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

• Received 10 May 1976

DOI:https://doi.org/10.1103/PhysRevLett.37.246