Search for the Chiral Magnetic Effect from the RHIC Beam Energy Scan II
PAs: Zhiwan Xu, Gang Wang, Huan Huang
Target Journal: PRL
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Paper Draft (To be updated)
Analysis Note (To be updated)
PWGC preview: slides 12/22/2023
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Abstract
The chiral magnetic effect (CME) in heavy-ion collisions probes the topological sectors of Quantum ChromoDynamics (QCD) through the chirality imbalance emerging from QCD vacuum transitions and the intense magnetic field (B) generated by spectator protons from colliding nuclei. The CME predicts electric charge separation of quarks along the B direction, manifestly violating local P and CP symmetries. In this letter, a three-point correlator, Δγ112, is employed to detect the pertinent separation between two charged hadrons. However, the measurement includes significant non-CME backgrounds related to the collective motion of final-state particles, characterized by elliptic flow. To mitigate the flow background, we apply a novel Event Shape Selection (ESS) approach to classify events based on their shapes and determine Δγ112 at the zero-flow limit. Furthermore, we use the spectator information to estimate the B direction, which minimizes nonflow backgrounds. We report the measurements of Δγ112 and a background indicator, Δγ132, in Au+Au collisions from the RHIC Beam Energy Scan II program. With the ESS approach, we achieve background suppression by more than a factor of five. We observed 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 and 9.2 GeV are consistent with zero within statistical errors.
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Figures for the paper
Figure 1. The chiral magnetic effect describes the charge separation of quarks in local domain of chirality imbalance, in the presence of the external magnetic field. QCD vacuum transitions can generate the quark chirality imbalances. Major background in the search for CME in heavy ion collisions arises from elliptic flow (v2) related background. We apply an event shape selection method to choose events with nearly azimuthally isotopic particle emissions to minimize background.
Figure 2. presents the Event Shape Selection method applied to RHIC Au+Au 14.6 GeV data at 30-40%. Δγ112 , Δγ132 and v2 are measured using hadron (excluding protons) with respect to spectator plan (η_{EP} > 3.0 ) and according to different event shape categories. Extrapolation to zero-flow background limit presented a finite intercept for Δγ112.
Figure 3. The centrality dependence of Δγ
112 and Δγ
132 ensemble average in comparison with Δγ
112ESS and Δγ
132ESS after BKG subtraction for RHIC Au+Au at 7.7 to 27 GeV (BES-II). A finite signal of 3σ significance for Δγ
112 is observed
in the 20-50% centrality range of Au+Au at \sqrt{sNN} = 11.5, 14.6, and 19.6 GeV.
Figure 4.
The beam energy dependence of Δγ
112 ensemble average in comparison with Δγ
112ESS and Δγ
132ESS in aftermath of BKG subtraction for 20-50% RHIC Au+Au at 7.7 to 27 GeV (BES-II) in panel(a). The ratios of Δγ
112ESS to Δγ
112 are plotted in panel (b).
While Δγ
132ESS
are consistent with zero at all energies, finite Δγ
112ESS
values are observed at and above 11.5 GeV in Au+Au collisions. The trend suggests disappearance of chiral symmetry near 7.7 GeV._____________________________________________________________________________________________________________________________________________________________________________
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. With the novel Event Shape Selection method, we mitigate the flow background in Δγ112 and Δγ132. We utilize the Event Plane Detector to estimate the spectator planes, which are closely correlated with the magnetic field direction, and effectively minimize the nonflow background. 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 and 9.2 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 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.
