Observation of Charge Asymmetry Dependence of Pion Elliptic Flow and the Possible Chiral Magnetic Wave in Heavy-Ion Collisions

We present measurements of ${\pi }^{-}$ and ${\pi }^{+}$ elliptic flow, $v_2$, at midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200$, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV, as a function of event-by-event charge asymmetry, $A_{\mathrm{ch}}$, based on data from the STAR experiment at RHIC. We find that ${\pi }^{-}$ (${\pi }^{+}$) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins at $\sqrt{s_{\mathrm{NN}}}=27\mathrm{GeV}$ and higher. At $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$, the slope of the difference of $v_2$ between ${\pi }^{-}$ and ${\pi }^{+}$ as a function of $A_{\mathrm{ch}}$ exhibits a centrality dependence, which is qualitatively similar to calculations that incorporate a chiral magnetic wave effect. Similar centrality dependence is also observed at lower energies.

Figure 1

(a) Distribution of observed charge asymmetry from STAR data and, (b) the relationship between the observed charge asymmetry and the charge asymmetry from hijing generated events, for 30%–40% central $\mathrm{Au}+\mathrm{Au}$ collisions at 200 GeV. In this centrality, the mean charge asymmetry $⟨A_{\mathrm{ch}}⟩$ of hijing events is about 0.004. The errors are statistical only.

Figure 2

(a) Pion $v_2\{2\}$ as a function of observed charge asymmetry and (b) $v_2$ difference between ${\pi }^{-}$ and ${\pi }^{+}$ as a function of charge asymmetry with the tracking efficiency correction, for 30%–40% central $\mathrm{Au}+\mathrm{Au}$ collisions at 200 GeV. The errors are statistical only.

Figure 3

The slope parameter, $r$, as a function of centrality for $\mathrm{Au}+\mathrm{Au}$ collisions at 200 GeV. Also shown is the UrQMD [33] simulation, and the calculations with CMW [34] with different duration times. The grey bands include the systematic errors due to the DCA cut, the tracking efficiency, and the $p_T$ range of particles involved in the event plane determination. The cross-hatched band indicates the STAR measurement with the $v_2\{4\}$ method and the height of this band shows only the statistical error.

Figure 4

The slope parameter $r$ as a function of centrality for all the collision energies under study. For comparison, we also show the calculations with CMW [34] with different duration times. The grey bands carry the same meaning as those in Fig. 3.