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

Exclusive photoproduction cross sections have been measured for the process \ud γ\ud p\ud →\ud p\ud π\ud 0\ud [\ud e\ud +\ud e\ud −\ud (\ud γ\ud )\ud ]\ud with the Dalitz decay final state using tagged photon energies in the range of \ud E\ud γ\ud =\ud 1.275\ud –\ud 5.425\ud GeV. The complete angular distribution of the final state \ud π\ud 0\ud , for the entire photon energy range up to large values of \ud t\ud and \ud u\ud , has been measured for the first time. The data obtained show that the cross section \ud d\ud σ\ud /\ud d\ud t\ud , at mid to large angles, decreases with energy as \ud s\ud −\ud 6.89\ud ±\ud 0.26\ud . This is in agreement with the perturbative QCD quark counting rule prediction of \ud s\ud −\ud 7\ud . 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 \ud s\ud =\ud 11\ud GeV\ud 2\ud . At the same time, the Regge-exchange-based models for \ud π\ud 0\ud photoproduction are more consistent with experimental data.