Measurement of Identified ${\pi }^0$ and Inclusive Photon Second-Harmonic Parameter $v_2$ and Implications for Direct Photon Production in $\sqrt{s_{NN}}=200\mathrm{GeV}$ $\mathrm{Au}+\mathrm{Au}$

The azimuthal distribution of identified ${\pi }^0$ and inclusive photons has been measured in $\sqrt{s_{NN}}=200\mathrm{GeV}$ $\mathrm{Au}+\mathrm{Au}$ collisions with the PHENIX experiment at the Relativistic Heavy-Ion Collider (RHIC). The second-harmonic parameter ($v_2$) was measured to describe the observed anisotropy of the azimuthal distribution. The measured inclusive photon $v_2$ is consistent with the value expected for the photons from hadron decay and is also consistent with the lack of direct photon signal over the measured $p_T$ range $1–6\mathrm{GeV}/c$. An attempt is made to extract $v_2$ of direct photons.

Figure 1

The measured $v_2$ of inclusive photon ($v_2^{\mathrm{inclusive}\gamma }$, solid circles) and $v_2$ of ${\pi }^0$ ($v_2^{{\pi }^0}$, open squares) for 4 centrality selections. The statistical (vertical error bars) and systematic errors (lines) are plotted separately. The highest $p_T$ point corresponds to $4–6\mathrm{GeV}/c$. Charged pion data are from previous measurements [10].

Figure 2

The measured $v_2$ of inclusive photons ($v_2^{\mathrm{inclusive}\gamma }$, solid circles) and expected photon $v_2$ from hadronic decay ($v_2^{\mathrm{bg}}$, open squares). A subtracted $v_2$ quantity $R{v}_2^{\mathrm{inclusive}\gamma }-v_2^{\mathrm{bg}}$ is plotted at the bottom of each panel (open circles), where $R=(N_{\mathrm{direct}\gamma }+N_{\mathrm{bg}})/N_{\mathrm{bg}}$. The quantity corresponds to a product of the direct photon $v_2$ and a positive factor $R-1$, ($v_2^{\mathrm{direct}\gamma }(R-1)$).

Figure 3

The ratio of the hadronic decay photon $v_2$ over inclusive photon $v_2$ ($v_2^{\mathrm{bg}}/v_2^{\mathrm{inclusive}\gamma }$, open circles) compared with the direct photon excess ratio $R=(N_{\mathrm{direct}\gamma }+N_{\mathrm{bg}})/N_{\mathrm{bg}}$, (solid circles).