Observation of charge asymmetry dependence of pion elliptic flow and the possible chiral magnetic wave in heavy-ion collisions

Abstract

We present measurements of ${\pi }^{-}$ and ${\pi }^{+}$ elliptic flow, $v_2$, at midrapidity in Au+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, $A_{ch}$, 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 $\sqrt{s_{{}_{\mathrm{NN}}}}=\text{27 GeV}$ and higher. At $\sqrt{s_{{}_{\mathrm{NN}}}}=\text{200 GeV}$, the slope of the difference of $v_2$ between ${\pi }^{-}$ and ${\pi }^{+}$ as a function of $A_{ch}$ 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.

Comments: 6 pages, 4 figures