For the search of the chiral magnetic effect (CME), STAR previously presented the results from isobar collisions (${}_{44}{}^{96}\mathrm{Ru}+{}_{44}{}^{96}\mathrm{Ru}, {}_{40}{}^{96}\mathrm{Zr}+{}_{40}{}^{96}\mathrm{Zr}$) obtained through a blind analysis. The ratio of results in $\mathrm{Ru}+\mathrm{Ru}$ to $\mathrm{Zr}+\mathrm{Zr}$ collisions for the CME-sensitive charge-dependent azimuthal correlator ($\mathrm{<ruby><rb>\Delta </rb><rt>でるた</rt></ruby>}\mathrm{<ruby><rb>\gamma </rb><rt>がんま</rt></ruby>}$), 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, $Y=\frac{{(\mathrm{<ruby><rb>\Delta </rb><rt>でるた</rt></ruby>}\mathrm{<ruby><rb>\gamma </rb><rt>がんま</rt></ruby>}/v_2)}^{\text{Ru}}}{{(\mathrm{<ruby><rb>\Delta </rb><rt>でるた</rt></ruby>}\mathrm{<ruby><rb>\gamma </rb><rt>がんま</rt></ruby>}/v_2)}^{\text{Zr}}}$, is naively expected to be $\frac{{(1/N)}^{\mathrm{Ru}}}{{(1/N)}^{\mathrm{Zr}}}$; 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 $\mathrm{<ruby><rb>\Delta </rb><rt>でるた</rt></ruby>}\mathrm{<ruby><rb>\gamma </rb><rt>がんま</rt></ruby>}$ measurement of approximately $10\%$ at a $95\%$ confidence level on in isobar collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$, with an expected 15% difference in their squared magnetic fields.

• Received 24 October 2023
• Accepted 25 March 2024

DOI:https://doi.org/10.1103/PhysRevC.110.014905