Observation of aπ0-Mesonic Decay of a Helium Hypernucleus

The observation of a hypernucleus decaying into an electron pair, a recoil, and one or more neutral particles, is reported here. The event was produced by a $K^{-}$ capture in emulsion. The charge of the hyper-nucleus was determined as $Z=2\mathrm{}$. The most probable interpretation of this unusual event is the reaction ${}_{\Lambda }{}^{}{}_{}{}^{}\mathrm{He}_{}^4{}_{}{}^{}\to {\pi }^0+{\mathrm{He}}^4$. However, ${}_{\Lambda }{}^{}{}_{}{}^{}\mathrm{He}_{}^{4,5}{}_{}{}^{}\to {\pi }^0+n+{\mathrm{He}}^{3,4}$ cannot be ruled out, since a reliable determination of the energy of the electrons at emission cannot be made.