Scaling Properties of Proton and Antiproton Production in $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$ $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ Collisions

We report on the yield of protons and antiprotons, as a function of centrality and transverse momentum, in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$ measured at midrapidity by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider. In central collisions at intermediate transverse momenta ($1.5<p_T<4.5\mathrm{G}\mathrm{e}\mathrm{V}/c$) a significant fraction of all produced particles are protons and antiprotons. They show a centrality-scaling behavior different from that of pions. The $\overline{p}/\pi$ and $p/\pi$ ratios are enhanced compared to peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$, $p+p$, and $e^{+}{e}^{-}$ collisions. This enhancement is limited to $p_T<5\mathrm{G}\mathrm{e}\mathrm{V}/c$ as deduced from the ratio of charged hadrons to ${\pi }^0$ measured in the range $1.5<p_T<9\mathrm{G}\mathrm{e}\mathrm{V}/c$.

Figure 1

$p/\pi$ (left) and $\overline{p}/\pi$ ratios for central (0–10%), midcentral (20–30%), and peripheral (60–92%) $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$. Open (filled) points are for ${\pi }^{\pm }$ (${\pi }^0$), respectively. Data from $\sqrt{s}=53\mathrm{G}\mathrm{e}\mathrm{V}$ $p+p$ collisions [7] are shown with stars. The dashed and dotted lines are $(\overline{p}+p)/({\pi }^{+}+{\pi }^{-})$ ratios in gluon and in quark jets [8].

Figure 2

$p$ and $\overline{p}$ invariant yields scaled by $N_{\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}}$. Error bars are statistical. Systematic errors on $N_{\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}}$ range from $\sim 10\%$ for central to $\sim 28\%$ for 60–92% centrality. Multiplicity dependent normalization errors are $\sim 3\%$.

Figure 3

Nuclear modification factor $R_{\mathrm{C}\mathrm{P}}$ for $(p+\overline{p})/2$ (filled circles) and ${\pi }^0$. Dashed and dotted lines indicate $N_{\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}}$ and $N_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}}$ scaling; the shaded bars show the systematic errors on these quantities.

Figure 4

Charged hadron to ${\pi }^0$ ratio in central (0–10% squares) and peripheral (60–92% circles) $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions. The peripheral data points are offset by $+130\mathrm{M}\mathrm{e}\mathrm{V}/c$ for clarity. The line at 1.6 is the $h/\pi$ ratio measured in $p+p$ collisions [7]. The lower panel shows the fractional normalization error common to both centrality selections (solid line) and the relative error between the two (dashed line).