Disentangling Centrality Bias and Final-State Effects in the Production of High- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>p</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> Neutral Pions Using Direct Photon in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>d</mml:mi> <mml:mo>+</mml:mo> <mml:mi>Au</mml:mi> </mml:mrow> </mml:math> Collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msqrt> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>N</mml:mi> <mml:mi>N</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msqrt> <mml:mo>=</mml:mo> <mml:mn>200</mml:mn> <mml:mtext> </mml:mtext> <mml:mtext> </mml:mtext> <mml:mi>GeV</mml:mi> </mml:mrow> </mml:math>
N. J. Abdulameer, U. Acharya, C. Aidala, Y. Akiba, M. Alfred, K. Aoki, N Apadula, C. Ayuso, V. Babintsev, K. N. Barish, S. Bathe, A. Bazilevsky, R. Belmont, A. Berdnikov, Y. Berdnikov, L. Bichon, B. Blankenship, D. Blau, M. Boër, J. S. Bok, V. Borisov, M. L. Brooks, J. Bryslawskyj, V. Bumazhnov, C. Butler, S. Campbell, V. Canoa Roman, M. Chiu, M. Connors, R Corliss, Y. Corrales Morales, M. Csanád, T. Csörgő, L. D. Liu, T. W. Danley, M. S. Daugherity, G. David, C. T. Dean, K. DeBlasio, K. Dehmelt, Alexander Denisov, Abhay Deshpande, E. J. Desmond, V. Doomra, J. H., A. Drees, K. A. Drees, M. Dumancic, J. M. Durham, A. Durum, T. Elder, A. Enokizono, R. Esha, B. Fadem, W. Fan, N. Feege, M. Finger, M. Finger, D. Firak, D. Fitzgerald, S. Fokin, J. E. Frantz, A. Franz, A. D. Frawley, Y. Fukuda, C. Gal, P. Garg, H. Ge, M. Giles, Y. Goto, N. Grau, S. V. Greene, T. Gunji, T. Hachiya, J. S. Haggerty, K. I. Hahn, S. Y. Han, M. Harvey, S. Hasegawa, T. O. S. Haseler, T. K. Hemmick, X. He, K. Hill, A. Hodges, K. Homma, B. Hong, T. Hoshino, N. Hotvedt, J. Huang, J. Imrek, M. Inaba, D. Isenhower, Y. Ito, D. Ivanishchev, B. V. Jacak, Z. Ji, B. M. Johnson, V. Jorjadze, D. Jouan, D. S. Jumper
Abstract
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 $9\text{ }\text{ }\mathrm{Gev}/c$.