# Chiral magnetic effect reveals the topology of gauge fields in heavy-ion collisions

## Abstract

The topological structure of vacuum is the cornerstone of non-Abelian gauge theories describing strong and electroweak interactions within the standard model of particle physics. However, transitions between different topological sectors of the vacuum (believed to be at the origin of the baryon asymmetry of the Universe) have never been observed directly. An experimental observation of such transitions in quantum chromodynamics (QCD) has become possible in heavy-ion collisions, where the chiral magnetic effect converts the chiral asymmetry (generated by topological transitions in hot QCD matter) into an electric current, under the presence of the magnetic field produced by the colliding ions. The Relativistic Heavy Ion Collider programme on heavy-ion collisions such as the zirconium–zirconium and ruthenium–ruthenium isobars thus has the potential to uncover the topological structure of vacuum in a laboratory experiment. This discovery would have far-reaching implications for the understanding of QCD, the origin of the baryon asymmetry in the present-day Universe, and other areas, including condensed matter physics.

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## Acknowledgements

This work is partly supported by the US Department of Energy, Office of Nuclear Physics, within the framework of the Beam Energy Scan Theory (BEST) Topical Collaboration. The authors also acknowledge support by the US Department of Energy, Office of Nuclear Physics contracts no. DE-FG-88ER40388 and no. DE-SC0012704 (DK), and by NSF grant no. PHY-1913729 (JL). We thank B. Liao, S. Mukherjee, S. Shi, P. Tribedy, G. Wang and H. Zhang for help.

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Kharzeev, D.E., Liao, J. Chiral magnetic effect reveals the topology of gauge fields in heavy-ion collisions. Nat Rev Phys 3, 55–63 (2021). https://doi.org/10.1038/s42254-020-00254-6

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