Azimuthal anisotropy of ${\pi }^0$ and $\eta$ mesons in Au + Au collisions at $\sqrt{s_{\mathit{\text{NN}}}}=200$ GeV

The azimuthal anisotropy coefficients $v_2$ and $v_4$ of ${\pi }^0$ and $\eta$ mesons are measured in Au + Au collisions at $\sqrt{s_{\mathit{\text{NN}}}}=200$ GeV as a function of transverse momentum $p_T$ (1–14 GeV/$c$) and centrality. The extracted $v_2$ coefficients are found to be consistent between the two meson species over the measured $p_T$ range. The ratio of $v_4/v_2^2$ for ${\pi }^0$ mesons is found to be independent of $p_T$ for 1–9 GeV/$c$, implying a lack of sensitivity of the ratio to the change of underlying physics with $p_T$. Furthermore, the ratio of $v_4/v_2^2$ is systematically larger in central collisions, which may reflect the combined effects of fluctuations in the initial collision geometry and finite viscosity in the evolving medium.

Figure 1

Resolutions for $v_2$ (left) and $v_4$ (right) calculated using various detectors for event plane.

Figure 2

The $m_{\gamma \gamma }$ distributions used to obtain $v_2^{\mathrm{obs}}$ and $v_4^{\mathrm{obs}}$ values. (a) Distributions of foreground $N_{\mathrm{frg}}$, background $N_{\mathrm{bkg}}$, and signal $N_{\mathrm{sig}}$, (b) the signal fraction $R=N_{\mathrm{sig}}/(N_{\mathrm{sig}}+N_{\mathrm{bkg}})$, (c) the $v_2^{\mathrm{obs}}$ distributions for foreground, background and signal, and (d) the $v_4^{\mathrm{obs}}$ distributions for foreground, background, and signal. The vertical dashed lines indicate the two-standard-deviation window around the ${\pi }^0$ peak.

Figure 3

Comparison of ${\pi }^0$ $v_2$ for the $dN/d\Delta \varphi$ and the invariant mass methods of analysis for several centrality selections (left panels). The corresponding ratios are shown in the right panels, with the dashed curves indicating a $\pm 10$% deviation from unity.

Figure 4

Comparison of ${\pi }^0$ $v_4$ for the $dN/d\Delta \varphi$ and the invariant mass methods of analysis for several centrality selections (left panels). The corresponding ratios are shown in the right panels, with the dashed curves indicating a $\pm 10$% deviation from unity.

Figure 5

Event plane resolution $\mathrm{Res}\left\{2\Psi \right\}$ for events containing a high-$p_T$ ${\pi }^0$. Results are plotted as a function of $p_T$ of the ${\pi }^0$ for several EP detectors in several centrality ranges.

Figure 6

Comparison of $v_2$ vs $p_T$ for ${\pi }^0$ mesons obtained at several centralities with event planes determined by detectors in different pseudorapidity ranges.

Figure 7

The $v_2$ (left panels) and nuclear modification factor $R_{\mathrm{AA}}$ (right panels) for ${\pi }^0$ and $\eta$ mesons in 0%–20% (top panels) and 20%–60% (bottom panels) centrality ranges. The $v_2$ results for ${\pi }^0$ mesons are identical to what was published in Ref. [22]. The $R_{\mathrm{AA}}$ data are taken from Ref. [37] (${\pi }^0$, $p_T<5$ GeV/$c$), Ref. [23] (${\pi }^0$, $p_T>5$ GeV/$c$), and Ref. [32] ($\eta$ meson). The uncertainties of $R_{\mathrm{AA}}$ associated with $N_{\mathrm{coll}}$ and normalization are common between ${\pi }^0$ and $\eta$ mesons, and are shown as shaded boxes around unity.

Figure 8

The $v_4$ coefficient vs $p_T$ for ${\pi }^0$ mesons for the indicated centrality selections. The shaded boxes represent the systematic uncertainties.

Figure 9

The ratio $v_4/v_2^2$ vs $p_T$ for ${\pi }^0$ mesons for the indicated centrality selections. The shaded boxes represent the systematic uncertainties.