Study of prompt e+e− , η0 , and ω0 production in low-energy p¯p annihilations

A study of prompt ${e}^{+}{e}^{\ensuremath{-}}$ production has been made using the Argonne 12-foot bubble chamber exposed to a low-energy $\overline{p}$ beam. We found eleven annihilation events with ${e}^{+}{e}^{\ensuremath{-}}$ pairs of invariant mass greater than the ${\ensuremath{\pi}}^{0}$ mass. The background to direct ${e}^{+}{e}^{\ensuremath{-}}$ pair production comes mainly from ${\ensuremath{\eta}}^{0}\ensuremath{\rightarrow}\ensuremath{\gamma}{e}^{+}{e}^{\ensuremath{-}}$ and ${\ensuremath{\omega}}^{0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{e}^{+}{e}^{\ensuremath{-}}$ Dalitz pairs. To determine these backgrounds, we have measured the inclusive ${\ensuremath{\eta}}^{0}$ and ${\ensuremath{\omega}}^{0}$ production rates using externally converted $\ensuremath{\gamma}$ rays. We find $\frac{{\ensuremath{\omega}}^{0}}{{\ensuremath{\pi}}^{0}}=0.20\ifmmode\pm\else\textpm\fi{}0.07$ and $\frac{{\ensuremath{\eta}}^{0}}{{\ensuremath{\pi}}^{0}}<0.11$ at the 90% confidence level. The (${\ensuremath{\eta}}^{0}+{\ensuremath{\omega}}^{0}$) Dalitz background plus other small backgrounds yield ${7.0}_{\ensuremath{-}3.0}^{+44}$ events. The remaining events yield a 90%-confidence-level upper limit for the ratio of prompt ${e}^{+}{e}^{\ensuremath{-}}$ pairs to ${\ensuremath{\pi}}^{0}$ production of 4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}5}$.