Testing the chiral magnetic effect with isobaric collisions

Abstract

The quark-gluon matter produced in relativistic heavy-ion collisions may contain local domains in which parity ($\mathcal{P}$) and combined charge conjugation and parity ($\mathcal{C}\mathcal{P}$) symmetries are not preserved. When coupled with an external magnetic field, such $\mathcal{P}$- and $\mathcal{C}\mathcal{P}$-odd domains will generate electric currents along the magnetic field---a phenomenon called the chiral magnetic effect (CME). Recently, the STAR Collaboration at the BNL Relativistic Heavy Ion Collider (RHIC) and the ALICE Collaboration at the CERN Large Hadron Collider (LHC) released data of charge-dependent azimuthal-angle correlators with features consistent with the CME expectation. However, the experimental observable is contaminated with significant background contributions from elliptic-flow-driven effects, which makes the interpretation of the data ambiguous. We show that the collisions of isobaric nuclei, $_{44}^{96}\mathrm{Ru}+_{44}^{96}\mathrm{Ru}$ and $_{40}^{96}\mathrm{Zr}+_{40}^{96}\mathrm{Zr}$, provide an ideal tool to disentangle the CME signal from the background effects. Our simulation demonstrates that the two collision types at $\sqrt{{s}_{NN}}=200$ GeV have more than $10%$ difference in the CME signal and less than $2%$ difference in the elliptic-flow-driven backgrounds for the centrality range of 20--60%.