Chiral Magnetic and Vortical Effects in High-Energy Nuclear Collisions --- A Status Report

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 quark-gluon 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.

• The paper demonstrates that quantum anomalies induce the chiral magnetic effect, driving non-dissipative currents in the quark-gluon plasma.
• It analyzes experimental results from RHIC and LHC that support CME predictions while addressing challenges in isolating the effect.
• The review links CME and CVE to advances in hydrodynamic modeling, emphasizing their role in probing QCD's topological features.

Exploration of Chiral Magnetic and Vortical Effects in High-Energy Nuclear Collisions

The document presented offers a comprehensive review of recent advancements related to the chiral magnetic effect (CME) and related phenomena occurring in high-energy nuclear collisions. The primary focus is on the interplay between quantum anomalies, external magnetic fields, and vorticity which gives rise to unique non-dissipative transport phenomena in systems that include chiral fermions, such as the quark-gluon plasma (QGP). Key contributions of this review include a detailed exploration of CME's impact on the hydrodynamics and transport properties of QGP, experimental evidences, alongside the current state of relevant theoretical endeavors.

The chiral magnetic effect arises in environments exhibiting chirality imbalance, inducing an electric current along an external magnetic field. The CME current is inherently tied to the topologically protected nature of gauge fields in quantum chromodynamics (QCD), leading to its non-dissipative behavior even amidst strong interactions. This property renders the CME of significant interest for probing QCD's topological features, which remain largely elusive.

The review further elaborates on the enhancement of CME observation in high-energy nuclear collisions—specifically in scenarios where large magnetic fields are generated due to colliding ions at relativistic speeds. Through experimental setups, such as those conducted by the STAR Collaboration at RHIC and ALICE at LHC, initial evidences aligned with the CME predictions have been recorded. Importantly, these studies underline the necessity of continuous examination and improved theoretical models to resolve persisting uncertainties embroiling experimental interpretations and underlying intrinsic QCD phenomena.

Another critical section of the review is the study of related anomalous effects, such as the chiral vortical effect (CVE), which arises in rotating QGP due to couplings between vorticity and spin. Chiral separating effects (CSE), which involve axial currents induced by a magnetic field in conjunction with vector chemical potentials, are also scrutinized for their similar macroscopic quantum manifestations and tangible experimental indicators.

The implications of these investigations span over a fascinating array of theoretical and practical domains. For theoretical physics, they herald a profound understanding of symmetry violations and transport coefficients arising within strongly interacting gauge theories. Practically, they hold the promise for new methodologies to probe the QGP state, an essential pursuit to comprehend the universe's evolution within microseconds post-big bang.

Looking to the future, projected endeavors encapsulate fine-tuning experimental designs for clearer isolation of CME signals from background noise. Enhancement of chiral magnetohydrodynamic models alongside event simulations could significantly bolster the understanding of the magnetic and vortical contributions in collision environments. Also, exploring the parameter space in colliders through varying beam energies and collision types—such as isotope or isobaric nuclei—offers promising avenues to substantiate the CME's signature in heavy-ion physics.

This review serves as an essential milestone in consolidating the understanding of anomalous chiral effects, and future advancements in this field are anticipated to reshape our comprehension of QCD and other gauge theories systematically. The pathways identified and explored here shall indubitably cast long shadows on the scientific landscape, where understanding of quantum anomalies evolves conterminously with experimental technology in high-energy physics.