A double-hyperfragment event has been found in a hybrid-emulsion experiment
. It is identified uniquely as the sequential decay of He-6(Lambda Lambda)
emitted from a Xi (-) hyperon nuclear capture at rest. ne mass of He-6(Lamb
da Lambda) and the Lambda-Lambda interaction energy DeltaB(Lambda Lambda) h
ave been measured for the first time devoid of the ambiguities due to the p
ossibilities of excited states. The value of DeltaB(Lambda Lambda) is 1-01
0.20(-0.11)(+0.18) mev. This demonstrates that the Lambda-Lambda interactio
n is weakly attractive.