Simple model for a study of Λ hypernuclei in medium and heavy mass regions

We propose a simple and versatile model for Λ hypernuclei in medium- and heavy-mass regions to obtain their low-lying energy levels and electromagnetic transitions. In this model, we treat the core nucleus as a simple indivisible union of nucleons, and its properties are considered based on the experimental data. The interaction between nucleons in the core and a Λ particle is assumed to be weak and is treated perturbatively. We study medium- and heavy-Λ hypernuclei, i.e., Λ 51 V, Λ 52 V, Λ 89 Y, Λ 139 La, and Λ 208 Pb. To check the applicability of the model to a lighter hypernucleus, we apply it to Λ 19 F, where γ-ray transitions have been experimentally observed. The present study demonstrates that the low-lying excited states of the core nucleus below 2 MeV, in addition to the ground state of the core nucleus have to be considered in the calculation of spectra and electromagnetic transitions in the low-lying states of Λ hypernuclei. The proposed model helps us theoretically study specific medium- and heavy-hypernuclei where large-scale shell-model studies are intractable to carry out.