ΛNN Three-Body Force due to Coherent Λ-Σ Coupling

The overbinding problem of Λ5He is solved by introducing a concept of coherent Λ‐Σ coupling which is equivalent to a ΛNN three‐body force. This three‐body force is coherently enhanced in the 0+ states of Λ4H and Λ4He. The 0+−1+ splitting in these hypernuclei is mainly due to coherent Λ‐Σ coupling and partly due to the ΛN spin‐spin interaction. A ΛNN three‐body potential is derived from the coupled‐channel treatment. The origin of the repulsive and attractive nature of the three‐body force is discussed. Coherent Λ‐Σ coupling becomes more important in neutron‐rich hypernuclei and especially in neutron‐star matter at high densities. The possible existence of “hyperheavy hydrogen”, Λ6H, is suggested.