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Energy levels of the double-Λ hypernuclei Λ 7 Λ He, Λ 7 Λ Li, Λ 8 Λ Li, Λ 9 Λ Li, Λ 9 Λ Be and Λ 10 Λ Be are predicted

Movitated by the on-going gamma-ray experiments at the Alternating Gradient Synchrotron at Brookhaven National Laboratory, we discuss the energy splittings of the 5/2(+)(1)-3/2(+)(1) doublet in (9)(Lambda)Be and the 3/2(-)(1)-1/2(-)(1) doublet in (13)(Lambda)C for which the LambdaN spin-orbit ( LS) and antisymmetric spin-orbit ( ALS) forces are relevant. In the microscopic 2alpha+Lambda ( 3alpha+Lambda) model for (9)(Lambda)Be ( (13)(Lambda)C), all the available Nijmegen one-boson-exchange (OBE) model LambdaN interactions lead to a wide range of splittings of 0.08-0.20 MeV in (9)(Lambda)Be and 0.39-0.96 MeV in (13)(Lambda)C. On the other hand, if we use information from quark-model LambdaN interactions which have generally large ALS force, the splittings become about half of the smallest OBE model prediction.

On the basis of the ⌳ 5 HeϩNϩN three-body model which has successfully been applied to a systematic study of the energy and nucleon halo structure of the Aϭ7 isotriplet hypernuclei, strengths of ␥ transitions in ⌳ 7 Li and ⌳ 7 He are calculated. The new model confirms that the B(E2;5/2 ϩ →1/2 ϩ ) value in ⌳ 7 Li is reduced remarkably in comparison with the corresponding B(E2;3 ϩ →1 ϩ ) in the core nucleus 6 Li. This is due to the gluelike role of the ⌳ particle which induces a contraction of the core nuclear size. It is suggested that a measurement of the 5/2 ϩ →1/2 ϩ E2 transition rate in ⌳ 7 Li ͑ongoing at KEK as E419͒ provides a unique opportunity to derive the hypernuclear size and hence to confirm the size contraction experimentally. The E2 and M 1 transition strengths are also predicted for low-lying states in the hypernucleus ⌳ 7 He whose core nucleus 6 He is known to have a neutron halo. Another prediction is made of much enhanced E2 transitions in ⌳ 7 Li from the 5/2 ϩ and 3/2 ϩ (Tϭ1) states which are expected to have a proton halo structure.

Energy levels of the double Λ hypernucleus (ΛΛ)(11)Be are calculated within the framework of a ααnΛΛ five-body model. Interactions between constituent particles are determined so as to reproduce reasonably the observed low-energy properties of the αα, ααn nuclei and the existing data for Λ-binding energies of the αΛ, ααΛ, αnΛ, and ααnΛ systems. An effective ΛΛ interaction is constructed so as to reproduce, within the αΛΛ three-body model, the B(ΛΛ) of (ΛΛ)(6)He, which was extracted from the emulsion experiment, the NAGARA event. With no adjustable parameters for the ααnΛΛ system, B(ΛΛ) of the ground and bound excited states of (ΛΛ)(11)Be are calculated with the Gaussian expansion method. The Hida event, recently observed at KEK-E373 experiment, is interpreted as an observation of the ground state of the (ΛΛ)(11)Be.