Antiproton-Nucleon Annihilation Process (Antiproton Collaboration Experiment)

Abstract

Thirty-five antiproton stars have been found in an emulsion stack exposed to a 700-Mev/c negative particle beam. Of these antiprotons, 21 annihilate in flight and three give large-angle scatters ($\ensuremath{\theta}>15\ifmmode^\circ\else\textdegree\fi{}$, ${T}_{\overline{p}}>50$ Mev), while 14 annihilate at rest. From the interactions in flight we obtain the total cross section for antiproton interaction: $\frac{{\ensuremath{\sigma}}_{\overline{p}}}{{\ensuremath{\sigma}}_{0}}=2.9\ifmmode\pm\else\textpm\fi{}0.7$, where ${\ensuremath{\sigma}}_{0}=\ensuremath{\pi}{{R}_{0}}^{2}$ and ${R}_{0}=1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{A}^{\frac{1}{3}}$ cm. This cross section was measured at an average antiproton energy of ${\overline{T}}_{\overline{p}}=140$ Mev.We also find that the antiproton-nucleon annihilation proceeds primarily through pion production with occasional emission of $K$ particles. On the average 5.3\ifmmode\pm\else\textpm\fi{}0.4 pions are produced in the primary process; of these, 1 pion is absorbed and 0.3 inelastically scattered. From the small fraction of pions absorbed, we conclude that the annihilation occurs mainly at the surface of the nucleus at a distance larger than the conventional radius.A total energy balance of particles emitted in the annihilation gives a ratio of charged to neutral pions consistent with charge independence. Conversely, assuming charge independence, we conclude that the energy going into electromagnetic radiation or neutrinos is small.Comparisons with the Fermi statistical model and the Lepore-Neuman statistical model have been made. Good agreement with the experimental results on the annihilation process can be obtained through appropriate choice of the interaction volume parameters.Several different estimates of the antiproton mass are in good agreement and suggest strongly that the antiproton mass is the same as the proton mass within an accuracy of 2\textonehalf{}%.A study of the elastic scattering of the antiprotons down to angles of 2\ifmmode^\circ\else\textdegree\fi{} suggests a possible destructive interference between nuclear and Coulomb scattering.