Low-momentum direct-photon measurement in Cu + Cu collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$

We measured direct photons for $p_T<5\mathrm{GeV}/c$ in minimum bias and 0%–40% most-central events at midrapidity for $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. The $e^{+}{e}^{-}$ contribution from quasireal direct virtual photons has been determined as an excess over the known hadronic contributions in the $e^{+}{e}^{-}$ mass distribution. A clear enhancement of photons over the binary scaled $p+p$ fit is observed for $p_T<4\mathrm{GeV}/c$ in $\mathrm{Cu}+\mathrm{Cu}$ data. The $p_T$ spectra are consistent with the $\mathrm{Au}+\mathrm{Au}$ data covering a similar number of participants. The inverse slopes of the exponential fits to the excess after subtraction of the $p+p$ baseline are $285\pm 53\left(\mathrm{stat}\right)\pm 57\left(\mathrm{syst}\right)\mathrm{MeV}/c$ and $333\pm 72\left(\mathrm{stat}\right)\pm 45\left(\mathrm{syst}\right)\mathrm{MeV}/c$ for minimum bias and 0%–40% most-central events, respectively. The rapidity density, $dN/dy$, of photons demonstrates the same power law as a function of $d{N}_{\mathrm{ch}}/d\eta$ observed in $\mathrm{Au}+\mathrm{Au}$ at the same collision energy.

Figure 1

The beam view of the PHENIX detector configuration in 2005.

Figure 2

The $E/p$ distribution for electrons from pairs with $p_T$ of $1-2\mathrm{GeV}/c$ after applying all cuts for electron identification except for $E/p$.

Figure 3

(a) Like-sign and (c) unlike-sign mass distributions of the data together with ${\mathrm{BG}}^{\mathrm{SUM}}$ normalized by the ${\mathrm{BG}}^{\mathrm{SUM}}$ fit for $1<p_T<5\mathrm{GeV}/c$. (b), (d) Ratios of data over ${\mathrm{BG}}^{\mathrm{SUM}}$.

Figure 4

Background pair distributions of $e^{+}{e}^{-}$ determined by like-sign subtraction method, ${\alpha }_{\mathrm{acc}}({\mathrm{FG}}_{--}+{\mathrm{FG}}_{++})$ (circle symbols), and ${\mathrm{BG}}^{\mathrm{SUM}}$ fit method (solid curves) for (a) $p_T=1-2\mathrm{GeV}/c$ and (b) $2\text{–}3\mathrm{GeV}/c$. The resulting contributions to ${\mathrm{BG}}_{+-}^{\mathrm{SUM}}$ are also shown by dashed, dotted, and dashed-dotted curves (see text and legend).

Figure 5

The $e^{+}{e}^{-}$ pair mass distribution in $\mathrm{Cu}+\mathrm{Cu}$ MB collisions for $1.5<p_T<2.0\mathrm{GeV}/c$. (a) The data [closed (black) circles], fit to the data $(1-r_{\gamma })f_c+r_{\gamma }{f}_{\mathrm{dir}}+f_{\mathrm{BG}}$ [thick (red) curve], hadronic contribution [thin (blue) curve], and ${\mathrm{BG}}_{+-}^{\mathrm{SUM}}$ [shaded (red) region]. (b) The data after ${\mathrm{BG}}_{+-}^{\mathrm{SUM}}$ subtraction [closed (black) circles], the fit [thick (red) curve], hadronic contribution [thin (blue) curve], and cocktail components (indicated curves; see legend).

Figure 6

Direct-photon fraction measured with the virtual photon method for different systems in $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$ collisions: (a) $p+p$ [9], (b) $d+\mathrm{Au}$ (MB) [9], (c) $\mathrm{Cu}+\mathrm{Cu}$ (MB), (d) $\mathrm{Au}+\mathrm{Au}$ (MB) [6]. Expectations from NLO pQCD calculations [24] are also shown by curves with different cutoff mass scales $\mu$.

Figure 7

The direct-photon spectra [closed (black) circles] for 200 GeV $\mathrm{Cu}+\mathrm{Cu}$ (a) MB and (b) 0%–40% centralities. The $T_{\mathrm{AA}}$-scaled $p+p$ data and fits together with uncertainties are shown as the open (red) circles symbols and the dotted (blue) curves and accompanying (red) boxes and (blue) bands. $\mathrm{Au}+\mathrm{Au}$ 40%–60% centrality data points, which have a similar $N_{\mathrm{part}}$ as the $\mathrm{Cu}+\mathrm{Cu}$ 0%–40% centrality data, are shown as the open (black) squares, where the $\mathrm{Au}+\mathrm{Au}$ points are scaled by the $N_{\mathrm{part}}$ ratio (66.4/56.0). An exponential fit to the $\mathrm{Cu}+\mathrm{Cu}$ data of the excess yield over the scaled $p+p$ fit [solid (red) curve] yields inverse slopes of $285\pm 53\left(\mathrm{stat}\right)\pm 57\left(\mathrm{syst}\right)\mathrm{MeV}/c$ for MB and $333\pm 72\left(\mathrm{stat}\right)\pm 45\left(\mathrm{syst}\right)\mathrm{MeV}/c$ for 0%–40%.

Figure 8

Rapidity densities of the excess yield of direct photons over the scaled $p+p$ fits for $p_T>1\mathrm{GeV}/c$ at midrapidity as a function of $d{N}_{\mathrm{ch}}/d\eta$. The $\mathrm{Au}+\mathrm{Au}$ data points with different centralities [7] and the power-law fit with the fixed power of 1.25 to both $\mathrm{Cu}+\mathrm{Cu}$ and $\mathrm{Au}+\mathrm{Au}$ data points are shown together.