Theoretical Motivation for the Chiral Magnetic Effect in Heavy-Ion Collisions

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 (CME) 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 program isobar run thus has the potential to uncover the topological structure of vacuum in a laboratory experiment. The study of CME in isobar collisions has to take into account the difference in the baseline in ZrZr and RuRu collisions, including the observed difference in hadron multiplicities. The observation of CME in heavy ion collisions 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.