The lightest neutral hypernuclei with strangeness -1 and -2

Our current knowledge of the baryon--baryon interaction suggests that the dineutron $(n,n)$ and its strange analogue $(n,\Lambda)$ are unstable. In contrast, the situation is more favorable for the strange three-body system $(n,n,\Lambda)$, and even better for the four-body system $T\equiv (n,n,\Lambda,\Lambda)$ with strangeness $-2$, which is likely to be stable under spontaneous dissociation. The recent models of the hyperon-nucleon and hyperon-hyperon interactions suggest that the stability of the $(n,n,\Lambda)$ and $T$ is possible within the uncertainties of our knowledge of the baryon-baryon interactions. This new nucleus $T$ could be produced and identified in central deuteron--deuteron collisions via reaction $d+d\to T+K^++K^+$, and the tetrabaryon $T$ could play an important role in catalyzing the formation of a strange core in neutron stars.