  The chiral magnetic effect (CME) is a phenomenon that arises from the QCD
anomaly in the presence of an external magnetic field. The experimental search
for its evidence has been one of the key goals of the physics program of the
Relativistic Heavy-Ion Collider. The STAR collaboration has previously
presented 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. The
isobar ratio ($Y$) of CME-sensitive observable, charge separation scaled by
elliptic anisotropy, is close to but systematically larger than the inverse
multiplicity ratio, the naive background baseline. This indicates the potential
existence of a CME signal and the presence of remaining nonflow background due
to two- and three-particle correlations, which are different between the
isobars. In this post-blind analysis, we estimate the contributions from those
nonflow correlations as a background baseline to $Y$, utilizing the isobar data
as well as Heavy Ion Jet Interaction Generator simulations. This baseline is
found consistent with the isobar ratio measurement, and an upper limit of 10%
at 95% confidence level is extracted for the CME fraction in the charge
separation measurement in isobar collisions at $\sqrt{s_{\rm NN}}=200$ GeV.
