Structure of light hypernuclei

Abstract

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 ( Λ 6 He , Λ 6 Li , Λ 7 Li , Λ 8 Li , Λ 8 Be , Λ 9 Be , Λ 13 C ) and s d -shell ones ( Λ 20 Ne , Λ 21 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 Λ 6 He and Λ 6 Li etc. The typical light hypernucleus Λ 9 Be is thoroughly investigated, including its production, structure and decay. Precise three-body and four-body calculations of Λ 7 Li , Λ 9 Be and Λ 13 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 Λ 4 H and Λ 4 He . The binding mechanism of Σ 4 He is discussed together with the possible existence of Σ 7 Li , emphasizing the fact that the study of Σ 7 Li is useful for extracting information on the Σ N interaction differing from that from Σ 4 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.