Production of aθ0Particle without an Associated Hyperon in aπ−−pCollision

An event is described that is interpreted as evidence for the simultaneous production of a ${\theta }^0$ and a ${\overline{\theta }}^0$ according to the scheme of Gell-Mann and Pais. This interpretation assumes the production of a normal ${\theta }^0$ and a $K^0$ with no associated hyperon, where the ${\theta }^0$ is observed and the $K^0$ is inferred from the rule of associated production of heavy unstable particles. The event was obtained by exposing a high-pressure diffusion cloud chamber to a 4.5-Bev/c ${\pi }^{-}$ meson beam from the Bevatron. The event is most reasonably interpreted as ${\pi }^{-}+p\to {\pi }^{-}+p+{\theta }^0+\left(\mathrm{neutral}\right)$. Energy and momentum conservation are satisfied by an undetected neutral particle having a mass of ${502}_{-124\mathrm{}}^{+91\mathrm{}}$ Mev. This is consistent with the associated production of a ${\theta }^0$ and a ${\overline{\theta }}^0$ according to the scheme of Gell-Mann and Pais. Details are given and alternative interpretations are discussed.