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

The invariant cross sections for direct positron production in p-p collisions have been measured to be $E{{d}³{σ}}{d{p}³}=({-}3±{}2)×{}{10}^{{-}32} and ({-}14±{}12)×{}{10}^{{-}32}$ ${cm}²$/sr ${(GeV/c)}²$ at 256 and 800 MeV, respectively. These results are consistent with zero (${{e}⁺}{{{π}}⁺}{}3×{}{10}^{{-}6}$ at 800 MeV) and are in marked contrast to results at higher energies.