2025/05/30 by the STAR Collaboration, The STAR Collaboration · 1 citation
Engineering · Physics and Astronomy · #FOS: Physical sciences #High Energy Physics - Experiment (hep-ex) #Nuclear Experiment (nucl-ex) #Nuclear Physics and Applications #Particle Accelerators and Free-Electron Lasers #Particle Detector Development and Performance
paper · pdf · doi:10.48550/arxiv.2506.00278
openalex publication_date 2025/05/30 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31
High-energy, heavy-ion collisions can create local domains of chirality-imbalanced quarks, reflecting the topological features of quantum chromodynamics. The chiral magnetic effect (CME) predicts an electric charge separation of quarks in such topological domains along the magnetic field (B) generated by the passing of two high-Z nuclei. We use a correlation observable Δγ112 between charged meson pairs to detect the CME-induced charge separation and a novel event shape selection (ESS) method to mitigate the background effects related to elliptic flow (v2). The ESS method classifies events based on the emission pattern of final-state particles and determines Δγ112\rm ESS from the zero-flow limit. We reconstruct the B field direction from the spectator nucleons, which minimizes backgrounds unrelated to the collective motion of the system. In this work, we report the measurements of Δγ112 and a background indicator Δγ132 in Au+Au collisions from the RHIC Beam Energy Scan phase II and at the top RHIC energy. After background suppression, Δγ132\rm ESS aligns with zero, and Δγ112\rm ESS is reduced to no more than 20% of Δγ112. We observe a finite residual charge separation with 2.6σ, 3.1σ, and 3.3σ significance in the 20%--50% centrality range of Au+Au collisions at 11.5, 14.6, and 19.6 GeV. The results at 17.3 and 27 GeV also show positive values but with a lower significance of 1.3σ and 1.1σ, respectively. The corresponding Δγ112\rm ESS values at 7.7, 9.2, and 200 GeV are consistent with zero within uncertainties.