For the search of the chiral magnetic effect (CME), STAR previously presentedthe results from isobar collisions(${^{96}_{44}\text{Ru}}+{^{96}_{44}\text{Ru}}$,${^{96}_{40}\text{Zr}}+{^{96}_{40}\text{Zr}}$) obtained through a blindanalysis. The ratio of results in Ru+Ru to Zr+Zr collisions for theCME-sensitive charge-dependent azimuthal correlator ($\Delta\gamma$),normalized by elliptic anisotropy ($v_{2}$), was observed to be close to butsystematically larger than the inverse multiplicity ratio. The backgroundbaseline for the isobar ratio, $Y =\frac{(\Delta\gamma/v_{2})^{\text{Ru}}}{(\Delta\gamma/v_{2})^{\text{Zr}}}$, isnaively expected to be $\frac{(1/N)^{\text{Ru}}}{(1/N)^{\text{Zr}}}$; however,genuine two- and three-particle correlations are expected to alter it. Weestimate the contributions to $Y$ from those correlations, utilizing both theisobar data and HIJING simulations. After including those contributions, wearrive at a final background baseline for $Y$, which is consistent with theisobar data. We extract an upper limit for the CME fraction in the$\Delta\gamma$ measurement of approximately $10\%$ at a $95\%$ confidence levelon in isobar collisions at $\sqrt{s_{\text{NN}}} = 200$ GeV, with an expected$15\%$ difference in their squared magnetic fields.
The chiral magnetic effect (CME) is a phenomenon that arises from the QCDanomaly in the presence of an external magnetic field. The experimental searchfor its evidence has been one of the key goals of the physics program of theRelativistic Heavy-Ion Collider. The STAR collaboration has previouslypresented the results of a blind analysis of isobar collisions(${^{96}_{44}\text{Ru}}+{^{96}_{44}\text{Ru}}$,${^{96}_{40}\text{Zr}}+{^{96}_{40}\text{Zr}}$) in the search for the CME. Theisobar ratio ($Y$) of CME-sensitive observable, charge separation scaled byelliptic anisotropy, is close to but systematically larger than the inversemultiplicity ratio, the naive background baseline. This indicates the potentialexistence of a CME signal and the presence of remaining nonflow background dueto two- and three-particle correlations, which are different between theisobars. In this post-blind analysis, we estimate the contributions from thosenonflow correlations as a background baseline to $Y$, utilizing the isobar dataas well as Heavy Ion Jet Interaction Generator simulations. This baseline isfound consistent with the isobar ratio measurement, and an upper limit of 10%at 95% confidence level is extracted for the CME fraction in the chargeseparation measurement in isobar collisions at $\sqrt{s_{\rm NN}}=200$ GeV.