Baryon resonances: A primary rho -->l+l- source in p+p and p+d at 4.9 GeV.

Abstract

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 ${\mathrm{\ensuremath{\Delta}}}^{\mathrm{*}}$ decays present the major contributions to the \ensuremath{\rho} and \ensuremath{\omega} meson yields. Pion annihilation yields only \ensuremath{\sim}1.5% of all \ensuremath{\rho}'s in p+d. The \ensuremath{\rho} 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 \ensuremath{\rho}, \ensuremath{\omega}, and \ensuremath{\eta} 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 \ensuremath{\rho}, \ensuremath{\omega}, and \ensuremath{\eta} mesons into photons.