Lepton-Pair Production in Virtual Compton Scattering off the Proton

We show that lepton-pair production in Virtual Compton Scattering offers, through interference with the well-known Bethe-Heitler process, a sensitive probe to learn the longitudinal response of resonances and the electromagnetic nucleon form factors. This interference can be measured directly in terms of an asymmetry. The role of off-shell effects in the N-N-$γ$ vertices is investigated as well. An additional N-N-$γ-γ$ contact term in the amplitude, included to ensure gauge invariance of the model, cancels a substantial part of the off-shell effects.

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