Exclusive photoproduction of ${\pi }^0$ up to large values of Mandelstam variables $s,t$ and $u$ with CLAS

Abstract

Exclusive photoproduction cross sections have been measured for the process $\gamma p\to p{\pi }^0(e^{+}{e}^{-}(\gamma ))$ 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 ${\pi }^0$, 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 $s^{-6.89\pm 0.26}$. 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 $s=11$ GeV${}^2$. At the same time, the Regge exchange based models for ${\pi }^0$ photoproduction are more consistent with experimental data.

Comments: 7 pages, 6 figures