Study of prompt $e^{+}{e}^{-}$, ${\eta }^0$, and ${\omega }^0$ production in low-energy $\overline{p}p$ annihilations

A study of prompt $e^{+}{e}^{-}$ 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 $e^{+}{e}^{-}$ pairs of invariant mass greater than the ${\pi }^0$ mass. The background to direct $e^{+}{e}^{-}$ pair production comes mainly from ${\eta }^0\to \gamma e^{+}{e}^{-}$ and ${\omega }^0\to {\pi }^0{e}^{+}{e}^{-}$ Dalitz pairs. To determine these backgrounds, we have measured the inclusive ${\eta }^0$ and ${\omega }^0$ production rates using externally converted $\gamma$ rays. We find $\frac{{\omega }^0}{{\pi }^0}=0.20\mathrm{}\pm 0.07$ and $\frac{{\eta }^0}{{\pi }^0}<0.11\mathrm{}$ at the 90% confidence level. The (${\eta }^0+{\omega }^0$) Dalitz background plus other small backgrounds yield ${7.0}_{-3.0\mathrm{}}^{+44\mathrm{}}$ events. The remaining events yield a 90%-confidence-level upper limit for the ratio of prompt $e^{+}{e}^{-}$ pairs to ${\pi }^0$ production of 4 × ${10}^{-5\mathrm{}}$.