Dilepton production in proton–proton collisions at BEVALAC energies

The dilepton production in elementary pp → e⁺e⁻X reactions at BEVALAC energies T_lab = 1–5 GeV is investigated. The calculations include direct e⁺e⁻ decays of the vector mesons ρ⁰, ω and ϕ, Dalitz decays of the π⁰-, η-, ρ-, ω- and ϕ-mesons and of the baryon resonances Δ(1232), N(1520),.... The subthreshold vector meson production cross sections in pp collisions are treated in a way sufficient to avoid double counting with the inclusive vector meson production. The vector meson dominance model for the transition form factors of the resonance Dalitz decays R → e⁺e⁻N is used in an extended form to ensure correct asymptotics which are in agreement with the quark counting rules. Such a modification gives a unified and consistent description of both R → Nγ radiative decays and R → Nρ(ω) meson decays. The effect of multiple pion production on the experimental efficiency for the detection of the dilepton pairs is studied. We find the dilepton yield in reasonable agreement with the experimental data for the set of intermediate energies whereas at the highest energy T_lab = 4.88 GeV, the number of dilepton pairs is likely to be overestimated experimentally in the mass range M = 300–700 MeV.