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

Abstract

This paper reports the observation of 164 definite and 20 ambiguous examples of the reaction ${K}^{\ensuremath{-}}+{\mathrm{He}}^{4}\ensuremath{\rightarrow}{\ensuremath{\Lambda}}^{0}+{\ensuremath{\pi}}^{\ensuremath{-}}+p+d$ produced by ${K}^{\ensuremath{-}}$-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 ${K}^{\ensuremath{-}}+{\mathrm{He}}^{4}\ensuremath{\rightarrow}(\ensuremath{\Sigma}+\mathrm{nucleus})+{\ensuremath{\pi}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\Lambda}}^{0}+p+d+{\ensuremath{\pi}}^{\ensuremath{-}}$. The momentum distributions of the baryonic particles (for events with ${\ensuremath{\pi}}^{\ensuremath{-}}$-meson momentum below 180 $\frac{\mathrm{MeV}}{c}$ are compared with the predictions of an impulse-model calculation. In this calculation the $\ensuremath{\Sigma}$-hyperon-nucleus interaction in the intermediate state is approximated by an effective potential acting on the $\ensuremath{\Sigma}$ hyperon. It is found that very good fits are obtained with a potential of central depth around 37 MeV. This value implies that the $\ensuremath{\Lambda}$-and $\ensuremath{\Sigma}$-hyperon-nuclear potentials are of similar strengths. We show that such a result is consistent with the data on free (${\ensuremath{\Sigma}}^{\ifmmode\pm\else\textpm\fi{}},p$) and ($\ensuremath{\Lambda},p$) 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 $\ensuremath{\Sigma}$ hyperons in medium-weight nuclei. There is therefore no evidence in the reaction observed for any final-state ($\ensuremath{\Lambda},p$) resonance as postulated by Bugg, Bhatt, and Cohn.