Fig. 2. (Color online.) Event-by-event photon multiplicity distributions (solid circles) for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ at $\sqrt{s_{\mathrm{NN}}}=62.4$ and $200 \mathrm{GeV} .$ The distributions for top $0-5 \%$ central $\mathrm{Au}+$ Au collisions and top $0-10 \%$ central $\mathrm{Cu}+\mathrm{Cu}$ collisions are also shown (open circles). The photon multiplicity distributions for central collisions are observed to be Gaussian (solid line). Only statistical errors are shown. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.

Fig. 2. (Color online.) Event-by-event photon multiplicity distributions (solid circles) for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ at $\sqrt{s_{\mathrm{NN}}}=62.4$ and $200 \mathrm{GeV} .$ The distributions for top $0-5 \%$ central $\mathrm{Au}+$ Au collisions and top $0-10 \%$ central $\mathrm{Cu}+\mathrm{Cu}$ collisions are also shown (open circles). The photon multiplicity distributions for central collisions are observed to be Gaussian (solid line). Only statistical errors are shown. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.

Fig. 2. (Color online.) Event-by-event photon multiplicity distributions (solid circles) for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ at $\sqrt{s_{\mathrm{NN}}}=62.4$ and $200 \mathrm{GeV} .$ The distributions for top $0-5 \%$ central $\mathrm{Au}+$ Au collisions and top $0-10 \%$ central $\mathrm{Cu}+\mathrm{Cu}$ collisions are also shown (open circles). The photon multiplicity distributions for central collisions are observed to be Gaussian (solid line). Only statistical errors are shown. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.

Fig. 3. (Color online.) Photon pseudorapidity distributions for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ at $\sqrt{\mathrm{s}_{\mathrm{NN}}}=62.4$ and $200\mathrm{GeV}$. The results for several centrality classes are shown. The solid curves are results of HIJING simulations for central $(0-5 \%$ for $\mathrm{Au}+\mathrm{Au}$ and $0-10 \%$ for $\mathrm{Cu}+\mathrm{Cu})$ and $30-40 \%$ mid-central collisions. The errors shown are systematic, statistical errors are negligible in comparison. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.

Fig. 3. (Color online.) Photon pseudorapidity distributions for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ at $\sqrt{\mathrm{s}_{\mathrm{NN}}}=62.4$ and $200\mathrm{GeV}$. The results for several centrality classes are shown. The solid curves are results of HIJING simulations for central $(0-5 \%$ for $\mathrm{Au}+\mathrm{Au}$ and $0-10 \%$ for $\mathrm{Cu}+\mathrm{Cu})$ and $30-40 \%$ mid-central collisions. The errors shown are systematic, statistical errors are negligible in comparison. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.

Fig. 3. (Color online.) Photon pseudorapidity distributions for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ at $\sqrt{\mathrm{s}_{\mathrm{NN}}}=62.4$ and $200\mathrm{GeV}$. The results for several centrality classes are shown. The solid curves are results of HIJING simulations for central $(0-5 \%$ for $\mathrm{Au}+\mathrm{Au}$ and $0-10 \%$ for $\mathrm{Cu}+\mathrm{Cu})$ and $30-40 \%$ mid-central collisions. The errors shown are systematic, statistical errors are negligible in comparison. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.

Fig. 3. (Color online.) Photon pseudorapidity distributions for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ at $\sqrt{\mathrm{s}_{\mathrm{NN}}}=62.4$ and $200\mathrm{GeV}$. The results for several classes are shown. The solid curves are results of HIJING simulations for central $(0-5 \%$ for $\mathrm{Au}+\mathrm{Au}$ and $0-10 \%$ for $\mathrm{Cu}+\mathrm{Cu})$ and $30-40 \%$ mid-central collisions. The errors shown are systematic, statistical errors are negligible in comparison. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.

Fig. 4. (Color online.) Top panel: The number of photons divided by $\left\langle N_{\text{part}}\right\rangle / 2$ as a function of average number of participating nucleons for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ at $\sqrt{\mathrm{s} _{\mathrm{NN}}}=62.4$ and $200 \mathrm{GeV}$ for $-3.7<\eta<-2.3 .$ Errors shown are systematic only and include those for $\left\langle N_{\text {part }}\right\rangle .$ Results from HIJING are shown as lines (solid for $\mathrm{Au}+\mathrm{Au}$ and dashed for $\mathrm{Cu}+\mathrm{Cu}$ ). Bottom panel: Same as above, for both photons and charged particles from PHOBOS [10]. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.

Fig. 4. (Color online.) Top panel: The number of photons divided by $\left\langle N_{\text{part}}\right\rangle / 2$ as a function of average number of participating nucleons for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ at $\sqrt{\mathrm{s} _{\mathrm{NN}}}=62.4$ and $200 \mathrm{GeV}$ for $-3.7<\eta<-2.3 .$ Errors shown are systematic only and include those for $\left\langle N_{\text {part }}\right\rangle .$ Results from HIJING are shown as lines (solid for $\mathrm{Au}+\mathrm{Au}$ and dashed for $\mathrm{Cu}+\mathrm{Cu}$ ). Bottom panel: Same as above, for both photons and charged particles from PHOBOS [10]. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.

Fig. 5. (Color online.) Photon pseudorapidity distributions normalized by the average number of participating nucleon pairs for different collision centralities are plotted as a function of pseudorapidity shifted by the beam rapidity (-5.36 for $200 \mathrm{GeV}$ and -4.19 for $62.4 \mathrm{GeV}$ ) for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{\mathrm{NN}}}=62.4$ and $200 \mathrm{GeV}$. Errors are systematic only, statistical errors are negligible in comparison. For clarity of presentation, results for only four centralities are shown. The $\mathrm{Cu}+$ Cu data are shifted by 0.1 unit in $\eta-y_{\text {beam }} .$ The solid line is a second order polynomial fit to the data (see text for details).

Fig. 5. (Color online.) Photon pseudorapidity distributions normalized by the average number of participating nucleon pairs for different collision centralities are plotted as a function of pseudorapidity shifted by the beam rapidity (-5.36 for $200 \mathrm{GeV}$ and -4.19 for $62.4 \mathrm{GeV}$ ) for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{\mathrm{NN}}}=62.4$ and $200 \mathrm{GeV}$. Errors are systematic only, statistical errors are negligible in comparison. For clarity of presentation, results for only four centralities are shown. The $\mathrm{Cu}+$ Cu data are shifted by 0.1 unit in $\eta-y_{\text {beam }} .$ The solid line is a second order polynomial fit to the data (see text for details).