Neutral Decay Modes of theη0Meson

Abstract

The branching ratios of the decay of the ${\eta }^0$ meson into neutral products have been measured. The ${\eta }^0$ is produced in the reaction ${\pi }^{-}p\to {\eta }^0+n$, and the decay products are observed as $\gamma$ rays by a system of optical spark chambers and counters in nearly $4\pi$ geometry. Emphasis was placed on examining evidence for the decay mode ${\eta }^0\to {\pi }^0\gamma \gamma (\to 4\gamma )$. Measurement of the neutron time of flight in the production process, together with detailed measurement and kinematical analysis of the pictures showing four $\gamma$ rays, enables us to test each event for consistency with such a decay mode, and also for the "background" process ${\pi }^{-}+p\to 2{\pi }^0+n$. From what is essentially a comparison of the observed rates for these processes, we obtain the upper limit for $\frac{({\eta }^0\to {\pi }^0\gamma \gamma )}{({\eta }^0\to \mathrm{all}\mathrm{neutrals})}\le 7\%$ (90% confidence level), a result which is relatively insensitive to estimates of the efficiency of the detecting system. We also obtain $\frac{({\eta }^0\to 3{\pi }^0)}{({\eta }^0\to 2\gamma )}=0.75\mathrm{}\pm 0.09$.