Weakly bound states in light hypernuclei

From the viewpoint of "critical stability", we discuss the three- and four-body structure of _Γ⁶He, _Γ⁷He, _Γ⁴He, and _ΓΓ⁴H. With the α + Γ + N three-body model, _Γ⁶He is found to have a three-layer structure of the matter distribution: α core, a Γ skin and neutron halo. Also the level structure of _Γ⁷He with the three-body model of _Γ⁵He + n + n is predicted. This stimulates a new study of neutron-rich and proton-rich hypernuclei. By performing a four-body calculation with both NNNΓ and NNNΣ channels and with both NNΓΓ and NNNΘ channels, we show that the ΓN-ΣN and ΓΓ-ΣN couplings are very important in critical stability of few-body hypernuclear systems.