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  • Exclusive photoproduction of \(\pi^0\) up to large values of Mandelstam variables \(s, t\) and \(u\) with CLAS
    Kunkel, M C; Amaryan, M J; Ritman, J; Goldstein, G R; Adhikari, S; Avakian, H; Balossino, I; Barion, L; Battaglieri, M; Batourine, V; Bedlinskiy, I; Biselli, A S; Boiarinov, S; Brooks, W K; Bueltmann, S; Burkert, V D; Cao, F; Carman, D S; Celentano, A; Charles, G; Chetry, T; Clark, L; Cole, P L; Contalbrigo, M; Dashyan, N; De Vita, R; De Sanctis, E; Degtyarenko, P V; Defurne, M; Deur, A; Dugger, M; Dupre, R; Egiyan, H; L El Fassi; Elouadrhiri, L; Fedotov, G; Fersch, R; Filippi, A; Fradi, A; Gavalian, G; Ghosh, S; Giovanetti, K L; Girod, F X; Gohn, W; Gothe, R W; Griffioen, K A; Guo, L; Guidal, M; Hakobyan, H; Harrison, N; Hattawy, M; Hyde, C E; Ireland, D G; Ishkhanov, B S; Jenkins, D; Kabir, M L; Khachatryan, G; Kim, A; Kim, W; Klein, A; Kubarovsky, V; Kuhn, S E; Lanza, L; Lenisa, P; Mbianda, G; McKinnon, B; Mokeev, V; C Munoz Camacho; Nadel-Turonski, P; Niccolai, S; Niculescu, G; Osipenko, M; Ostrovidov, A I; Paolone, M; Park, K; Payette, D; Phelps, W; Procureur, S; Protopopescu, D; Ripani, M; Ritchie, B G; Rosner, G; Roy, A; Sabatie, F; Salgado, C; Schadmand, S; Sharabian, Y G; Skorodumina, Iu; Sokhan, D; Sober, D I; Strauch, S; Taiuti, M; Ungaro, M; Voskanyan, H; Voutier, E; Watts, D P; Weinstein, L; Wei, X; Zachariou, N; The CLAS Collaboration

    arXiv.org, 12/2017
    Paper

    Exclusive photoproduction cross sections have been measured for the process \(\gamma p \rightarrow p\pi^0(e^+e^-(\gamma))\) with the Dalitz decay final state using tagged photon energies in the range of \(E_{\gamma} = 1.275-5.425\) GeV. The complete angular distribution of the final state \(\pi^0\), 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 \(d\sigma/dt\), at mid to large angles, decreases with energy as \(s^{-6.89\pm 0.26} \). 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 \(s=11\) GeV\(^2\). At the same time, the Regge exchange based models for \(\pi^0\) photoproduction are more consistent with experimental data.

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