Remark on the kinematics of the one-photon production in electron-proton scattering

We clarify some misleading mistakes in the literature about the phase space for the one-photon production in electron-proton scattering and present the correct overall phase space factor in the cross section formula. Our conclusion is that the earlier diquark model predictions for the cross sections and spin asymmetries of the one-photon production in electron-proton scattering are unreliable. Further, we specify the phase space boundaries. In relation to the recent proposal to measure the off-diagonal quark distribution functions in the deeply virtual Compton scattering off the proton, we point out that the kinematics strongly disfavors such experiments.