Coherent Λ-Σ coupling in s-shell hypernuclei

It is found that the suppression due to two-body Lambda N-Sigma N coupling solves the overbinding problem in He-5(Lambda) 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 Lambda NN three-body force. This three-body force becomes strong in the 0(+) states of H-4(Lambda) and He-4(Lambda) according to the coherently added enhancement. The 0(+)-1(+) splitting in H-4(Lambda) and He-4(Lambda) is found partly due to the Lambda N spin-spin interaction and partly due to the Lambda-Sigma coupling in the recent Nijmegen soft-core potential.