Search for the Direct Production of Positrons by 256- and 800-MeV Protons

Abstract

The invariant cross sections for direct positron production in $p\ensuremath{-}p$ collisions have been measured to be $E\frac{{d}^{3}\ensuremath{\sigma}}{d{p}^{3}}=(\ensuremath{-}3\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}32} \mathrm{and} (\ensuremath{-}14\ifmmode\pm\else\textpm\fi{}12)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}32}$ ${\mathrm{cm}}^{2}$/sr ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ at 256 and 800 MeV, respectively. These results are consistent with zero ($\frac{{e}^{+}}{{\ensuremath{\pi}}^{+}}\ensuremath{\lesssim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ at 800 MeV) and are in marked contrast to results at higher energies.