Title: Measurement of chiral magnetic effect-sensitive charge separation with full background removal in Au+Au collisions at √s_NN=200 GeV

PAs: Yicheng Feng, Haojie Xu, Jie Zhao, Han-Sheng Li, Fuqiang Wang

Targeted journal: 

Abstract: 
Vacuum fluctuations of the topological gluon field will produce a chirality imbalance of quarks because of quark-gluon interactions in quantum chromodynamics. Such a chirality imbalance will result in an electric current along an external magnetic field, a phenomenon called the chiral magnetic effect (CME). We measure reaction plane (RP)- and charge-dependent azimuthal correlations (∆γ), sensitive to the CME-induced charge separation, in Au+Au collisions at nucleon-nucleon center-of-mass energy of √s_NN=200 GeV at BNL's Relativistic Heavy Ion Collider. The elliptic flow (v₂)-induced background is eliminated by comparing the ∆γ measurements with respect to the harmonic event planes reconstructed from the zero-degree calorimeters and from the time projection chamber in the STAR experiment. The effects of nonflow contamination in v₂ (V_2p) and the RP-independent three-particle correlations (C_3p) are removed by flow/nonflow decomposition of data and by utilizing model calculations of C_3p/V_2p, respectively. We report CME signal fractions in the measured ∆γ after removal of the backgrounds from flow, nonflow, and RP-independent three-particle correlations.

Conclusion: 
After the removal of flow-induced backgrounds, nonflow contamination, and RP-independent three-particle correlation contributions, we report the final CME signal fractions in Au+Au collisions at √s_NN=200 GeV. Assuming the RP-independent three-particle correlations to scale with two-particle correlations as in Pythia and Hijing models, the CME signal fraction averaged over the 20–50% centrality range is ⟨f_CME⟩ = (29 ± 8)%, and that averaged over 20–80% centrality is ⟨f_CME⟩ = (29 ± 6)%.

Status: 
After PWGC preview

Paper draft: 
drupal.star.bnl.gov/STAR/system/files/FcmeAuAu200GeV_v13.5.pdf

Analysis note: 
drupal.star.bnl.gov/STAR/system/files/FcmeAuAu200GeV_Note_v13.5.pdf

CVS: 

Previous presentations:
2025/07/xx, Chirality Workshop, Fuqiang, drupal presentation: drupal.star.bnl.gov/STAR/presentations/Chirality-2025/Measurements-azimuthal-correlations-spectator-and-participant-planes--0
2025/06/25, FCV meeting, Fuqiang's slides, drupal.star.bnl.gov/STAR/system/files/Fcme_20250625_FCV.pdf
2025/06/13, CME-fg meeting, discussion, drupal.star.bnl.gov/STAR/system/files/Fcme20250613.pdf
2025/06/10, FCV meeting, 2p and 3p nonflow ratio in models to scale data & preliminary requests, drupal.star.bnl.gov/STAR/system/files/Fcme20250610.pdf
2025/05/02, CME-fg meeting, <N^2 C3>, NV2nf scaling, and HIJING with BWB, drupal.star.bnl.gov/STAR/system/files/Fcme20250502_v2.pdf
2025/02/25, FCV meeting, update the v2 nonflow and small systems, drupal.star.bnl.gov/STAR/system/files/Fcme20250225.pdf
2024/11/14, PWGC preview, drupal.star.bnl.gov/STAR/system/files/Fcme20241115_PWGC_preview_v2.pdf
                    Email summary: lists.bnl.gov/sympa/arc/star-fcv-l/2024-11/msg00163.html
                    Reply to comments: drupal.star.bnl.gov/STAR/system/files/AuAuFcme_Pwgc_Preview.pdf
2024/10/23, STAR Collaboration meeting, drupal.star.bnl.gov/STAR/system/files/Fcme20241021.pdf
2024/08/21, FCV meeting, paper proposal, drupal.star.bnl.gov/STAR/system/files/Fcme20240818_v3.pdf
2024/08/14, FCV meeting, drupal.star.bnl.gov/STAR/system/files/Fcme20240812_v2.pdf 
2024/08/09, CME-fg meeting, drupal.star.bnl.gov/STAR/system/files/Fcme20240807_v3.pdf 
2024/07/24, FCV meeting, drupal.star.bnl.gov/STAR/system/files/FlowDecor20240721.pdf 
2024/05/15, CME-fg meeting, drupal.star.bnl.gov/STAR/system/files/Fcme20240514_v4.pdf 
2024/04/02, FCV meeting, drupal.star.bnl.gov/STAR/system/files/Fcme20240402.pdf 

Figures: 
Please see the paper draft for now