Electromagnetic fields and anomalous transports in heavy-ion collisions --- A pedagogical review (1509.04073v3)

Abstract: The hot and dense matter generated in heavy-ion collisions may contain domains which are not invariant under P and CP transformations. Moreover, heavy-ion collisions can generate extremely strong magnetic fields as well as electric fields. The interplay between the electromagnetic field and triangle anomaly leads to a number of macroscopic quantum phenomena in these P- and CP-odd domains known as the anomalous transports. The purpose of the article is to give a pedagogical review of various properties of the electromagnetic fields, the anomalous transports phenomena, and their experimental signatures in heavy-ion collisions.

• The paper provides a pedagogical review of how intense electromagnetic fields generate anomalous transport phenomena such as the CME, CSE, and CESE in heavy-ion collisions.
• It details the evolution of ultra-strong magnetic (up to 10^20 Gauss) and electric fields, emphasizing their rapid decay influenced by collision geometry and energy.
• The review bridges theoretical QCD foundations and experimental observables, enhancing our understanding of the quark-gluon plasma’s topological features.

Overview of Electromagnetic Fields and Anomalous Transports in Heavy-Ion Collisions

The paper of heavy-ion collisions offers a unique opportunity to explore the properties of quantum chromodynamics (QCD) under extreme conditions, such as those encountered in the early universe. The article by Xu-Guang Huang provides a comprehensive review focusing on the intriguing interplay between electromagnetic (EM) fields and anomalous transport phenomena in such collisions. It explores how these transport phenomena emerge in P- and CP-odd domains within the quark-gluon plasma (QGP), assessing both theoretical foundations and experimental efforts towards their detection.

Properties of Electromagnetic Fields

Heavy-ion collisions are known to generate exceptionally strong magnetic fields, and to a lesser extent, electric fields. The source of these fields is the fast-moving charged ions, which produce significant electromagnetic radiation. At the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), the magnetic field in non-central collisions can reach up to $10^{20}$ Gauss, an intensity unparalleled in laboratory settings.

The paper emphasizes that the fields are strongest at very early times after the collision and decay rapidly as the spectators move away. The characteristics and evolution of these fields depend significantly on the collision system, energy, and geometry. In particular, the Cu+Au collision system is highlighted for its ability to produce both in-plane electric fields and out-of-plane magnetic fields, due to its asymmetrical nuclear charge distribution.

Anomalous Transport Phenomena

Anomalous transport phenomena in QGP arise due to the coupling of these strong EM fields with the topological features of the QCD vacuum. Key transport phenomena discussed include:

• Chiral Magnetic Effect (CME): This effect involves the generation of an electric current along an external magnetic field in chiral-imbalanced media. The CME results from topological fluctuations in QCD and is fundamentally tied to the axial anomaly. It leads to charge separation in heavy-ion collisions, providing a mechanism for parity violation.
• Chiral Separation Effect (CSE): The CSE describes the induction of an axial current in the presence of a magnetic field, contingent upon a non-zero vector chemical potential. It serves as a dual to the CME and plays a role in the vector and axial current dynamics.
• Chiral Electric Separation Effect (CESE): CESE pertains to the generation of axial currents by electric fields in the presence of both axial and vector charge densities. The paper discusses the potential observables for testing CESE in heavy-ion collisions, particularly in asymmetric systems like Cu+Au.

Experimental Implications

Experimentally, the detection of these effects provides insight into the QCD phase structure and topological properties of the QGP. For instance, charge-dependent azimuthal correlations, such as $\gamma$ and $\delta$ observables, have been proposed and measured at RHIC and LHC to probe the CME. These observables reflect the spatial and momentum-space distribution of charged particles emitted from the collisions. The current research efforts are exploring energy and system size dependence of these signals to distinguish potential CME signals from confounding effects like collective flow and local charge conservation.

Future Outlook

As the understanding of the role of EM fields and topological fluctuations in QCD deepens, experimental techniques and theoretical models continue to evolve, improving the capability to identify and quantify these phenomena. Further exploration of anomalous transport effects in controlled conditions at future colliders or through complementary studies in condensed matter systems like Weyl semimetals could enhance the understanding of these fundamental interactions.

In summary, the review articulates a detailed picture of the state-of-the-art in the paper of EM fields and anomalous transport phenomena in high-energy nuclear physics. It outlines both the opportunities and challenges in deciphering the complex, yet enlightening, anomalies that emerge in the extreme environments produced in heavy-ion collisions.