Observation of Charge Asymmetry Dependence of Pion Elliptic Flow and the Possible Chiral Magnetic Wave in Heavy-Ion Collisions
We present measurements of π− and π+ elliptic flow, v2, at midrapidity in Au+Au collisions at √sNN=200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV, as a function of event-by-event charge asymmetry, Ach, based on data from the STAR experiment at RHIC. We find that π− (π+) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins at √sNN=27 GeV and higher. At √sNN=200 GeV, the slope of the difference of v2 between π− and π+ as a function of Ach 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.