Di-electron bremsstrahlung in intermediate-energy pn and Dp

Invariant mass spectra of di-electrons stemming from bremsstrahlung processes are calculated in a covariant diagrammatical approach for the exclusive reaction Dp to p_spnpe⁺e^- with detection of a forward spectator proton, p_sp. We employ an effective nucleon-meson theory for parameterizing the sub-reaction np to p_spnpe⁺e^- and, within the Bethe-Salpeter formalism, derive a factorization of the cross section in the form dsigma_{Dp to pspnpe⁺e^-} /dM = dsigma_{np to pspnpe⁺e^-} /dM x kinematical factor related solely to the deuteron (M is the e⁺e^- invariant mass). The effective nucleon-meson interactions, including the exchange mesons pi, sigma, omega and rho as well as excitation and radiative decay of Delta(1232), have been adjusted to the process pp to pp e⁺e^- at energies below the vector meson production threshold. At higher energies, contributions from omega and rho meson excitations are analyzed in both, NN and Dp collisions. A relation to two-step models is discussed. Subthreshold di-electron production in Dp collisions at low spectator momenta is investigated as well. Calculations have been performed for kinematical conditions envisaged for forthcoming experiments at HADES.