Revisit the Chiral Magnetic Effect Expectation in Isobaric Collisions at the Relativistic Heavy Ion Collider

TL;DR

This study reevaluates the detectability of the chiral magnetic effect in isobaric collisions at RHIC, suggesting that current data may be insufficient to observe the CME due to background and signal scaling issues, with large theoretical uncertainties.

Contribution

The paper provides a revised expectation for CME detection in isobar collisions, incorporating background and signal scaling, and highlights the limitations of current experimental sensitivity.

Findings

CME may not be detectable with current isobar data within 2σ significance.

Background in isobar collisions is roughly twice that in Au+Au collisions.

CME signal is approximately half due to weaker magnetic fields.

Abstract

Isobaric $^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr collisions at $\sqrt{s_{_{NN}}}=200$ GeV have been conducted at the Relativistic Heavy Ion Collider to circumvent the large flow-induced background in searching for the chiral magnetic effect (CME), predicted by the topological feature of quantum chromodynamics (QCD). Considering that the background in isobar collisions is approximately twice that in Au+Au collisions (due to the smaller multiplicity) and the CME signal is approximately half (due to the weaker magnetic field), we caution that the CME may not be detectable with the collected isobar data statistics, within $\sim$2$\sigma$ significance, if the axial charge per entropy density ($n_5/s$) and the QCD vacuum transition probability are system independent. This expectation is generally verified by the Anomalous-Viscous Fluid Dynamics (AVFD) model. While our estimate provides an approximate "experimental" baseline, theoretical uncertainties on the CME remain large.