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 Au+Au collisions at $sqrt{s_{_{rm NN}}} =$ 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 $sqrt{s_{_{rm NN}}} = text{27 GeV}$ and higher. At $sqrt{s_{_{rm NN}}} = text{200 GeV}$, 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.