Background control and upper limit on the chiral magnetic effect in isobar collisions at √S_NN = 200 GeV from STAR

Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907

^* e-mail: feng216@purdue.edu

Published online: 26 June 2024

Abstract

The STAR Collaboration has reported results from a blind analysis of isobar collisions (₄₄⁹⁶Ru + ₄₄⁹⁶Ru , ₄₀⁹⁶Zr + ₄₀⁹⁶Zr) at √S_NN = 200 GeV on the search for the chiral magnetic effect (CME). Significant differences were observed in the measured multiplicity (N) and elliptic anisotropy (v₂) between the two isobar systems. In these proceedings, we present two post-blind analyses aimed at mitigating remaining background effects. The first involves employing an event weighting procedure to match the distributions in N and v₂ and then comparing the CME-sensitive Δγ correlator and signed balance functions. The second analysis investigates the contributions of the two- and three-particle nonflow to the isobar ratio of Δγ/v₂. The estimated background baseline is consistent with the isobar measurements, and an upper limit is extracted on the CME signal.