Centrality Dependence of Direct Photon Production in sqrt(s_NN) = 200 GeV Au+Au Collisions

Abstract

The first measurement of direct photons in Au+Au collisions at sqrt(s_NN) = 200 GeV is presented. The direct photon signal is extracted as a function of the Au+Au collision centrality and compared to NLO pQCD calculations. The direct photon yield is shown to scale with the number of nucleon-nucleon collisions for all centralities.

Figures (3)

• Figure 1: Double ratio of measured $(\gamma/\pi^0)_{\mathrm{Measured}}$ invariant yield ratio to the background decay $(\gamma/\pi^0)_{\mathrm{Background}}$ ratio as a function of $p_T$ for minimum bias and for five centralities of Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV (0-10% is the most central). Statistical and total errors are indicated separately on each data point by the vertical bar and shaded region, respectively. The solid curves are the ratio of pQCD predictions described in the text to the background photon invariant yield based on the measured $\pi^0$ yield for each centrality class. The shaded region around the curves indicate the variation of the pQCD calculation for scale changes from $p_T/2$ to $2p_T$, plus the $\langle N_{coll}\rangle$ uncertainty.
• Figure 2: Direct $\gamma$ invariant yields as a function of transverse momentum for 9 centrality selections and minimum bias Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. The vertical error bar on each point indicates the total error. Arrows indicate measurements consistent with zero yield with the tail of the arrow indicating the 90% confidence level upper limit. The solid curves are pQCD predictions described in the text.
• Figure 3: Ratio of Au+Au yield to $p+p$ yield normalized by the number of binary nucleon collisions as a function of centrality given by $N_{part}$ for direct $\gamma$ (closed circles) and $\pi^0$ (open circles) yields integrated above 6 GeV$/c$. The $p+p$ direct photon yield is taken as the NLO pQCD prediction described in the text. The error bars indicate the total error excluding the error on $\langle N_{coll} \rangle$ shown by the dashed lines and the scale uncertainty of the NLO calculation shown by the shaded region at the right.