LEPTON-PAIR PRODUCTION IN VIRTUAL COMPTON-SCATTERING OFF THE PROTON

We show that lepton-pair production in virtual Compton scattering offe rs, through interference with the well-known Bethe-Heitler process, a sensitive probe to separate longitudinal and transverse response at po sitive photon invariant masses. This interference can be measured dire ctly in terms of an asymmetry. The role of off-shell effects in the NN gamma vertices is investigated as well. An additional NN gamma gamma contact term in the amplitude, included to ensure gauge invariance of the model, cancels a substantial part of the off-shell effects, but in troduces ambiguities which affect observables. The response of nucleon resonances is also investigated. (C) 1997 Elsevier Science B.V.