Four-body cluster structure of [Formula Presented] double-Λ hypernuclei

Energy levels of the double-Λ hypernuclei [Formula Presented] [Formula Presented] [Formula Presented] and [Formula Presented] are predicted on the basis of an [Formula Presented] four-body model, where [Formula Presented] and [Formula Presented] respectively. Interactions between the constituent particles are determined so as to reproduce reasonably the observed low-energy properties of the [Formula Presented] nuclei [Formula Presented] and the existing data for Λ-binding energies of the [Formula Presented] and [Formula Presented] systems [Formula Presented] An effective ΛΛ interaction is constructed so as to reproduce, within the α+Λ+Λ model, the [Formula Presented] of [Formula Presented] which was extracted from the emulsion experiment, the NAGARA event. With no adjustable parameters for the [Formula Presented] system, [Formula Presented] of ground and bound excited states of the double-Λ hypernuclei with [Formula Presented] are calculated within the Gaussian-basis coupled-rearrangement-channel method. The Demachi-Yanagi event, identified recently as [Formula Presented] is interpreted as an observation of its [Formula Presented] excited state on the basis of the present calculation. Structural changes in the [Formula Presented] core nuclei, due to the interaction of the Λ particles, are found to be substantial, and these play an important role in estimating the ΛΛ bond energies of those hypernuclei.