A double-hyperfragment event has been found in a hybrid-emulsion experiment. It is identified uniquely as the sequential decay of ${}_{\Lambda \Lambda }^6\mathrm{He}$ emitted from a ${\Xi }^{-}$ hyperon nuclear capture at rest. The mass of ${}_{\Lambda \Lambda }^6\mathrm{He}$ and the $\Lambda -\Lambda$ interaction energy $\Delta B_{\Lambda \Lambda }$ have been measured for the first time devoid of the ambiguities due to the possibilities of excited states. The value of $\Delta B_{\Lambda \Lambda }$ is $1.01\pm {0.20}_{-0.11\mathrm{}}^{+0.18\mathrm{}}\mathrm{MeV}$. This demonstrates that the $\Lambda -\Lambda$ interaction is weakly attractive.

• Received 17 July 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.212502