Exclusive photoproduction cross sections have been measured for the process γp ->0 [e+ e- (γ)] with the Dalitz decay final state using tagged photon energies in the range of = 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.

Document Type


Publication Date


Publisher Statement

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.

Please note that downloads of the article are for private/personal use only.

Included in

Physics Commons