Antiproton-Nucleon Annihilation Process. II

Abstract

We have continued work on antiproton interactions in photographic emulsions. Most of the data come from an exposure at the Bevatron to an enriched antiproton beam of 700 Mev/c momentum. In this paper we present the analysis of 221 antiproton stars, 95 of which occurred in flight. We find an average antiproton cross section of $(1.9\mathrm{}\pm 0.2){\sigma }_0$, where ${\sigma }_0=\pi {(1.2\mathrm{}\times {10}^{-13\mathrm{}}{A}^{\frac{1}{3}})}^2$ ${\mathrm{cm}}^2$, for all the elements in emulsion, excluding hydrogen. The primary antiproton annihilation gives rise to 5.36±0.3 pions on the average. In annihilations at rest, 1.3 of the pions formed interact with the parent nucleus; in reactions in flight, 1.9 of the pions. For stars at rest the energy available in the annihilation in complex nuclei is divided up among the products as follows: charged pions, 48±6%, neutral particles (other than neutrons and $K^0$ mesons) 28±7%; $K$ mesons 3±1.5%; and cascade nucleons and nuclear excitation 21±2%. For the stars in flight the corresponding percentages are: 45±7%, 22±7%, 3±1.5%, and 30±2%. To fit the average pion multiplicity, the interaction radius of the Fermi statistical model must be taken as $\frac{2.5\hslash }{m_{\pi }c}$. Other proposals to explain the large multiplicity are discussed. We deduce from the fraction of pions interacting in the same nucleus that the annihilation takes place at the outer fringes of the nucleus.