Estimate of Background Baseline and Upper Limit on the Chiral Magnetic Effect in Isobar Collisions at $\sqrt{s_{\text{NN}}}=200$ GeV at the Relativistic Heavy-Ion Collider

Abstract

For the search of the chiral magnetic effect (CME), STAR previously presented the results from isobar collisions (${}_{44}^{96}\text{Ru}+{}_{44}^{96}\text{Ru}$, ${}_{40}^{96}\text{Zr}+{}_{40}^{96}\text{Zr}$) obtained through a blind analysis. The ratio of results in Ru+Ru to Zr+Zr collisions for the CME-sensitive charge-dependent azimuthal correlator ($\hat{I}”{\hat{I}}^3$), normalized by elliptic anisotropy ($v_2$), was observed to be close to but systematically larger than the inverse multiplicity ratio. The background baseline for the isobar ratio, $Y=\frac{(\hat{I}”{\hat{I}}^3/v_2{)}^{\text{Ru}}}{(\hat{I}”{\hat{I}}^3/v_2{)}^{\text{Zr}}}$, is naively expected to be $\frac{(1/N{)}^{\text{Ru}}}{(1/N{)}^{\text{Zr}}}$; however, genuine two- and three-particle correlations are expected to alter it. We estimate the contributions to $Y$ from those correlations, utilizing both the isobar data and HIJING simulations. After including those contributions, we arrive at a final background baseline for $Y$, which is consistent with the isobar data. We extract an upper limit for the CME fraction in the $\hat{I}”{\hat{I}}^3$ measurement of approximately $10\%$ at a $95\%$ confidence level on in isobar collisions at $\sqrt{s_{\text{NN}}}=200$ GeV, with an expected $15\%$ difference in their squared magnetic fields.

19 pages, 14 figures