Analysis of the exclusive final state npe+e- in the quasi-free np reaction

Abstract

We 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 π0 mass in the exclusive channel dp→ npe+e-(pspect) 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- Δ excitation combined with intermediate ρ-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.