Disentangling centrality bias and final-state effects in the production of high-$p_T$ $\pi^0$ using direct $\gamma$ in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

• Main Authors: Acharya, U, Babintsev, V, Barish, K. N, Bathe, S, Berdnikov, A, Berdnikov, Y, Blau, D. S, Borisov, V, Brooks, M. L, Roman, V. Canoa, Connors, M, Csanád, M, David, G, DeBlasio, K, Deshpande, A, Drees, A, Durum, A, Fadem, B, Fan, W, Finger,Jr, M, Finger, M, Firak, D, Gal, C, Garg, P, Ge, H, Grau, N, Haggerty, J. S, Hahn, K. I, Han, S. Y, Hasegawa, S, Haseler, T. O. S, Hong, B, Isenhower, D, Ito, Y, Jorjadze, V, Jouan, D, Kim, D. J, Kim, M. H, Kingan, A, Koblesky, T, Kovacs, L, Kurita, K, Lallow, E. O, Lee, S. H, Lewis, N. A, Manko, V. I, Mannel, E, Mignerey, A. C, Milov, A, Mitchell, J. T, Mitrankov, Iu, Mondal, M. M, Moon, T, Morrow, S. I, Muhammad, A, Murakami, T, Murata, J, Nagashima, K, Nakagawa, I, Nakano, K, Nattrass, C, Nouicer, R, Novotny, R, Nyanin, A. S, O'Brien, E, Oh, J, Orosz, M, Osborn, J. D, Oskarsson, A, Park, J. S, Patel, M, Peng, W, Perepelitsa, D. V, Perera, G. D. N, PerezLara, C. E, Petti, R, Phipps, M, Potekhin, M, Rowan, Z, Samsonov, V, Shein, I, Shibata, M, Shibata, T. -A, Silva, C. L, Silvermyr, D, Slunečka, M, Smith, K. L, Syed, S, Tanida, K, Tannenbaum, M. J, Tarnai, G, Tomášek, M, Towell, R. S, Ueda, Y, Vazquez-Carson, S, Velkovska, J, Xu, C, Yamaguchi, Y. L, Yushmanov, I. E, Zajc, W. A
• Format: Journal Article
• Language: English
• Published: 22.03.2023
• Subjects: Physics - Nuclear Experiment
• DOI: 10.48550/arxiv.2303.12899

PHENIX presents a simultaneous measurement of the production of direct $\gamma$ and $\pi^0$ in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV over a $p_T$ range of 7.5 to 18 GeV/$c$ for different event samples selected by event activity, i.e. charged-particle multiplicity detected at forward rapidity. Direct-photon yields are used to empirically estimate the contribution of hard-scattering processes in the different event samples. Using this estimate, the average nuclear-modification factor $R_{d\rm Au,EXP}^{\gamma^{\rm dir}}$ is $0.925{\pm}0.023({\rm stat}){\pm}0.15^{\rm (scale)}$, consistent with unity for minimum-bias (MB) $d$$+$Au events. For event classes with moderate event activity, $R_{d\rm Au,EXP}^{\gamma^{\rm dir}}$ is consistent with the MB value within 5\% uncertainty. These results confirm that the previously observed enhancement of high-$p_T$ $\pi^0$ 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, $R_{d\rm Au,EXP}^{\gamma^{\rm dir}}$ is suppressed by 20\% relative to the MB value with a significance of $4.5\sigma$, which may be due to final-state effects.