Dilepton production in pp and np collisions at 1.25 GeV

The inclusive reactions $pp\to e^{+}{e}^{-}X$ and $np \rightarrow e^+ e^- X$ at the laboratory kinetic energy of 1.25 GeV are investigated in a model of dominance of nucleon and $\mathrm{\Delta }$ resonances. Experimental data for these reactions have recently been reported by the HADES Collaboration. In the original model, the dileptons are produced either from the decays of nucleon and $\mathrm{\Delta }$ resonances $R\to N{e}^{+}{e}^{-}$ or from the Dalitz decays of ${\pi }^0$- and $\eta$-mesons created in the $R\to N{\pi }^0$ and $R\to N\eta$ decays. We found that the distribution of dilepton invariant masses in the $pp \rightarrow e^+ e^- X$reactioniswellreproducedbythecontributionsof$R \rightarrow N e^+ e^-$decaysand$R \rightarrow N \pi^0$,$\pi^0 \to \gamma e^+e^-$ decays. Among the resonances, the predominant contribution comes from the $\mathrm{\Delta }(1232)$, which determines both the direct decay channel $R \rightarrow N e^+ e^-$andthepiondecaychannel\mathrm{.}Inthecollisions$np \rightarrow e^+ e^- X$,additionalsignificantcontributionsarisefromthe$\eta$-mesonDalitzdecays,producedinthe$np \rightarrow np\eta$and$np \rightarrow d\eta$reactions,theradiativecapture$np \rightarrow d e^+ e^-$, and the $np\to np{e}^{+}{e}^{-}$ bremsstrahlung. These mechanisms may partly explain the strong excess of dileptons in the cross section for collisions of $np$ versus $pp$, which ranges from 7 to 100 times for the dilepton invariant masses of 0.2 to 0.5 GeV.