Elliptic Flow of Identified Hadrons in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ Collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$

The anisotropy parameter ($v_2$), the second harmonic of the azimuthal particle distribution, has been measured with the PHENIX detector in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$ for identified and inclusive charged particle production at central rapidities ($|\eta |<0.35$) with respect to the reaction plane defined at high rapidities ($|\eta |=3–4$). We observe that the $v_2$ of mesons falls below that of (anti)baryons for $p_T>2\mathrm{G}\mathrm{e}\mathrm{V}/c$, in marked contrast to the predictions of a hydrodynamical model. A quark-coalescence model is also investigated.

Figure 1

Correlation of reaction planes between two beam counters for the second moment is shown as a function of centrality (bottom-left panel) and the correction factor for the combined reaction plane resolution of two beam counters is shown as a function of centrality (top-left panel). The value of $v_2$ for charged particles is shown as a function of centrality (middle panel) and as a function of $p_{\mathrm{T}}$ (right panel).

Figure 2

Transverse momentum dependence of $v_2$ for identified particles, ${\pi }^{-}$, $K^{-}$, $\overline{p}$ (top-left panel) and ${\pi }^{+}$, $K^{+}$, $p$ (top-right panel). The circles show $p$ and $\overline{p}$, the squares show $K^{+}$ and $K^{-}$, and the triangles show ${\pi }^{+}$ and ${\pi }^{-}$ for minimum-bias events. Statistical errors are represented by error bars and overall systematic error due to all sources by the solid lines in the top two panels. The combined positive particles and negative particles are shown in the bottom-left panel, and the lines there represent the result of a hydrodynamical calculation [20] including a first-order phase transition with a freeze-out temperature of 120 MeV for $\pi$, $K$, and $p$ from upper to lower curves, respectively. The bottom-right panel shows the quark $v_2$ as a function of the quark $p_T$ by scaling both axes with the number of quarks for each particle, as motivated by a quark-coalescence model [21].

Figure 3

Transverse momentum dependence of $v_2$ for combined ${\pi }^{-}$ and $K^{-}$ (top) or ${\pi }^{+}$ and $K^{+}$ (bottom) compared with $\overline{p}$ (top) and $p$ (bottom). In addition, results for inclusive negative (top) and positive (bottom) charged particle distributions are plotted as open squares. From left to right, the different centrality selections are shown for 0%–20% (left), 20%–40% (middle), and 40%–60% (right).