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.

Abilene Christian University Abilene Texas 79699 USA

Advanced Science Research Center Japan Atomic Energy Agency 2 4 Shirakata Shirane Tokai mura Naka gun Ibaraki ken 319 1195 Japan

Baruch College City University of New York New York New York 10010 USA

Bhabha Atomic Research Centre Bombay 400 085 India

Brookhaven National Laboratory Collider Accelerator Department Upton New York 11973 5000 USA

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Center for Nuclear Study Graduate School of Science University of Tokyo 7 3 1 Hongo Bunkyo Tokyo 113 0033 Japan

Charles University Faculty of Mathematics and Physics 180 00 Troja Prague Czech Republic

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IPN Orsay Univ Paris Sud CNRS IN2P3 Université Paris Saclay BP1 F 91406 Orsay France

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Jeonbuk National University Jeonju 54896 Korea

KEK High Energy Accelerator Research Organization Tsukuba Ibaraki 305 0801 Japan

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PNPI Petersburg Nuclear Physics Institute Gatchina Leningrad region 188300 Russia

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RIKEN Nishina Center for Accelerator Based Science Wako Saitama 351 0198 Japan

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Tomonaga Center for the History of the Universe University of Tsukuba Tsukuba Ibaraki 305 Japan

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