Neutron-rich hypernuclei: Lambda-6H and beyond

Recent experimental evidence presented by the FINUDA Collaboration for a particle-stable Lambda-6H has stirred renewed interest in charting domains of particle-stable neutron-rich Lambda hypernuclei, particularly for unbound nuclear cores. We have studied within a shell-model approach several neutron-rich Lambda hypernuclei in the nuclear p shell that could be formed in (pi-,K+) or in (K-,pi+) reactions on stable nuclear targets. Hypernuclear shell-model input is taken from a theoretically inspired successful fit of gamma-ray transitions in p-shell Lambda hypernuclei which includes also Lambda-N <-> Sigma-N coupling (Lambda<->Sigma coupling). The particle stability of Lambda-6H is discussed and predictions are made for binding energies of Lambda-9He, Lambda-10Li, Lambda-12Be, Lambda-14B. None of the large effects conjectured by some authors to arise from Lambda<->Sigma coupling is borne out, neither by these realistic p-shell calculations, nor by quantitative estimates outlined for heavier hypernuclei with substantial neutron excess.