HADES

Investigation of di-electron pair production in quasi-free $n-p$ interactions using deuterium beams on proton target at kinetic energy of 1.25 GeV/u will be presented. Detection of spectator proton from deuterium break-up at forward angles $(0.3^\circ < \theta < 7^\circ )$ and electron-positron pairs and proton in High Acceptance Dielectron Spectromter (HADES) located at GSI allows for detailed analysis of the exclusive pair production in the $np\to np e^+ e^-$ reaction. Obtained exclusive distributions will be compared to the corresponding one obtained from $pp$ collisions at the same energy. The results will be compared with predictions obtained from available calculations. In particular interpretations of a striking difference in the pair production in both reactions will be discussed.