Title: Post-blind Analysis of Isobar Collisions and Background-controlled Upper Limit on the Chiral Magnetic Effect from STAR


The STAR Collaboration has reported results from a blind analysis of isobar collisions (${^{96}_{44}\text{Ru}}+{^{96}_{44}\text{Ru}}$, ${^{96}_{40}\text{Zr}}+{^{96}_{40}\text{Zr}}$) at $\sqrt{s_{\textsc{nn}}}=200$ GeV
on the search for the chiral magnetic effect (CME). Significant differences were observed in the measured multiplicity ($N$) and elliptic anisotropy ($v_{2}$) between the two isobar systems [1]. The isobar ratio (Ru/Zr) of CME-sensitive observable, $v_2$-scaled charge separation ($\Delta\gamma/v_2$) is close to but systematically larger than the inverse multiplicity ($1/N$) ratio. This indicates the potential existence of a CME signal, as well as the presence of remaining background
that is different between the isobars [2]. In this contribution, we present two post-blind analyses of the isobar data
that address the remaining backgrounds and attempt to extract any possible
CME signal. One of the analysises applies a weighting procedure such that the two isobar systems have identical distributions of $N$ and $v_{2}$, and then compares the CME-sensitive observables ($\Delta\gamma$ correlator and signed Balance Functions [3]) between two isobars with matched $N$ and $v_{2}$. The other analysis examines the two- and three-particle nonflow contributions to the isobar ratio of $\Delta\gamma / v_{2}$ using real data as well as HIJING simulation. This allows the estimation of a modified background baseline for the
$\Delta\gamma/v_2$ ratio.  The overall contribution of nonflow is found to be positive, resulting in a background baseline larger than the inverse multiplicity ratio and generally consistent with the isobar measurements.
We extract an upper limit of the CME signal in isobar collisions.

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\noindent[1] M.~Abdallah \textit{et al.} (STAR Collaboration), Phys. Rev. C \textbf{105}, 014901 (2022)  \\
\noindent[2] D.~E.~Kharzeev, J.~Liao and S.~Shi, Phys. Rev. C \textbf{106}, L051903 (2022) \\
\noindent[3] S. Choudhury \textit{et al.}, Chinese Phys. C \textbf{46} 014101 (2022)