The contribution of the low-lying nucleon resonances $P_{33}(1232)$, $P_{11}(1440)$, $D_{13}(1520)$, and $S_{11}(1535)$ to the invariant-mass spectra of di-electrons stemming from the exclusive processes $\mathit{pp}\to \mathit{pp}\hspace{0.5em}{e}^{+}{e}^{-}$ and $\mathit{pn}\to \mathit{pn}\hspace{0.5em}{e}^{+}{e}^{-}$ is investigated within a fully covariant and gauge-invariant diagrammatical approach. We employ, within the one-boson exchange approximation, effective nucleon-meson interactions including the exchange mesons $\pi$, $\eta$, $\sigma$, $\omega$, and $\rho$ as well as excitations and radiative decays of the above low-lying nucleon resonances. The total contribution of these resonances is dominant; however, bremsstrahlung processes in $\mathit{pp}$ and, in particular, $\mathit{pn}$ collisions at beam energies of $1–2$ GeV are still significant in certain phase-space regions.

• Received 17 March 2009

DOI:https://doi.org/10.1103/PhysRevC.80.064003