Cross-Section Measurement of Charged-Pion Photoproduction from Hydrogen and Deuterium

We have measured the differential cross section for the γn→π^-p and γp→π⁺n reactions at θ_c.m.=90° in the photon energy range from 1.1 to 5.5GeV at Jefferson Lab (JLab). The data at E_γ≳3.3 GeV exhibit a global scaling behavior for both π^- and π⁺ photoproduction, consistent with the constituent counting rule and the existing π⁺ photoproduction data. Possible oscillations around the scaling value are suggested by these new data. The data show enhancement in the scaled cross section at a center-of-mass energy near 2.2GeV. The cross section ratio of exclusive π^- to π⁺ photoproduction at high energy is consistent with the prediction based on one-hard-gluon-exchange diagrams.