We provide the general expression of the cross section for exclusive deeply virtual photon electroproduction from a spin 1/2 target using current parameterizations of the off-forward correlation function in a nucleon for different beam and target polarization configurations up to twist three accuracy. All contributions to the cross section including deeply virtual Compton scattering, the Bethe-Heitler process, and their interference, are described within a helicity amplitude based framework which is also relativistically covariant and readily applicable to both the laboratory frame and in a collider kinematic setting. Our formalism renders a clear physical interpretation of the various components of the cross section by making a connection with the known characteristic structure of the electron scattering coincidence reactions. In particular, we focus on the total angular momentum, Jz, and on the orbital angular momentum, Lz. On one side, we uncover an avenue to a precise extraction of Jz, given by the combination of generalized parton distributions, H + E, through a generalization of the Rosenbluth separation method used in elastic electron proton scattering. On the other, we single out for the first time, the twist three angular modulations of the cross section that are sensitive to Lz. The proposed generalized Rosenbluth technique adds constraints and can be extended to additional observables relevant to the mapping of the 3D structure of the nucleon. *
We use a generalization of the Rosenbluth separation method for a model independent simultaneous extraction of the Compton Form Factors H and E, from virtual Compton scattering data on an unpolarized target. A precise evaluation of H and E, enabled by the proposed method, is the first step towards pinning down the the distribution of angular momentum inside the proton.
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