Experimental review on the chiral magnetic effect in relativistic heavy ion collisions

TL;DR

This review critically assesses two decades of CME searches in relativistic heavy-ion collisions, emphasizing that the Δγ correlator is dominated by flow-related and RP-independent backgrounds rather than a clear CME signal. It surveys observables, background sources, and mitigation techniques, including mixed-harmonic correlators, event-shape engineering, isobar comparisons, and SP/PP analyses, and summarizes experimental results from STAR, ALICE, CMS, and others. Despite intriguing hints, high-precision, background-controlled measurements remain necessary to confirm or exclude a CME at the percent level. The work highlights that next-generation analyses and detector capabilities, particularly in forward regions and high-statistics runs, are essential to disentangle topology-driven QCD effects from conventional background processes and establish the CME’s existence and magnitude.

Abstract

The chiral magnetic effect (CME) refers to a predicted phenomena in quantum chromodynamics that manifests as a charge separation along an external magnetic field, driven by an imbalance of quark chirality. Searches for the CME has been carried out by azimuthal particle correlations in relativistic heavy ion collisions where such a chirality imbalance is anticipated and a strong magnetic field is created in the initial stage. No conclusive experimental evidence on the CME has been established so far because of large background contributions to azimuthal correlation observables. We review the status of the experimental search for the CME, covering the observables used, the techniques to mitigate backgrounds, and the strengths and limitations of various experimental approaches, and outline a future prospect of the CME search in high-energy nuclear collisions.

Figures (11)

• Figure 1: (Color online) Schematic view of the transverse plane in a collision of two heavy nuclei -- one emerging from and one going into the page. The azimuthal angles of the reaction plane and produced particles with charges $\alpha$ and $\beta$ as used in Eqs. (\ref{['eq:gamma']}) and (\ref{['eq:c3']}) are depicted here. Drawing is taken from Ref. STAR:2009wot.
• Figure 2: (Color online) The first measurements of the opposite-sign (OS) and same-sign (SS) $\gamma$ correlators in Au+Au and Cu+Cu collisions at $\sqrt{s_{_{\textsc{nn}}}}=200$ GeV by STAR (left panel) STAR:2009wotSTAR:2009tro and in Pb+Pb collisions at $\sqrt{s_{_{\textsc{nn}}}}=2.76$ TeV by ALICE (right panel) Abelev:2012pa. See Refs. STAR:2009wotSTAR:2009troAbelev:2012pa for more details. Plots are taken from Refs. STAR:2009wotSTAR:2009tro and Ref. Abelev:2012pa, respectively.
• Figure 3: (Color online) The three-point correlators ${\gamma_{\textsc{os}}\xspace}$ and ${\gamma_{\textsc{ss}}\xspace}$ as a function of centrality for Au+Au collisions at $\sqrt{s_{_{\textsc{nn}}}}=7.7$-–62.4 GeV STAR:2014uiw from STAR. Figure is taken from Ref. STAR:2014uiw.
• Figure 4: (Color online) Upper panel: The multiplicity-scaled $\Delta\gamma$ from Blast-Wave model calculations Schlichting:2010qia for realistic LCC effect at freeze-out (red dots) and perfect local charge conservation (blue dots), compared to the STAR data (black dots). For explanation of the dashed curves, presenting separate contributions, see Ref. Schlichting:2010qia. Plot is taken from Ref. Schlichting:2010qia. Lower panel: The BW+LCC description for the ALICE data. Plot is taken from Ref. Wu:2022fwz.
• Figure 5: (Color online) The ${\gamma_{\textsc{os}}\xspace}$ and ${\gamma_{\textsc{ss}}\xspace}$ correlators in small systems compared to those in large systems as functions of multiplicity in $p$+Pb and Pb+Pb collisions at $\sqrt{s_{_{\textsc{nn}}}}=5.02$ TeV by CMS (left panel) CMS:2016wfo, and in $p$+Au, $d$+Au, and Au+Au collisions at $\sqrt{s_{_{\textsc{nn}}}}=200$ GeV by STAR (right panel) STAR:2019xzd. Plots are taken from Ref. CMS:2016wfo and Ref. STAR:2019xzd, respectively.