Disentangling Centrality Bias and Final-State Effects in the Production of High-pT Neutral Pions Using Direct Photon in d plus Au Collisions at √sNN=200 GeV

Abstract

PHENIX presents a simultaneous measurement of the production of direct gamma and pi(0) in d + Au collisions at root S-NN = 200 GeV over a p(T) range of 7.5 to 18 GeV/c for different event samples selected by event activity, i.e., charged-particle multiplicity detected at forward rapidity. Direct-photon yields are used to empirically estimate the contribution of hard-scattering processes in the different event samples. Using this estimate, the average nuclear-modification factor, R-dAu,EXP(pi 0) is 0.925 +/- 0.023(stat) +/- 0.15(scale), consistent with unity for minimum- bias (MB) d + Au collisions. For event classes with low and moderate event activity, R-dAu,EXP(pi 0) is consistent with the MB value within 5% uncertainty. This result confirms that the previously observed enhancement of high-pT pi(0) production found in small-system collisions with low event activity is a result of a bias in interpreting event activity within the Glauber framework. In contrast, for the top 5% of events with the highest event activity, R-dAu,EXP(pi 0) is suppressed by 20% relative to the MB value with a significance of 4.5s, which may be due to final-state effects. This suppression corresponds to a p(T) shift of delta p(T) = 0.213 +/- 0.055 Gev/c at 9 Gev/c.