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 $\Lambda\Lambda$ 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 $N\Lambda$ to $N\Sigma$, or of $\Lambda\Lambda$ to $N\Xi$, and the role of tensor forces. The new nucleus $(n,n,\Lambda,\Lambda)$ 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.