Production of Heavy Unstable Particles by 1.37-Bev Pions

Five additional events interpreted as the production of heavy unstable particles by 1.37-Bev pions have been observed in a hydrogen-filled diffusion cloud chamber. In each case the observations are most naturally interpreted as due to the associated production of a hyperon and $K$-meson. These and earlier observations can be made consistent with known hyperon and $K$-meson masses if the reaction ${\pi }^{-}+p\to {\Sigma }^0+{\theta }^0$ is assumed to be a possibility with an immediate $\gamma$-decay ${\Sigma }^0\to {\Lambda }^0+\gamma$ as suggested by Gell-Mann and Pais. The total cross section for production of heavy unstable particles is estimated to be about 0.9 millibarn. The five new events confirm the previous indications that the ${\Lambda }^0\text{'}\mathrm{s}$ tend to be emitted backwards in the center-of-mass system of the initial collision and to decay in a plane at a small angle to that of the production plane. The orientations of ${\Lambda }^0\text{'}\mathrm{s}$ from the wall also support these angular correlations.