Analysis of the exclusive final state npe${}^{+}$e${}^{-}$ in quasi-free np reaction

We report on the investigation of dielectron production in tagged quasi-free neutron-proton collisions by using a deuteron beam of kinetic energy 1.25 GeV/u inpinging on a liquid hydrogen target. Our measurements with HADES confirm a significant excess of $e^{+}{e}^{-}$ pairs above the ${\pi }^0$ mass in the exclusive channel $dp\to np{e}^{+}{e}^{-}(p_{spect})$ as compared to the exclusive channel $pp{e}^{+}{e}^{-}$ measured in proton-proton collisions at the same energy. That excess points to different bremsstrahlung production mechanisms. Two models were evaluated for the role of the charged pion exchange between nucleons and double-$\mathrm{\Delta }$ excitation combined with intermediate $\rho$-meson production. Differential cross sections as a function of the $e^{+}{e}^{-}$ invariant mass and of the angles of the virtual photon, proton and electrons provide valuable constraints and encourage further investigations on both experimental and theoretical side.