Baryon resonances: A primary ρ→l+l−source inp+pandp+dat 4.9 GeV

Dilepton spectra for p+p and p+d reactions at 4.9 GeV are calculated. We consider electromagnetic bremsstrahlung also in inelastic reactions. ${\mathit{N}}^{\mathrm{*}}$ and ${\mathrm{\Delta }}^{\mathrm{*}}$ decays present the major contributions to the ρ and ω meson yields. Pion annihilation yields only ∼1.5% of all ρ’s in p+d. The ρ mass spectrum is strongly distorted due to phase space effects, populating dominantly dilepton masses below 770 MeV. It is found that inclusive production cross sections of ρ, ω, and η mesons in p+p are 3–4 times higher than the (measured) exclusive cross sections for pp+meson. This prediction could be checked by measuring the decays of ρ, ω, and η mesons into photons.