It is found that the suppression due to two-body LambdaN-SigmaN coupling solves the overbinding problem in (5)(Lambda)He but it, in turn, causes a severe underbinding in the four-body systems. The shortage of this binding is overcome by introducing explicitly the Lambda-Sigma coupling which is equivalent to the LambdaNN three-body force. This three-body force becomes strong in the 0(+) states of (4)(Lambda)H and (4)(Lambda)He according to the coherently added enhancement. The 0(+)-1(+) splitting in (4)(Lambda)H and (4)(Lambda)He is found partly due to the LambdaN spin-spin interaction and partly due to the Lambda-Sigma coupling in the recent Nijmegen soft-core potential.

The significance of ΛΛ-ΞN coupling in double-Λ hypernuclei has been studied. The Pauli suppression effect due to this coupling in 6 ΛΛ He has been found to be 0.43 MeV for the coupling strength of the NSC97e potential. This indicates that the freespace ΛΛ interaction is stronger by about 5 • phase shift than that deduced from the empirical data of 6 ΛΛ He without including the Pauli suppression effect. In 5 ΛΛ He and 5 ΛΛ H, an attractive term arising from ΛΛ-ΞN conversion is enhanced by the formation of an alpha particle in intermediate Ξ states. According to this enhancement, we have found that the ΛΛ binding energy (∆B ΛΛ ) of 5 ΛΛ He is about 0.27 MeV larger than that of 6 ΛΛ He for the NSC97e coupling strength. This finding deviates from a general picture that the heavier is the core nucleus, the larger is ∆B ΛΛ .

We describe ab initio calculations of doubly strange, S = -2, s-shell hypernuclei (4(LambdaLambda)H, 5LambdaLambda)H, 5(LambdaLambda)He, and 6(LambdaLambda)He) as a first attempt to explore the few-body problem of the full-coupled channel scheme for these systems. The wave function includes LambdaLambda, LambdaSigma, NXi, and SigmaSigma channels. Minnesota NN, D2' YN, and simulated YY potentials based on the Nijmegen hard-core model are used. Bound-state solutions of these systems are obtained. We find that a set of phenomenological B8B8 interactions among the octet baryons in S = 0,-1, and -2 sectors, which is consistent with all of the available experimental binding energies of S = 0,-1, and -2 s-shell (hyper)nuclei, can predict a particle stable bound state of 4(LambdaLambda)H. For 5(LambdaLambda)H and 5(LambdaLambda)He, LambdaN-SigmaN and XiN-LambdaSigma potentials significantly affect the net LambdaLambda-NXi coupling, and a large Xi probability is obtained even for a weaker LambdaLambda-NXi potential.

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