Three-body model study ofA=6–7 hypernuclei: Halo and skin structures

Low-lying states of A=6 hypernuclear doublets (_Λ⁶He and _Λ⁶Li) and A=7 triplets (_Λ⁷He, _Λ⁷Li, and _Λ⁷Be) are studied with an accurate three-body model calculation in which all the rearrangement Jacobian coordinates are equally taken into account. Since most of the hypernuclear states concerned are weakly bound states, focus is placed on the binding energies with respect to the particle breakup thresholds and on the density distributions in the surface and exterior regions. With the {α}+{Λ}+N model, the observed binding energies of the ground states of _Λ⁶He and _Λ⁶Li are well reproduced. _Λ⁶He is found to have a three-layer structure of the matter distribution: the {α} nuclear core, a {Λ} skin, and a neutron halo. The A=7 hypernuclei are shown to be well described with the _Λ⁵He + N+N model. Using a realistic NN interaction, the correlation between the valence nucleons is fully taken into account; this is essentially important to make the proton-rich three-body system _Λ⁷Be = _Λ⁵He + p+p bound although none of the two-body subsystems is bound. The observed binding energies of _Λ⁷Li and _Λ⁷Be are well reproduced, and energies are predicted for _Λ⁷He whose core nucleus is a neutron halo nucleus, ⁶He. We discuss the validity of the assumption of frozen deuterons adopted in the previous {α}+d+{Λ} models for the T=0 states of _Λ⁷Li. {} 1996 The American Physical Society.