Exclusive photoproduction of π0 up to large values of Mandelstam variables s, t, and u with CLAS

Kunkel, M. C.; Amaryan, M. J.; Strakovsky, I. I.; Ritman, J.; Goldstein, G. R.; Adhikari, K. P.; Adhikari, S.; Avakian, H.; Ball, J.; Balossino, I.; Barion, L.; Battaglieri, M.; Batourine, V.; Bedlinskiy, I.; Biselli, A. S.; Boiarinov, S.; Briscoe, W. J.; Brooks, W. K.; Bültmann, S.; Burkert, V. D.; Cao, F.; Carman, D. S.; Celentano, A.; Charles, G.; Chetry, T.; Ciullo, G.; Clark, L.; Cole, P. L.; Contalbrigo, M.; Cortes, O.; Crede, V.; D'Angelo, A.; Dashyan, N.; De Vita, R.; De Sanctis, E.; Degtyarenko, P. V.; Defurne, M.; Deur, A.; Djalali, C.; Dugger, M.; Dupre, R.; Egiyan, H.; El Alaoui, A.; El Fassi, L.; Elouadrhiri, L.; Eugenio, P.; Fedotov, G.; Fersch, R.; Filippi, A.; Fradi, A.; Gavalian, G.; Ghandilyan, Y.; Ghosh, S.; Gilfoyle, G. P.; Giovanetti, K. L.; Girod, F. X.; Glazier, D. I.; Gohn, W.; Golovatch, E.; Gothe, R. W.; Griffioen, K. A.; Guo, L.; Guidal, M.; Hafidi, K.; Hakobyan, H.; Harrison, N.; Hattawy, M.; Hicks, K.; Holtrop, M.; Hyde, C. E.; Ireland, D. G.; Ishkhanov, B. S.; Isupov, E. L.; Jenkins, D.; Joo, K.; Kabir, M. L.; Keller, D.; Khachatryan, G.; Khachatryan, M.; Khandaker, M.; Kim, A.; Kim, W.; Klein, A.; Klein, F.; Kubarovsky, V.; Kuhn, S. E.; Laget, J. M.; Lanza, L.; Lenisa, P.; Lersch, D.; Livingston, K.; Macgregor, I. J. D.; Markov, N.; Mbianda Njencheu, G.; Mckinnon, B.; Mineeva, T.; Mokeev, V.; Montgomery, R. A.; Movsisyan, A.; Munoz Camacho, C.; Nadel-Turonski, P.; Niccolai, S.; Niculescu, G.; Osipenko, M.; Ostrovidov, A. I.; Paolone, M.; Park, K.; Pasyuk, E.; Payette, D.; Phelps, W.; Pogorelko, O.; Poudel, J.; Price, J. W.; Procureur, S.; Prok, Y.; Protopopescu, D.; Ripani, M.; Ritchie, B. G.; Rizzo, A.; Rosner, G.; Roy, A.; Sabatié, F.; Salgado, C.; Schadmand, S.; Schumacher, R. A.; Sharabian, Y. G.; Skorodumina, Iu.; Stepanyan, S.; Ireland, D. G.; Sokhan, D.; Sober, D. I.; Sparveris, N.; Strauch, S.; Taiuti, M.; Tan, J. A.; Ungaro, M.; Voskanyan, H.; Voutier, E.; Watts, D. P.; Weinstein, L.; Wei, X.; Weygand, D. P.; Zachariou, N.; Zhang, J.; Zhao, Z. W.

doi:10.1103/PhysRevC.98.015207

Exclusive photoproduction cross sections have been measured for the process γp→pπ0[e+e-(γ)] with the Dalitz decay final state using tagged photon energies in the range of Eγ=1.275-5.425 GeV. The complete angular distribution of the final state π0, for the entire photon energy range up to large values of t and u, has been measured for the first time. The data obtained show that the cross section dσ/dt, at mid to large angles, decreases with energy as s-6.89±0.26. This is in agreement with the perturbative QCD quark counting rule prediction of s-7. Paradoxically, the size of angular distribution of measured cross sections is greatly underestimated by the QCD-based generalized parton distribution mechanism at highest available invariant energy s=11GeV2. At the same time, the Regge-exchange-based models for π0 photoproduction are more consistent with experimental data.