Structure of S =-2 Hypernuclei and hyperon-hyperon interactions

We review recent progress in S=-2 hypernuclei such as double-Λ hypernuclei and Ξ hypernuclei, which are composed of a nucleus and one or two hyperons, such as a Λ or a Ξ particle. Through observations of ⁶ _λλHe, such as the Nagara event, we obtain important information about the ΛΛ interaction. Using this information, we perform a four-body calculation of ααΛΛ for ¹⁰ _ΛΛBe, which was observed at KEK as the Demachi-Yanagi event. We interpret this event as the 2⁺ excited state. We calculate energy levels of Be within the framework of an ααnΛΛ five-body cluster model, and then interpret the Hida event, which was observed at KEK as the ground state of ¹¹ _ΛΛBe. Motivated by observation of the Kiso event of C, by using the Skyrme-Hartree-Fock and relativistic mean-field frameworks, we calculate the energy spectra of this hypernucleus. We interpret this event as the ¹⁴N(g.s.)+Ξ - (0p) state. Finally, we propose an experiment on H and Li to extract information about the spin- and isospin-averaged parts of the ΞN interaction.