Neutron-rich hypernuclei: HΛ6 and beyond

Recent experimental evidence presented by the FINUDA Collaboration for a particle-stable HΛ6 has stirred renewed interest in charting domains of particle-stable neutron-rich Λ hypernuclei, particularly for unbound nuclear cores. We have studied within a shell-model approach several neutron-rich Λ hypernuclei in the nuclear p shell that could be formed in (π^-, K⁺) or in (K^-, π⁺) reactions on stable nuclear targets. Hypernuclear shell-model input is taken from a theoretically inspired successful fit of γ-ray transitions in p-shell Λ hypernuclei which includes also ΛN↔σN coupling (Λσ coupling). The particle stability of HΛ6 is discussed and predictions are made for binding energies of HeΛ9, LiΛ10, BeΛ12, BΛ14. None of the large effects conjectured by some authors to arise from Λσ coupling is borne out, neither by these realistic p-shell calculations, nor by quantitative estimates outlined for heavier hypernuclei with substantial neutron excess.