a foundational understanding of anomalous chiral transport phenomena, particularly the Chiral Magnetic Effect (CME), in systems with chiral fermions.
• To assess the current experimental status of CME detection in relativistic heavy-ion collisions at RHIC and the LHC.
• To identify and clarify the major uncertainties in interpreting experimental data as evidence for the CME.
• To outline the theoretical and experimental challenges that must be overcome to confirm the CME as a non-dissipative, topologically protected effect.
• To guide future research by identifying key open questions in both theory and experiment for validating the CME in quark-gluon plasma.

Proposed method

• Theoretical analysis of quantum anomalies in chiral gauge theories, focusing on the interplay between chirality imbalance, magnetic fields, and vorticity.
• Application of topological field theory to derive the CME current, which is non-dissipative due to its topological protection from global gauge field configurations.
• Review of experimental observables sensitive to the CME, such as charge-dependent azimuthal correlations in heavy-ion collisions.
• Comparison of predictions from anomalous hydrodynamics with data from the STAR and ALICE collaborations at RHIC and the LHC.
• Use of effective field theory and transport models to simulate CME signals and disentangle them from background effects like non-flow correlations.
• Evaluation of the robustness of observed correlations under various assumptions about initial conditions and medium response.

Experimental results

Research questions

• RQ1 To what extent do observed charge-dependent correlations in heavy-ion collisions provide unambiguous evidence for the Chiral Magnetic Effect?
• RQ2 How do non-dissipative, topologically protected currents like the CME manifest in the hydrodynamic and transport behavior of the quark-gluon plasma?
• RQ3 What are the dominant background contributions that could mimic CME-like signals, and how can they be distinguished experimentally?
• RQ4 How do magnetic fields and vorticity in heavy-ion collisions influence the generation and detection of chiral transport effects?
• RQ5 What theoretical and experimental improvements are necessary to conclusively confirm the existence of the CME in the quark-gluon plasma?

Key findings

• Experimental data from the STAR and ALICE collaborations show charge-dependent azimuthal correlations consistent with CME predictions, though with significant uncertainties.
• The CME current is topologically protected and non-dissipative, even in the presence of strong interactions, due to its origin in global gauge field topology.
• Theoretical models predict that the CME should generate a net electric current along the direction of the external magnetic field when a chirality imbalance is present.
• The observed correlations in heavy-ion collisions are sensitive to initial conditions, medium response, and non-equilibrium effects, complicating unambiguous identification of the CME.
• Current data do not yet rule out alternative explanations such as non-flow correlations or local charge separation effects.
• Future experiments with improved detector resolution, better background subtraction techniques, and more precise theoretical modeling are essential to confirm the CME.