Extraction of generalized parton distribution observables from deeply virtual electron proton scattering experiments

We provide the general expression of the cross section for exclusive deeply virtual photon electro-production from a spin-1/2 target using current parametrizations 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 collidcr 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, J(z), and on the orbital angular momentum, L-z. On one side, we uncover an avenue to a precise extraction of J(z), 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 side, we single out for the first time, the twist-three angular modulations of the cross section that are sensitive to L-z. The proposed generalized Rosenbluth technique adds constraints and can be extended to additional observables relevant to the mapping of the three-dimensional structure of the nucleon.