The onset of ΛΛ hypernuclear binding

Binding energies of light, A≤6, ΛΛ hypernuclei are calculated using the stochastic variational method in a pionless effective field theory (π̸EFT) approach at leading order with the purpose of assessing critically the onset of binding in the strangeness S=−2 hadronic sector. The π̸EFT input in this sector consists of (i) a ΛΛ contact term constrained by the ΛΛ scattering length a_ΛΛ, using a range of values compatible with ΛΛ correlations observed in relativistic heavy ion collisions, and (ii) a ΛΛN contact term constrained by the only available A≤6 ΛΛ hypernucler binding energy datum of _ΛΛ ⁶He. The recently debated neutral three-body and four-body systems _ΛΛ ³n and _ΛΛ ⁴n are found unbound by a wide margin. A relatively large value of |a_ΛΛ|≳1.5 fm is needed to bind _ΛΛ ⁴H, thereby questioning its particle stability. In contrast, the particle stability of the A=5 ΛΛ hypernuclear isodoublet _ΛΛ ⁵H–_ΛΛ ⁵He is robust, with Λ separation energy of order 1 MeV.