Structure of light hypernuclei

The structure of light hypernuclei with strangeness S = - 1 and - 2 is investigated with the microscopic cluster model and the Gaussian expansion method (GEM). We emphasize that the cluster picture as well as the mean-field picture is invaluable to understand the structure of Λ hypernuclei, Σ hypernuclei and double Λ hypernuclei. A variety of aspects of Λ hypernuclei is demonstrated through a systematic study of p-shell hypernuclei (_Λ⁶ He,_Λ⁶ Li, _Λ⁷ Li, _Λ⁸ Li, _Λ⁸ Be, _Λ⁹ Be, _Λ¹³ C) and s d-shell ones (_Λ²⁰ Ne, _Λ²¹ Ne): for example, the appearance of genuine hypernuclear states with new spatial symmetry which cannot be seen in ordinary nuclei, the glue-like role of the Λ particle which shrinks the size of nuclear core and thus reduces the B(E2) value, and the halo and skin structures in _Λ⁶ He and _Λ⁶ Li etc. The typical light hypernucleus _Λ⁹ Be is thoroughly investigated, including its production, structure and decay. Precise three-body and four-body calculations of _Λ⁷ Li, _Λ⁹ Be and _Λ¹³ C using GEM provide important information on the spin structure of the underlying Λ N interaction, by comparing with recent experimental data from γ-ray hypernuclear spectroscopy. The Λ - Σ coupling effect is studied in _Λ⁴ H and _Λ⁴ He. The binding mechanism of _Σ⁴ He is discussed together with the possible existence of _Σ⁷ Li, emphasizing the fact that the study of _Σ⁷ Li is useful for extracting information on the Σ N interaction differing from that from _Σ⁴ He. A systematic study of double-Λ hypernuclei, constrained by the NAGARA data (_{Λ Λ}⁶ He) within a four-body cluster model indicates that the recently observed Demachi-Yanagi event can be interpreted as the 2⁺ state of _{Λ Λ}¹⁰ Be. The effect of hyperon mixing in _{Λ Λ}⁶ He and _{Λ Λ}¹⁰ Be is investigated using one-boson-exchange potentials and quark-cluster-model interactions for the S = - 2 sector. A close relation between nuclear deep hole states and hypernuclei is discussed, emphasizing the selection rule for fragmentation of the s-hole in light nuclei, which is promising for understanding the production mechanism of double-Λ and twin-Λ hypernuclei via Ξ-atomic capture.