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.
Copyright © 2018 American Physical Society. This article first appeared in Physical Review C 98, no. 1 (July 25, 2018): 015207: 1-8. doi: 10.1103/PhysRevC.98.015207.
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Kunkel, M. C., M. J. Amaryan, I. I. Strakovsky, J. Ritman, G. R. Goldstein, K. P. Adhikari, S. Adhikari, G. Gilfoyle, et al. “Exclusive Photoproduction of Pi Degrees up to Large Values of Mandelstam Variables s, t, and u with CLAS.” Physical Review C 98, no. 1 (July 25, 2018): 015207. https://doi.org/10.1103/PhysRevC.98.015207.