Exclusive Photoproduction Of Pi Degrees Up To Large Values Of Mandelstam Variables S, T, And U With Clas

Exclusive photoproduction cross sections have been measured for the process gamma p -> p pi degrees[e(+) e(-)(gamma)] with the Dalitz decay final state using tagged photon energies in the range of E-gamma = 1.275-5.425 GeV. The complete angular distribution of the final state pi degrees, 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 d sigma / 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 pi degrees photoproduction are more consistent with experimental data.