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 = 11 GeV2. At the same time, the Regge-exchange-based models for π0 photoproduction are more consistent with experimental data.
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Funded by SCOAP3.
This work was supported in part by the Italian Istituto Nazionale di Fisica Nucleare, the French Centre National de la Recherche Scientifique and Commissariat à l’Energie Atomique, the United Kingdom’s Science and Technology Facilities Council (STFC), the US DOE and NSF, and the National Research Foundation of Korea. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract No. DE-AC05-06OR23177.
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.; and Gohn, Wesley P., "Exclusive Photoproduction of π0 Up to Large Values of Mandlestam Variables s, t, and u with CLAS" (2018). Physics and Astronomy Faculty Publications. 617.