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 ΛΛ~~6 He (the Nagara emulsion event) in a realistic Λ-Λ-⁴He three-body model, we find that the ΛΛ~~6 He→ H + ⁴ He strong-interaction lifetime increases beyond 10⁻¹⁰ s for mH < mΛ+mₙ, 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 Δ S=1 nonmesonic weak-interaction decay rates, we follow EFT methods to evaluate the Δ S=2 H→ nn weak-decay lifetime of H in the mass range 2mₙ mH < mΛ+mₙ. The resulting H lifetime is of order 10⁵ s, many orders of magnitude shorter than required to qualify for a dark-matter candidate.