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Search for the Chiral Magnetic Effect in Au+Au collisions at 27 GeV with the STAR forward Event Plane Detectors

Updated on Tue, 2023-03-07 14:37 by huyu. Originally created by ptribedy on 2019-12-17 01:12.

Title: Search for the Chiral Magnetic Effect in Au+Au collisions at $\sqrt{s_{_{\rm{NN}}}}=27$ GeV with the STAR forward Event Plane Detectors

PA's: S.Choudhury, Y.Hu, M.Lisa, P.Tribedy

Target Journal: Physics Letter B

The proof of the letter:
https://drupal.star.bnl.gov/STAR/system/files/plb137779.pdf

Latest paper draft:
https://drupal.star.bnl.gov/STAR/system/files/Paperdraft_27GeV_v15.pdf

Review and reponse draft:
https://drupal.star.bnl.gov/STAR/system/files/PLB_27gev_CME_response_v3.pdf

Submission to PLB:
Confirmation email:
https://drupal.star.bnl.gov/STAR/system/files/Berkeley%20Lab%20%28Univ%20of%20California%29%20Mail%20-%20PLB-D-22-01308%20-%20Confirming%20your%20submission%20to%20Physics%20Letters%20B.pdf

Link to arxiv:
https://arxiv.org/abs/2209.03467

Paper draft (latest):

Differences between paper draft version 11 (old) and version 12 (new):
https://drupal.star.bnl.gov/STAR/system/files/Paperdraft_27GeV_diff_v11v12.pdf

Supplementary Material:
https://drupal.star.bnl.gov/STAR/system/files/27gev_cme_supplementary_v4.pdf

Differences between supplementary material version 3 (old) and version 4 (new):
https://drupal.star.bnl.gov/STAR/system/files/27gev_cme_supplement_doc_diff_v3v4.pdf

Analysis note (latest):
https://drupal.star.bnl.gov/STAR/system/files/27GeV_analysis_note_v5s.pdf

Note for the collaboration comments and our response:
https://drupal.star.bnl.gov/STAR/system/files/27gev_cme_institution_reviews.pdf

Note for GPC comments & Our reply:
Latest note for GPC comments & Our reply (round7):
https://drupal.star.bnl.gov/STAR/system/files/27gev_cme_gpc_comments_round7.pdf
Round5-6
https://docs.google.com/document/d/1zwAVyen_1V_c9iCdp5_kpC_KwXeJ582_RFP8LewXeIg/edit?usp=sharing
Round-4, reply to Kong:
https://drupal.star.bnl.gov/STAR/system/files/GPCcomments_round4%20_Kong.pdf
Round-4, reply to Richard:
https://drupal.star.bnl.gov/STAR/system/files/GPC_Comments_from_Richard.pdf
Round-3:
https://drupal.star.bnl.gov/STAR/system/files/27gev_cme_gpc_comments_doc_all_r3.pdf
Only 33-35 page:
https://drupal.star.bnl.gov/STAR/system/files/27gev_cme_gpc_comments_doc_page_33-35.pdf

Paper proposal presentation :
https://drupal.star.bnl.gov/STAR/system/files/PaperProposal_PWG_Meeting_06_10_2020.pdf

-------------------------------

Note for PWGC comments & Our reply:
https://drupal.star.bnl.gov/STAR/system/files/27GeV_analysis_note_PWGC_comments.pdf

Note for GPC comments & Our reply:
Round-1:
https://drupal.star.bnl.gov/STAR/system/files/27gev_cme_gpc_comments_doc_r1.pdf
Round-2:
https://drupal.star.bnl.gov/STAR/system/files/27gev_cme_gpc_comments_doc_r2.pdf

Document for how to run our code:
https://drupal.star.bnl.gov/STAR/system/files/document_ourcode_27gev.pdf

Abstract:

decisive experimental test of the Chiral Magnetic Effect (CME) is considered one of the major scientific goals at the Relativistic Heavy-Ion Collider (RHIC) towards understanding the nontrivial topological fluctuations of the Quantum Chromodynamics vacuum. In heavy-ion collisions, the CME is expected to result in a charge separation phenomenon across the reaction plane, whose strength could be strongly energy dependent. The previous CME searches have been focused on top RHIC energy collisions. In this Letter, we present a low energy search for the CME in Au+Au collisions at $\sqrt{s_{_{\rm{NN}}}}=27$ GeV. We measure elliptic flow scaled charge-dependent correlators relative to the event planes that are defined at both mid-rapidity $|\eta|<1.0$ and at forward rapidity $2.1 < |\eta|<5.1$. We compare the results based on the directed flow plane ($\Psi_1$) at forward rapidity and the elliptic flow plane ($\Psi_2$) at both central and forward rapidity. The CME scenario is expected to result in a larger correlation relative to $\Psi_1$ than to $\Psi_2$, while a flow driven background scenario would lead to a consistent result for both event planes~\cite{Xu:2017qfs,Voloshin:2018qsm}. In 10-50\% centrality, results using three different event planes are found to be consistent within experimental uncertainties, suggesting a flow driven background scenario dominating the measurement. We obtain an upper limit on the deviation from a flow driven background scenario at the 95\% confidence level. This work opens up a possible road map towards future CME search with the high statistics data from the RHIC Beam Energy Scan Phase-II.

Motivation:

Observables:

Figures that will go in the paper:

Fig. 2 : v₂ and ratio for different definitions

Fig. 3: Δγ and ratio for different definitions

Fig. 4 : Δγ/v₂ and ratio for different definitions

Fig. 5 : Upper limits of excess charge separation along spectator rich plane at 95% CL

Summary :

In this letter, we present measurements of charge separation with respect to reaction plane using Au+Au $\sqrt{s_{_{\rm NN}}}$ $=$27 GeV collisions. At this collision energy, the STAR forward EPDs can measure the event plane associated with a large directed flow from beam fragments with high resolution. This directed flow plane ($\Psi_1$) is expected to be more correlated with the direction of magnetic field than the elliptic flow planes ($\Psi_2$) of produced particles as shown in UrQMD simulations. Therefore, we measured the charge separation scaled by ellipticity $R(\Psi_n)$ across the $\Psi_1$ determined at forward rapidity and compare it with the corresponding measurements using $\Psi_2$ reconstructed out of produced particles at both forward and mid-rapidity. Within our measurement uncertainties we find the ratio $R(\Psi_1)/R(\Psi_2)$ to be consistent with unity, which agrees with the expectations from a flow driven background scenario. For further quantification, we derive an upper limit at the 95\% confidence level on the quantity $\mathcal{D}=R(\Psi_1)/R(\Psi_2)-1$ for possible deviation from a flow driven background scenario. In 10-50$\%$ centrality we find the upper limits of $\mathcal{D}$ to be 10$\%$ and 16$\%$ using $\Psi_2$ at forward and mid-rapidity, respectively. In this analysis we argued that the information of the directed flow near the beam rapidity using EPDs and the elliptic flow at mid-rapidity allows us to control the flow driven CME background in 27 GeV Au+Au collisions and explore effects beyond background. Due to the acceptance of the EPDs ($2.1<\eta<5.1$), the same can be done for several data sets ($\sqrt{s_{_{NN}}}=9.2, 11.5, 13.7, 14.5, 17.3, 19.6$ GeV) collected by the STAR under RHIC Beam Energy Scan Phase II program. The use of forward event planes in this work not only pioneers a high-precision CME search from the RHIC Beam Energy Scan Phase II program, but also opens up opportunities to search for other magnetic field driven effects at RHIC.

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