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Paper Proposal: Search for the Chiral Magnetic Effect from the RHIC Beam Energy Scan II

Updated on Fri, 2024-06-21 15:26. Originally created by zhiwanxu on 2022-05-06 16:50.
Search for the Chiral Magnetic Effect from the RHIC Beam Energy Scan II

Paper title: Charge Separation Measurements in Au+Au collisions at 7.7--200 GeV in Search of the Chiral Magnetic Effect (short)
Long Paper in accompany: Search for the chiral magnetic effect through beam energy dependence of charge separation using event shape selection


PAs: Zhiwan Xu, Gang Wang, Huan Huang
Target Journal: PRL, 
                        Accompanying long paper targeting PRC
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Paper Draft: version1 short paper
Analysis Note: version1

PWGC preview: slides 12/22/2023
Updates in response to PWGC: slides (Gang Wang) + slides (Zhiwan Xu) 03/20/2024

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Abstract

In high-energy heavy-ion collisions, the chiral magnetic effect (CME) may arise from the interplay between domains of chirality imbalanced quarks in the quark-gluon plasma and the strong magnetic field (B) generated by spectator protons. The CME is predicted to induce an electric charge separation along the B direction, manifestly violating local P and CP symmetries. We use the  Δγ112 correlator between two charged hadrons to detect such a charge separation across the reaction plane.  To mitigate the background induced by elliptic flow, we adopt a novel event shape selection (ESS) approach that classifies events based on their shapes and allows us to determine Δγ112ESS at the zero-flow limit. Furthermore, we use the spectator information to reconstruct the B direction, thereby minimizing nonflow backgrounds. 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 the flow-background suppression, Δγ132ESS aligns with zero, and Δγ112ESS is reduced from inclusive Δγ112 by more than five-fold.  The measured Δγ112ESS value in the 20\%--50\% centrality range is positively finite with an over 3σ significance at each of center-of-mass energies 11.5, 14.6, and 19.6 GeV, whereas the corresponding values at 7.7, 9.2 and 200 GeV are consistent with zero within uncertainties.
 



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 Figures for the paper
      

Figure 1. Left: schematic diagram of potential energy in gluon fields. The straight and curved arrows represent two modes of topological vacuum transitions in QCD, marked by the change in Chern-Simons number (NCS). Right: a sketch of the CME in a two-nucleus collision. Forward- and backward-moving balls denote spectator nucleons that do not participate in the collision but create a strong magnetic field B. In the participant zone (the overlap region), the  CME induces an electric current (J) across the x-z plane in chiral domains (μ5 \neq 0). 




Figure 2.Upper: illustration of various particle emission patterns originating from the same initial geometry. The ESS projects the CME observable to isotopic emission, characterized by zero elliptic flow ($v_2$). Lower: (a) Δγ112 and (b) Δγ132 as a function of v2 with events categorized by q22,PPOI in the 30--40\% centrality range of Au+Au collisions at \sqrt{sNN} = 14.6 GeV. POI are charged hadrons, excluding (anti)protons. The error bars are statistical only. Linear fits (dashed lines) are used to extract the y-intercepts.






Figure 3. Centrality dependence of (a--h) NpartΔγ112 and (i--p) NpartΔγ132 in Au+Au collisions at \sqrt sNN = 7.7--200 GeV. Both the ensemble averages (stars) and the ESS results (circles) are presented. The error bars and boxes represent the statistical and systematic uncertainties, respectively. The dashed lines represent constant fits over the 20--50% centrality range, with the shaded bands denoting the fit uncertainties. 

Figure 4. 

(a) Beam-energy dependence of Npart<Δγ112 >(stars), NpartΔγ112ESS (full circles), and NpartΔγ132ESS (open circles) integrated over the 20--50% centrality range in Au+Au collisions. For comparison, we also add the results for NpartΔH(κ{bg}=2.5) (diamonds) with a dashed curve to connect points.  

(b) The corresponding ratio of Δγ112ESS to <Δγ112 >. Some points are slightly shifted horizontally to enhance clarity. The error bars and boxes represent the statistical and systematic uncertainties, respectively.


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Conclusion:

We have measured the centrality dependence of the CME sensitive Δγ112 and the background indicator, Δγ132, in Au+Au collisions at \sqrt{sNN} =  7.7 -- 27 GeV from the RHIC Beam Energy Scan II, and at RHIC top energy of 200 GeV. With the novel Event Shape Selection method, we mitigate the flow background in Δγ112 and Δγ132. Meanwhile, we utilize the Event Plane Detector to estimate the spectator planes for BES-II energies, which are closely correlated with the magnetic field direction, and effectively minimize the nonflow background. At 200 GeV, the ZDC-SMD is utilized to reconstruct the spectator plane. At the zero-flow limit,  Δγ132ESS is consistent with zero at all energies, whereas Δγ112ESS reduces to at most 20\% of Δγ112. We report a finite  Δγ112ESS value with a 3σ significance in the 20-50% centrality range of  Au+Au at \sqrt{sNN} =  11.5, 14.6, and 19.6 GeV, while the corresponding results at 7.7, 9.2 GeV , and at 200 GeV are consistent with zero within statistical errors. The data at 27 GeV lack the statistical significance to be definitive.
The finite Δγ112ESS values observed in Au+Au collisions at and above 11.5 GeV may suggest an intriguing scenario where the chirality imbalance of quarks and the intense magnetic field may coexist at these energies, resulting in the CME-induced charge separation. Towards 7.7 GeV, partonic degrees of freedom and/or chiral symmetry restoration may diminish in the collision system, so that the precondition for the quark chirality effect vanishes.

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Previous Presentations: 


Related Preliminary Requests:

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Related Method paper (using AMPT + AVFD)

  1. Event Shape Selection Method in Search of the Chiral Magnetic Effect in Heavy-ion Collisions
  2. Utilization of event shape in search of the chiral magnetic effect in heavy-ion collisions

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--Last update 19/06/2024
 

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