Forward Neutral Pion Production in p+p and d+Au Collisions at sqrt(s_NN)=200 GeV

Abstract

Measurements of the production of forward pi0 mesons from p+p and d+Au collisions at sqrt(s_NN)=200 GeV are reported. The p+p yield generally agrees with next-to-leading order perturbative QCD calculations. The d+Au yield per binary collision is suppressed as eta increases, decreasing to ~30% of the p+p yield at <eta>=4.00, well below shadowing expectations. Exploratory measurements of azimuthal correlations of the forward pi0 with charged hadrons at eta~0 show a recoil peak in p+p that is suppressed in d+Au at low pion energy. These observations are qualitatively consistent with a saturation picture of the low-x gluon structure of heavy nuclei.

Figures (4)

• Figure 1: Inclusive $\pi^0$ cross section for p+p collisions versus 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 pQCD calculations using two sets of fragmentation functions (FF).
• Figure 2: Inclusive $\pi^0$ cross section per binary collision for d+Au collisions, as in Fig. \ref{['fig:inclusive']}. 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 ($R_{\rm dAu}$) for minimum-bias d+Au collisions versus transverse momentum ($p_T$). The solid circles are for $\pi^0$ mesons. The open circles and boxes are for negative hadrons BRAHMS. The error bars are statistical, while the shaded boxes are point-to-point systematic errors. (Inset) $R_{\rm dAu}$ for $\pi^0$ mesons with the ratio of curves in Figs. \ref{['fig:dau']} and \ref{['fig:inclusive']}.
• Figure 4: Coincidence probability versus azimuthal angle difference between the forward $\pi^0$ and a leading charged particle at midrapidity with $p_T>0.5\,$GeV/c. The left (right) column is p+p (d+Au) data. The curves are fits described in the text, including the area of the back-to-back peak ($S$).