Observation of the ReactionK−+He4→Λ0+π−+p+d

This paper reports the observation of 164 definite and 20 ambiguous examples of the reaction $K^{-}+{\mathrm{He}}^4\to {\Lambda }^0+{\pi }^{-}+p+d$ produced by $K^{-}$-meson absorptions at rest in the Northwestern University 20-cm liquid-helium bubble chamber. We confirm that the reaction is dominated by the two-step process $K^{-}+{\mathrm{He}}^4\to (\Sigma +\mathrm{nucleus})+{\pi }^{-}\to {\Lambda }^0+p+d+{\pi }^{-}$. The momentum distributions of the baryonic particles (for events with ${\pi }^{-}$-meson momentum below 180 $\frac{\mathrm{MeV}}{c}$ are compared with the predictions of an impulse-model calculation. In this calculation the $\Sigma$-hyperon-nucleus interaction in the intermediate state is approximated by an effective potential acting on the $\Sigma$ hyperon. It is found that very good fits are obtained with a potential of central depth around 37 MeV. This value implies that the $\Lambda$-and $\Sigma$-hyperon-nuclear potentials are of similar strengths. We show that such a result is consistent with the data on free (${\Sigma }^{\pm },p$) and ($\Lambda ,p$) interactions at low energies, and data on hypernuclear binding energies. This potential depth is also consistent with the meager information on the effective potential felt by $\Sigma$ hyperons in medium-weight nuclei. There is therefore no evidence in the reaction observed for any final-state ($\Lambda ,p$) resonance as postulated by Bugg, Bhatt, and Cohn.