Di-Electrons from Resonances in Nucleon-Nucleon Collisions

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 $pp\to pp e^+e^-$ and $pn\to pn e^+e^-$ 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 $pp$ and, in particular, $pn$ collisions at beam energies of 1 - 2 GeV are still significant in certain phase space regions.