Analysis of the exclusive final state npe$^+$e$^-$ in quasi-free np reaction
nucl-ex
/ Authors
J. Adamczewski-Musch, O. Arnold, E. T. Atomssa, C. Behnke, A. Belounnas, A. Belyaev, J. C. Berger-Chen, J. Biernat, A. Blanco, C. Blume
and 107 more authors
M. Böhmer, P. Bordalo, S. Chernenko, L. Chlad, C. Deveaux, J. Dreyer, A. Dybczak, E. Epple, L. Fabbietti, O. Fateev, P. Filip, P. Finocchiaro, P. Fonte, C. Franco, J. Friese, I. Fröhlich, T. Galatyuk, J. A. Garzón, R. Gernhäuser, M. Golubeva, F. Guber, M. Gumberidze, S. Harabasz, T. Heinz
/ Abstract
We report on the investigation of dielectron production in tagged quasi-free neutron-proton collisions by using a deuteron beam of kinetic energy 1.25 GeV/u inpinging on a liquid hydrogen target. Our measurements with HADES confirm a significant excess of $e^+e^-$ pairs above the $π^{0}$ mass in the exclusive channel $dp \to npe^{+}e^{-}(p_{spect})$ 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-$Δ$ 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 side.