Dilepton production in proton-proton and quasifree proton-neutron reactions at 1.25 GeV

We investigate the pp -> ppe(+)e(-) and quasifree pn -> pne(+)e(-) reactions within an effective Lagrangian model at a laboratory kinetic energy of 1.25 GeV for which experimental data have recently been reported by the HADES Collaboration. The model uses a meson-exchange approximation to describe the initial nucleon-nucleon (NN) scattering. Contributions to the reaction amplitudes are included from the NN bremsstrahlung as well as from the excitation, propagation, and radiative decay of the Lambda(1230) isobar state. It is found that the HADES data on the e(+)e(-) invariant mass distribution in the pp -> ppe(+)e(-) reaction are excellently reproduced by our model where the Delta isobar term dominates the spectrum. In the case of the quasifree pn -> pne(+)e(-) reaction, a strong sensitivity to the pion electromagnetic form factor is observed which helps to bring the calculated cross sections closer to the data in the higher dilepton mass region.