Disentangling Centrality Bias and Final-State Effects in the Production of High-p_{T} Neutral Pions Using Direct...

PHENIX presents a simultaneous measurement of the production of direct γ and π^{0} in d+Au collisions at sqrt[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}^{π^{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}^{π^{0}} is consistent with the MB value within 5% uncertainty. This result confirms that the previously observed enhancement of high-p_{T} π^{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}^{π^{0}} is suppressed by 20% relative to the MB value with a significance of 4.5σ, which may be due to final-state effects. This suppression corresponds to a p_{T} shift of δp_{T}=0.213±0.055  Gev/c at 9  Gev/c.