Measurement of Direct Photons in $\mathrm{Au}\mathbf{+}\mathrm{Au}$ Collisions at $\sqrt{s_{NN}}\mathbf{=}200\mathrm{GeV}$

We report the measurement of direct photons at midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The direct photon signal was extracted for the transverse momentum range of $4\mathrm{GeV}/c<p_T<22\mathrm{GeV}/c$, using a statistical method to subtract decay photons from the inclusive photon sample. The direct photon nuclear modification factor $R_{\mathrm{AA}}$ was calculated as a function of $p_T$ for different $\mathrm{Au}+\mathrm{Au}$ collision centralities using the measured $p+p$ direct photon spectrum and compared to theoretical predictions. $R_{\mathrm{AA}}$ was found to be consistent with unity for all centralities over the entire measured $p_T$ range. Theoretical models that account for modifications of initial direct photon production due to modified parton distribution functions in Au and the different isospin composition of the nuclei predict a modest change of $R_{\mathrm{AA}}$ from unity. They are consistent with the data. Models with compensating effects of the quark-gluon plasma on high-energy photons, such as suppression of jet-fragmentation photons and induced-photon bremsstrahlung from partons traversing the medium, are also consistent with this measurement.

Figure 1

Ratio $R_{\gamma }$ for different centrality selections, for the PbGl and the PbSc analysis. The error bars indicate point-to-point uncertainties, the boxes around the points indicate $p_T$ correlated uncertainties which are also mostly correlated between centralities.

Figure 2

(a) Direct photon spectra for all centrality selections in $\mathrm{Au}+\mathrm{Au}$, and for $p+p$ measured in [23]. The error bars indicate point-to-point uncertainties, the boxes around the points indicate $p_T$-correlated uncertainties. The lines depict a $T_{\mathrm{AA}}$ scaled fit for $p_T>8\mathrm{GeV}/c$ to the $p+p$ cross section, they are dashed for the range where the fit is extrapolated to lower $p_T$. (b) Comparison of the PbGl and PbSc results with the combined result for the 0–5% most central events. The error bars show the total uncertainties.

Figure 3

Direct photon nuclear modification factor $R_{\mathrm{AA}}$ for three different centrality selections. The error bars show point-to-point uncertainties, the boxes around the points depict $p_T$ correlated uncertainties. The boxes on the left show the uncertainty of the total inelastic $p+p$ cross section, the boxes on the right show the uncertainty in $N_{\mathrm{coll}}$. Note that all errors from the $p+p$ reference spectrum are correlated between the centralities.

Figure 4

Direct photon nuclear modification factor $R_{\mathrm{AA}}$ for 0%–5% most central events, compared with theoretical calculations [2, 3, 4, 5] for different scenarios. The boxes depict the same uncertainties as in Fig. 3. Note that the EPS09 PDF curve is calculated for minimum bias collisions.