Disentangling Centrality Bias and Final-State Effects in the Production of High- p T Neutral Pions Using Direct Photon in d + Au Collisions at s N N = 200 GeV

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PHENIX presents a simultaneous measurement of the production of direct $\ensuremath{\gamma}$ and ${\ensuremath{\pi}}^{0}$ in $d+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{GeV}$ over a ${p}_{T}$ range of 7.5 to $18\text{ }\text{ }\mathrm{GeV}/c$ 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, ${R}_{d\mathrm{Au},\mathrm{EXP}}^{{\ensuremath{\pi}}^{0}}$, is $0.925\ifmmode\pm\else\textpm\fi{}0.023(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.15(\mathrm{scale})$, consistent with unity for minimum-bias (MB) $d+\mathrm{Au}$ collisions. For event classes with low and moderate event activity, ${R}_{d\mathrm{Au},\mathrm{EXP}}^{{\ensuremath{\pi}}^{0}}$ is consistent with the MB value within 5% uncertainty. This result confirms that the previously observed enhancement of high-${p}_{T}$ ${\ensuremath{\pi}}^{0}$ 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, ${R}_{d\mathrm{Au},\mathrm{EXP}}^{{\ensuremath{\pi}}^{0}}$ is suppressed by 20% relative to the MB value with a significance of $4.5\ensuremath{\sigma}$, which may be due to final-state effects. This suppression corresponds to a ${p}_{T}$ shift of $\ensuremath{\delta}{p}_{T}=0.213\ifmmode\pm\else\textpm\fi{}0.055\text{ }\text{ }\mathrm{Gev}/c$ at $9\text{ }\text{ }\mathrm{Gev}/c$.

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