The fundamental γN → πN processes at JLab energies

The γn → π-p and γp → π+n reactions are essential probes of the transition from meson-nucleon degrees of freedom to quark-gluon degrees of freedom in exclusive processes. The cross sections of these processes are also advantageous, for investigation of the oscillatory behavior around the quark counting prediction, since they decrease relatively slower with energy compared with other photon-induced processes. Moreover, these photoreactions in nuclei can probe the QCD nuclear filtering effects. In this talk, I discuss the preliminary results on the γn → π-p process at a center-of-mass angle of 90deg from Jefferson Lab experiment E94-104. I also discuss a new proposal in which singles γp → π+n measurement from hydrogen, and coincidence γn → π-p measurements at the quasifree kinematics from deuterium and 12C for photon energies between 2.25 GeV to 5.8 GeV in fine steps at a center-of-mass angle of 90deg are planned. The proposed measurement will allow the detailed investigation of the oscillatory scaling behavior in the photopion production differential cross-section and the study of the nuclear dependence of rather mysterious oscillations with energy that previous experiments have indicated. The various nuclear and perturbative QCD approaches, ranging from Glauber theory, to quark-counting, to Sudakov-corrected independent scattering, make dramatically different predictions for the experimental outcomes. (author)