Structure of A=7 iso-triplet Λ hypernuclei studied with the four-body cluster model

The structure of the T=1 isotriplet hypernuclei, He-7(Lambda), Li-7(Lambda), and Be-7(Lambda) within the framework of an alpha+Lambda+N+N four-body cluster model is studied. Interactions between the constituent subunits are determined so as to reproduce reasonably well the observed low-energy properties of the alpha N, alpha Lambda, alpha NN, and alpha Lambda N subsystems. Furthermore, the two-body Lambda N interaction is adjusted so as to reproduce the 0(+)-1(+) splitting of H-4(Lambda). Also, a phenomenological Lambda N charge symmetry breaking (CSB) interaction is introduced. The Lambda binding energy of the ground state in He-7(Lambda) is predicted to be 5.16(5.36) MeV with (without) the CSB interaction. The calculated energy splittings of the 3/2(+)-5/2(+) states in He-7(Lambda) and Li-7(Lambda) are around 0.1 MeV. We point out that there is a three-layer structure of the matter distribution, alpha particle, Lambda skin, and proton or neutron halo, in the He-7(Lambda)(J=5/2(+)), Li-7(Lambda)(J=5/2(+)), and Be-7(Lambda)(J=1/2(+)) states.