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Possibility of a new neutral hypernucleus (n,n,Lambda,Lambda)

The status of the light hypernuclei is reviewed, and discussed with models based either on the Nijmegen-RIKEN baryon--baryon interaction or on recent studies using chiral effective field theory. The latter suggests a significantly shorter range for the $ΛΛ$ interaction, and this favours the formation of a Borromean state made of two neutrons and two hyperons. Various corrections are discussed, in particular the coupling of $NΛ$ to $NΣ$, or of $ΛΛ$ to $NΞ$, and the role of tensor forces. The new nucleus $(n,n,Λ,Λ)$ could be produced in various reactions, in particular deuteron--deuteron scattering with the simultaneous production of two charged kaons, for which an estimate of the cross section is provided.