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Description
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 $\sqrt{s_{\mathrm{NN}}}=200$ 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.
[1] M. Abdallah $\textit{et al.}$ (STAR Collaboration), Phys. Rev. C $\textbf{105}$, 014901 (2022)
[2] D.E.Kharzeev, J.Liao and S.~Shi, Phys. Rev. C $\textbf{106}$, L051903 (2022)
[3] S. Choudhury $\textit{et al.}$, Chinese Phys. C $\textbf{46}$ 014101 (2022)
| Category | Experiment |
|---|---|
| Collaboration (if applicable) | STAR Collaboration |
