Figure 1

(a) $P_2 (\Deta,\Dphi) $ in the 5% most central Pb-Pb collisions. The region $|\Deta| < 0.15$ and $|\Delta\varphi| < 0.13$ rad, where the weight technique used in this analysis does not provide a reliable efficiency correction, is excluded. (b) $P_2 (\Dphi)$ for $|\Delta\eta| \geq 0.9$ Systematic errors are shown as gray boxes. Note the statistically significant dip at $\Delta\varphi \sim \pi$.
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