Results from an improved RΨ2 observable in isobar collisions at STAR

The RΨ2 (∆S) observable, which quantifies charge separation (∆S) fluctuations perpendicular to the reaction plane relative to those parallel [1], has been used to search for the chiral magnetic effect (CME) in isobar (96 44Ru + 96 44Ru and 96 40Zr + 96 8 40Zr) collisions at nucleon-nucleon center-of-mass energy √ 9 sNN = 200 GeV by STAR [2]. Each event measured by the STAR Time Projection Chamber is divided into two subevents according to the particle pseudorapidity, where one is used to reconstruct an event plane as the proxy for the reaction plane and the other subevent is used to calculate ∆S, and vice versa. The ∆S is normalized by the width of the charge-shuffled ∆S parallel to the event plane, and the ∆S values from the two subevents are averaged. The Ru+Ru/Zr+Zr ratio of the squared inverse width of the RΨ2 (∆S) distribution, 1/σ2 RΨ2, is consistent with unity, indicating no evidence for the pre-defined CME signal [2]. In this talk, we present an improved definition of RΨ2 (∆S), where the ∆S values perpendicular and parallel to the event plane are normalized by the respective charge-shuffled widths, and the ∆S from the two subevents are not averaged but treated as independent quantities in the analysis [3]. We report preliminary results on this improved RΨ2 (∆S) observable. We make connections of the improved RΨ2 (∆S) to the ∆γ observable, and discuss our results in the context of the CME search. [1] N.N. Ajitanand, R.A. Lacey, A. Taranenko and J.M. Alexander, Phys. Rev. C 83, 011901 (2011). [2] M. Abdallah et al. (STAR Collaboration), Phys. Rev. C 105, 014901 (2022). [3] Y. Feng, J. Zhao, H.-j. Xu and F. Wang, Phys. Rev. C 103, 034912 (2021).