3-BODY MODEL STUDY OF A=6-7 HYPERNUCLEI - HALO AND SKIN STRUCTURES

Low-lying states of A = 6 hypernuclear doublets (He-6(Lambda) and Li-6 (Lambda)) and A = 7 triplets (He-7(Lambda), Li-7(Lambda), and Be-7(Lam bda)) are studied with an accurate three-body model calculation in whi ch all the rearrangement Jacobian coordinates are equally taken into a ccount. Since most of the hypernuclear states concerned are weakly bou nd states, focus is placed on the binding energies with respect to the particle breakup thresholds and on the density distributions in the s urface and exterior regions. With the alpha + Lambda + N model, the ob served binding energies of the ground state of He-6(Lambda) and Li-6(L ambda) are well reproduced. He-6(Lambda) is found to have a three-laye r structure of the matter distribution: the alpha nuclear core, a Lamb da skin, and a neutron halo. The A = 7 hypernuclei are shown to be wel l described with the He-5(Lambda) + N + N model. Using a realistic NN interaction, the correlation between the valence nucleons is fully tak en into account; this is essentially important to make the proton-rich three-body system Be-7(Lambda) = He-5(Lambda) + p + p bound although none of the two-body subsystems is bound. The observed binding energie s of Li-7(Lambda) and Be-7(Lambda) are well reproduced, and energies a re predicted for He-7(Lambda) whose core nucleus is a neutron halo nuc leus, He-6. We discuss the validity of the assumption of frozen deuter ons adopted in the previous alpha + d + Lambda models for the T = 0 st ates of Li-7(Lambda).