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

TL;DR

This study measures pion elliptic flow in heavy-ion collisions across various energies, revealing charge asymmetry dependence consistent with the possible presence of a chiral magnetic wave effect.

Contribution

It provides the first systematic measurement of charge asymmetry dependence of pion elliptic flow over a wide energy range, suggesting evidence for the chiral magnetic wave in heavy-ion collisions.

Findings

Elliptic flow of $\pi^-$ increases with charge asymmetry.

Elliptic flow of $\pi^+$ decreases with charge asymmetry.

Centrality dependence of flow difference aligns with chiral magnetic wave predictions.

Abstract

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.