Beam-Energy Dependence of Charge Separation along the Magnetic Field in $\mathrm{Au}+\mathrm{Au}$ Collisions at RHIC

Local parity-odd domains are theorized to form inside a quark-gluon plasma which has been produced in high-energy heavy-ion collisions. The local parity-odd domains manifest themselves as charge separation along the magnetic field axis via the chiral magnetic effect. The experimental observation of charge separation has previously been reported for heavy-ion collisions at the top RHIC energies. In this Letter, we present the results of the beam-energy dependence of the charge correlations in $\mathrm{Au}+\mathrm{Au}$ collisions at midrapidity for center-of-mass energies of 7.7, 11.5, 19.6, 27, 39, and 62.4 GeV from the STAR experiment. After background subtraction, the signal gradually reduces with decreased beam energy and tends to vanish by 7.7 GeV. This implies the dominance of hadronic interactions over partonic ones at lower collision energies.

Figure 1

Schematic depiction of the transverse plane for a collision of two heavy ions (the left one emerging from and the right one going into the page). Particles are produced in the overlap region (blue-colored nucleons). The azimuthal angles of the reaction plane and a produced particle used in the three-point correlator $\gamma$ are depicted here.

Figure 2

The three-point correlator $\gamma$ as a function of centrality for $\mathrm{Au}+\mathrm{Au}$ collisions at $7.7–624\mathrm{GeV}$. Note that the vertical scales are different for different rows. The filled boxes (starting from the central values) represent one type of systematic uncertainty (as discussed in the text). Charge-independent results from the model calculations of mevsim [27] are shown as grey curves. ${\gamma }_{\mathrm{OS}}$ and ${\gamma }_{\mathrm{SS}}$ from UrQMD calculations [28] are also shown as shaded bands for 27 and 39 GeV.

Figure 3

The two-particle correlation as a function of centrality for $\mathrm{Au}+\mathrm{Au}$ collisions at $7.7–62.4\mathrm{GeV}$. Note that the vertical scales are different for different rows. The filled boxes bear the same meaning as those in Fig. 2 and are described in the text. mevsim and UrQMD calculations are also shown for comparison.

Figure 4

$H_{\mathrm{SS}}-H_{\mathrm{OS}}$, as a function of beam energy for three centrality bins in $\mathrm{Au}+\mathrm{Au}$ collisions. The default values (dotted curves) are for $H^{\kappa =1}$, and the solid (dash-dotted) curves are obtained with $\kappa =1.5$ ($\kappa =2$). For comparison, the results for $\mathrm{Au}+\mathrm{Au}$ collisions at 200 GeV [11] and $\mathrm{Pb}+\mathrm{Pb}$ collisions at 2.76 TeV [16] are also shown. The systematic errors of the STAR data (filled boxes) bear the same meaning as those in Fig. 2. UrQMD calculations with $\kappa =1$ are also shown as solid shaded bars for 27 and 39 GeV.