Does the chiral magnetic effect occur in heavy-ion collisions?
In plain words
In the strong magnetic field of a heavy-ion collision, regions with unequal numbers of left- and right-handed quarks should push positive and negative charges in opposite directions along the field, an effect called the chiral magnetic effect. Measured charge-separation signals can so far be explained by ordinary background effects.
Precise statement
The effect predicts charge separation along the magnetic field, measured by the correlator $\Delta \gamma = \gamma_{\mathrm{OS}} - \gamma_{\mathrm{SS}}$. The STAR blind analysis of Ru+Ru versus Zr+Zr at $\sqrt{s_{NN}} = 200\,\mathrm{GeV}$ found no signature meeting predefined criteria (arXiv 2109.00131), and a later analysis set an upper limit of 10 percent at 95 percent confidence on the effect's fraction of the measured correlator (Phys. Rev. Research 6, L032005, 2024). Answer: a nonzero fraction at 5 standard deviations, or an upper limit below the range predicted by anomalous-hydrodynamics models with realistic magnetic-field lifetimes.
What would settle it
Charge-separation measurements relative to spectator and participant planes in Au+Au and isobar collisions with backgrounds fixed by event-shape selection, compared with anomalous-hydrodynamics predictions.
Status in the literature
Unverified note
The 2024 STAR upper limit of 10 percent left room for a small contribution; no established signal existed as of 2026.