Estimate of nonflow baseline for the chiral magnetic effect in isobar collisions at STAR

Recently, STAR reported the isobar (⁹⁶Ru+⁹⁶Ru, ⁹⁶Zr+⁹⁶Zr) results for the chiral magnetic effect (CME) search [1]. The Ru+Ru to Zr+Zr ratio of the CME-sensitive observable Δγ, normalized by elliptic anisotropy (v₂), is observed to be close to the inverse multiplicity (N) ratio. In other words, the ratio of the NΔγ/v₂ observable is close to the naive background baseline of unity. However, nonflow correlations are expected to cause the baseline to deviate from unity. To further understand the isobar results, we study nonflow effects using the isobar data by studying two-particle correlations as functions of pseudorapidity and azimuthal angle differences (Δη, Δφ) of the pairs. We fit this 2D distribution of same-sign pairs and attempt to extract the "true" v₂, whose difference from the measured v₂ is estimated as the nonflow contribution to the latter. We decompose the nonflow contributions to NΔγ/v₂ (isobar ratio) into three terms [2] and quantify each term by using the nonflow in v₂ measurement, published STAR data [1] and HIJNG simulations. From these estimates, we arrive at a nonflow baseline of the isobar ratio of NΔγ/v₂ for the CME. We report this nonflow baseline and discuss its implications.

[1] M. Abdallah et al. [STAR], Phys. Rev. C 105, 014901 (2022)

[2] Y. Feng, J. Zhao, H. Li, H. j. Xu and F. Wang, Phys. Rev. C 105, 024913 (2022)