HADES collaboration (117 authors)Blanco, A.Bordalo, P.Fonte, P.Franco, C.Lopes, L.Mangiarotti, A.Ramos, S.Silva, L.2019-02-042019-02-042017-07-18http://dx.doi.org/10.1140/epja/i2017-12341-3http://hdl.handle.net/10400.26/27315We report on the investigation of dielectron production in tagged quasi-free neutron-proton collisions by using a deuteron beam of kinetic energy 1.25GeV/u impinging 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 $dp \rightarrow npe^{+}e^{-}(p_{spect})$ as compared to the exclusive channel $ ppe^{+}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- $\Delta$ 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 sides.engjournal article2019-02-04