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

The collaboration
Phys.Rev.Lett. 114 (2015) 252302, 2015

Abstract (data abstract)
BNL-RHIC. 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.

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Data from Figure 1a

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The distribution of observed charge asymmetry from STAR data.

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Data from Figure 2a

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Pion $v_2${2} as a function of observed charge asymmetry.

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Data from Figure 2b

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$v_2$ difference between $\pi^-$ and $\pi^+$ as a function of charge asymmetry with the tracking efficiency correction, for 30-40% central...

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Data from Figure 4

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The slope parameter r as a function of centrality for collision energy of 200 GeV.

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Data from Figure 4

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The slope parameter r as a function of centrality for collision energy of 62.4 GeV.

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Data from Figure 4

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The slope parameter r as a function of centrality for collision energy of 39 GeV.

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Data from Figure 4

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The slope parameter r as a function of centrality for collision energy of 27 GeV.

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Data from Figure 4

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The slope parameter r as a function of centrality for collision energy of 19.6 GeV.

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Data from Figure 4

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The slope parameter r as a function of centrality for collision energy of 11.5 GeV.

• #### Table 10

Data from Figure 4

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The slope parameter r as a function of centrality for collision energy of 7.7 GeV.