Hypernuclear constraints on the existence and lifetime of a deeply bound H dibaryon

We study to what extent the unique observation of ΛΛ hypernuclei by their weak decay into known Λ hypernuclei, with lifetimes of order 10⁻¹⁰ s, rules out the existence of a deeply bound doubly-strange (S = − 2) H dibaryon. Treating _ΛΛ⁶He (the Nagara emulsion event) in a realistic Λ−Λ−⁴He three-body model, we find that the HeΛΛ6→H+⁴He strong-interaction lifetime increases beyond 10⁻¹⁰ s for m_H<m_Λ+m_n, about 176 MeV below the ΛΛ threshold, so that such a deeply bound H is not in conflict with hypernuclear data. Constrained by Λ hypernuclear ΔS=1 nonmesonic weak-interaction decay rates, we follow EFT methods to evaluate the ΔS=2 H→nn weak-decay lifetime of H in the mass range 2m_n≲m_H<m_Λ+m_n. The resulting H lifetime is of order 10⁵ s, many orders of magnitude shorter than required to qualify for a dark-matter candidate.