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Title: Estimate of Background Baseline and Upper Limit on the Chiral Magnetic Effect in Isobar Collisions at √s_NN=200 GeV at the BNL Relativistic Heavy-Ion Collider

PAs: Yicheng Feng, Fuqiang Wang

Targeted journals:
short paper: Phys. Rev. Lett
long paper: Phys. Rev. C

Abstract:
We have reported the isobar (⁹⁶₄₄Ru+⁹⁶₄₄Ru, ⁹⁶₄₀Zr+⁹⁶₄₀Zr) results from a blind analysis on the search for the chiral magnetic effect (CME). The Ru+Ru to Zr+Zr ratio of the CME-sensitive charge-dependent azimuthal correlator (Δγ), normalized by elliptic anisotropy (v₂), is observed to be close to but systematically larger than the inverse multiplicity (N) ratio. The background baseline for the isobar ratio Y=(Δγ/v₂)^Ru/(Δγ/v₂)^Zr is naively expected to be (1/N)^Ru/(1/N)^Zr, but genuine two- and three-particle correlations are expected to alter it. We estimate the contributions from those correlations, utilizing the STAR isobar data as well as HIJING simulations. The estimated baseline for Y is found consistent with the isobar data, and an upper limit of ~10% at 95% confidence level is extracted for the CME fraction in the Δγ measurement in isobar collisions at 200 GeV.

Conclusion:
In this study we have estimated nonflow contributions in v₂ by fitting two-particle (Δη,Δφ) distributions, and the deviations from simple multiplicity scaling of the three-particle correlator using the STAR isobar data. The 3-particle nonflow correlation contributions to C₃ are evaluated by HIJING simulations. Using these inputs, we arrive at an improved background estimate of the Ru+Ru to Zr+Zr ratio of the Δγ/v₂ variable. The estimated background baselines are consistent with the STAR measurements from both the full-event and subevent methods. We extract an upper limit of the CME fraction in Δγ measurements to be ~10% with 95% confidence level in isobar collisions at 200 GeV.

Status:
2024/07/09: both papers have been published online
Short paper on Phys.Rev.Research: journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.6.L032005
Long paper on Phys.Rev.C: journals.aps.org/prc/abstract/10.1103/PhysRevC.110.014905

2024/05/14: short paper transferred to PRResearch

2023/10/24: two papers are submitted jointly: short paper to PRL, long paper to PRC
starpapers-l is informed: lists.bnl.gov/mailman/private/starpapers-l/2023-October/008556.html
2023/10/23: long paper on arXiv: arxiv.org/abs/2310.13096
2023/08/31: short paper on arXiv: arxiv.org/abs/2308.16846

Long paper draft, latest version:
drupal.star.bnl.gov/STAR/system/files/IsobarNfBkg_L_v44.pdf

Short paper draft, latest version:
drupal.star.bnl.gov/STAR/system/files/IsobarNfBkg_S_v28.pdf

Analysis note:
drupal.star.bnl.gov/STAR/system/files/IsobarNfBkg_Note_v16_0.pdf
STAR note: drupal.star.bnl.gov/STAR/starnotes/private/PSN0811

CVS:
www.star.bnl.gov/cgi-bin/protected/viewvc.cgi/cvsroot/offline/paper/psn0811/

Figures.zip:
2023/10/26: long paper figures
→long paper figures: drupal.star.bnl.gov/STAR/system/files/IsobarNfBkg_L_v28_fig.zip
2023/09/14: short paper figures
→short paper figures: drupal.star.bnl.gov/STAR/system/files/IsobarNfBkg_S_v22_fig.zip

FCV:
2023/04/04:
→long paper draft: drupal.star.bnl.gov/STAR/system/files/IsobarNfBkg_L_v9.pdf
→latexdiff: drupal.star.bnl.gov/STAR/system/files/diff_IsobarNfBkg_L_v8v9.pdf
2023/03/30:
→long paper draft: drupal.star.bnl.gov/STAR/system/files/IsobarNfBkg_L_v8.pdf
→latexdiff: drupal.star.bnl.gov/STAR/system/files/diff_IsobarNfBkg_L_v7v8.pdf

Supplementary materials:
FCV analysis meeting: 2023/06/26, drupal.star.bnl.gov/STAR/blog/fengyich/Analysis-meeting-QM2023-topics-Isobar-CME-backgrounds
PWGC preview, 2023/02/17, drupal.star.bnl.gov/STAR/system/files/IsobarNfBkg_PwgcPreview_v5.pdf
drupal.star.bnl.gov/STAR/event/2023/02/17/STAR-PWGC-meeting#comment-2205
FCV meeting, 2023/01/25, paper proposal, drupal.star.bnl.gov/STAR/system/files/PPRP20230125_v2.pdf
CME focus meeting, 2023/01/20, drupal.star.bnl.gov/STAR/system/files/PPRP20230119_v3.pdf
CME focus meeting, 2022/12/16, paper proposal, drupal.star.bnl.gov/STAR/system/files/PPRP20221216pp_v3.pdf
CME focus meeting, 2022/11/11, drupal.star.bnl.gov/STAR/system/files/PPRP20221111.pdf
DNP2022, talk, 2022/10/29, meetings.aps.org/Meeting/DNP22/Session/JC.1
SQM2022, talk, 2022/06/14, indico.cern.ch/event/1037821/contributions/4842110/
QM2022, poster, 2022/04/06, indico.cern.ch/event/895086/contributions/4721260/

Table (for long paper):

Figure (for long paper):

Figure (for short paper):

Figure 2 in short paper is the same to Fig.12 of long paper.
Figure 3 in short paper is the same to Fig.13 of long paper.
Figure 4 in short paper is the same to Fig.14(a) of long paper.

Cartoons:

or through links below:
400x400: drupal.star.bnl.gov/STAR/system/files/IsobarDiffText_Pixel400_v1.png
1000x1000: drupal.star.bnl.gov/STAR/system/files/IsobarDiffText_v1.png

The anticipated chiral magnetic effect (CME)-related charge separation was not measured to be larger in ${^{96}_{44}\text{Ru}}+{^{96}_{44}\text{Ru}}$ than in ${^{96}_{40}\text{Zr}}+{^{96}_{40}\text{Zr}}$ collisions because the flow-induced backgrounds do not fully cancel due to nuclear structure differences between the two isobar nuclei. Properly taking this, as well as the next-level non-flow-related backgrounds, into account, the charge separation results are consistent between the isobar systems and an upper limit of 10% CME signal is extracted at 95% confidence level.

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