  For the search of the chiral magnetic effect (CME), STAR previously presented
the results from isobar collisions
(${^{96}_{44}\text{Ru}}+{^{96}_{44}\text{Ru}}$,
${^{96}_{40}\text{Zr}}+{^{96}_{40}\text{Zr}}$) obtained through a blind
analysis. The ratio of results in Ru+Ru to Zr+Zr collisions for the
CME-sensitive charge-dependent azimuthal correlator ($\Delta\gamma$),
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, $Y =
\frac{(\Delta\gamma/v_{2})^{\text{Ru}}}{(\Delta\gamma/v_{2})^{\text{Zr}}}$, 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
$\Delta\gamma$ 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.
