Charge-Exchange Production of Antineutrons and their Annihilation in Hydrogen

The charge exchange of antiprotons into antineutrons and the subsequent annihilation of antineutrons have been studied in the 72-in. liquid-hydrogen bubble chamber. The antiprotons were produced internally in the Bevatron; channeled externally by collimation, quadrupole focusing magnets, and bending magnets; and separated from other negatively charged particles by a system of three velocity spectrometers. Analysis of the data for a run with an antiproton momentum of 1.61 Bev/c has been completed. Three charge-exchange reactions have been studied: (a) $\overline{p}+p\to \overline{n}+n$, (b) $\overline{p}+p\to \overline{n}+n+{\pi }^0$, and (c) $\overline{p}+p\to \overline{n}+p+{\pi }^{-}$. The cross section for Reaction (a) plus Reaction (b) was found to be strongly peaked forward with a value for the angular differential cross section at zero degrees of 4.6±0.5 mb/sr. The total cross section for these two reactions was found to be 7.82±0.55 mb. The total cross section for Reaction (c) was found to be 0.99±0.24 mb; the statistical model would predict the cross section for (b) to be about the same as (c).

Of the antineutrons produced in reactions (a) plus (b), 122 were annihilated in the bubble chamber; the resulting annihilation cross section was found to be 45.2±5.4 mb. The kinetic energy of these antineutrons was distributed such that 80% of them had energies between 800 and 1000 Mev.

The average charged-pion multiplicity in the antineutron annihilations was found to be 3.5±0.3. The ratio of the number of antineutron annihilations containing five charged pions to the number containing three charged pions, and the momentum distribution of the pions, have been compared with predictions of a statistical model. Reasonable agreement was obtained for a volume five times that of a sphere with a radius of one-pion Compton wave-length. The center-of-mass angular distribution of the pions in the antineutron annihilations was found to be, within statistics, an isotropic distribution. Three events were found that fitted ${\mathrm{K}}^0$-meson production in antineutron annihilation.