Forward Neutral Pion Production in $p+p$ and $d+\mathrm{Au}$ Collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$

Measurements of the production of forward ${\pi }^0$ mesons from $p+p$ and $d+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ are reported. The $p+p$ yield generally agrees with next-to-leading order perturbative QCD calculations. The $d+\mathrm{Au}$ yield per binary collision is suppressed as $\eta$ increases, decreasing to $\sim 30\%$ of the $p+p$ yield at $⟨\eta ⟩=4.00$, well below shadowing expectations. Exploratory measurements of azimuthal correlations of the forward ${\pi }^0$ with charged hadrons at $\eta \approx 0$ show a recoil peak in $p+p$ that is suppressed in $d+\mathrm{Au}$ at low pion energy. These observations are qualitatively consistent with a saturation picture of the low-$x$ gluon structure of heavy nuclei.

Figure 1

Inclusive ${\pi }^0$ cross section for $p+p$ collisions vs the leading ${\pi }^0$ energy ($E_{\pi }$) averaged over 5 GeV bins at fixed pseudorapidity ($\eta$). The error bars combine statistical and point-to-point systematic errors. The curves are NLO $p\mathrm{QCD}$ calculations using two sets of fragmentation functions (FF).

Figure 2

Inclusive ${\pi }^0$ cross section per binary collision for $d+\mathrm{Au}$ collisions, as in Fig. 1. The curves are calculations described in the text. (Inset) Diphoton invariant mass spectrum for data (stars), normalized to simulation (histogram).

Figure 3

Nuclear modification factor ($R_{d\mathrm{Au}}$) for minimum-bias $d+\mathrm{Au}$ collisions vs transverse momentum ($p_T$). The solid circles are for ${\pi }^0$ mesons. The open circles and boxes are for negative hadrons [10]. The error bars are statistical, while the shaded boxes are point-to-point systematic errors. (Inset) $R_{d\mathrm{Au}}$ for ${\pi }^0$ mesons with the ratio of curves in Figs. 1 and 2.

Figure 4

Coincidence probability vs azimuthal angle difference between the forward ${\pi }^0$ and a leading charged particle at midrapidity with $p_T>0.5\mathrm{GeV}/c$. The left (right) column are $p+p$ ($d+\mathrm{Au}$) data. The curves are fits described in the text, including the area of the back-to-back peak ($S$).