Zhu, L.Y.Arrington, J.Averett, T.Beise, E.Calarco, J.Chang, T.Chen, J.P.Chudakov, E.Coman, M.Clasie, B.Crawford, C.Dieterich, S.Dohrmann, F.Dutta, D.Fissum, K.Frullani, S.Gao, H.Gilman, R.Glashausser, C.Gomez, J.Hafidi, K.Hansen, O.Higinbotham, D.W.Holt, R.J.de Jager, C.W.Jiang, X.Kinney, E.Kramer, K.Kumbartzki, G.LeRose, J.Liyanage, N.Mack, D.Markowitz, P.McCormick, K.Meekins, D.Meziani, Z.-E.Michaels, R.Mitchell, J.Nanda, S.Potterveld, D.Ransome, R.Reimer, P.E.Reitz, B.Saha, A.Schulte, E.C.Seely, J.Sirca, S.Strauch, S.Sulkosky, V.Vlahovic, B.Weinstein, L.B.Wijesooriya, K.Williamson, C.Wojtsekhowski, B.Xiang, H.Xiong, F.Xu, W.Zeng, J.Zheng, X.Jefferson Lab Hall A CollaborationJefferson Lab E94-104 CollaborationThe differential cross sections for the γn→π-p and the γp→π+n processes were measured at Jefferson Lab. The photon energies ranged from 1.1 to 5.5 GeV, corresponding to center-of-mass energies from 1.7 to 3.4 GeV. The pion center-of-mass angles varied from 50° to 110°. The π- and π+ photoproduction data both exhibit a global scaling behavior at high energies and high transverse momenta, consistent with the constituent counting rule prediction and the existing π+ data. The data suggest possible substructure of the scaling behavior, which might be oscillations around the scaling value. The data show an enhancement in the scaled cross section at center-of-mass energy near 2.2 GeV. The differential cross section ratios [dσ/dt(γn→π-p)/dσ/dt(γp→π+n)] at high energies and high transverse momenta can be described by calculations based on one-hard-gluon-exchange diagrams.PhysicsPhotonArticle