Exclusive Photoproduction of \u3cem\u3eπ\u3c/em\u3e\u3csup\u3e0\u3c/sup\u3e Up to Large Values of Mandlestam Variables \u3cem\u3es\u3c/em\u3e, \u3cem\u3et\u3c/em\u3e, and \u3cem\u3eu\u3c/em\u3e with CLAS

Abstract

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