Suppression of back-to-back hadron pairs at forward rapidity in d+Au Collisions at sqrt(s_NN)=200 GeV

TL;DR

Back-to-back hadron pair yields in d+Au and p+p collisions at √s(NN)=200 GeV were measured with the PHENIX detector at the Relativistic Heavy Ion Collider and the observed suppression as a function of nuclear thickness, p(T), and η points to cold nuclear matter effects arising at high parton densities.

Abstract

Back-to-back hadron pair yields in d+Au and p+p collisions at sqrt(s_NN)=200 GeV were measured with the PHENIX detector at the Relativistic Heavy Ion Collider. Rapidity separated hadron pairs were detected with the trigger hadron at pseudorapidity |eta|<0.35 and the associated hadron at forward rapidity (deuteron direction, 3.0<eta<3.8). Pairs were also detected with both hadrons measured at forward rapidity; in this case the yield of back-to-back hadron pairs in d+Au collisions with small impact parameters is observed to be suppressed by a factor of 10 relative to p+p collisions. The kinematics of these pairs is expected to probe partons in the Au nucleus with low fraction x of the nucleon momenta, where the gluon densities rise sharply. The observed suppression as a function of nuclear thickness, p_T, and eta points to cold nuclear matter effects arising at high parton densities.

Figures (3)

• Figure 1: (color online). Pedestal-subtracted $\pi^{0}$-$\pi^{0}$ per-trigger correlation functions for, as indicated, $p$+$p$, $d+$Au peripheral (60--88% centrality) and $d+$Au central (0--20% centrality) collisions at $\sqrt{s_{NN}}=200$ GeV; the associated $\pi^{0}$'s of $p_T=0.5$--0.75 GeV/$c$ are measured at forward rapidity $(3.0<\eta<3.8)$ and the triggered $\pi^{0}$'s are measured at midrapidity $(|\eta|<0.35)$ for the indicated $p_T$ ranges. The subtracted pedestal values, b$_0$, are also indicated.
• Figure 3: (color online). Relative yield $J_{dA}$ versus $\langle\hbox{$N_{\rm coll}$}\xspace\rangle$ for forward-rapidity ($3.0<{\eta}<3.8$) $\pi^{0}$'s paired with (left) midrapidity ($|\eta|<0.35$) hadrons and $\pi^{0}$'s and (right) forward-rapidity ($3.0<{\eta}<3.8$) cluster-$\pi^{0}$ pairs for the indicated combinations of $p_T$ ranges. Also plotted as inverted solid triangles are the values of the forward $\pi^0$$R_{dA}$. Around each data point the vertical bars indicate statistical uncertainties and the open boxes indicate point-to-point systematic uncertainties. The gray bar at the left in each panel represents a global systematic scale uncertainty of 9.7%. Additional centrality dependent systematic uncertainties of 7.5%, 5.1%, 4.1%, and 4.8% for the peripheral to central bins, respectively, are not shown. The $\langle \hbox{$N_{\rm coll}$}\xspace \rangle$ values within a centrality selection are offset from their actual values for visual clarity (see text for actual $\langle\hbox{$N_{\rm coll}$}\xspace\rangle$ values).
• Figure 4: (color online). $J_{dA}$ versus $x_{\rm Au}^{\rm frag}$ for peripheral (60--88%) and central (0--20%) $d+$Au collisions at $\sqrt{s_{NN}}=200$ GeV. The statistical error bars and systematic uncertainty boxes are the same as in Fig. \ref{['fig:jda']}. Above $x_{\rm Au}^{\rm frag}>10^{-3}$, some data points were offset from their true $x_{\rm Au}^{\rm frag}$ to avoid overlap. The leftmost point in each group of three is at the correct $x_{\rm Au}^{\rm frag}$.