Observation of a ΛΛ6He double hypernucleus -: art. no. 212502

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(Lambda Lambda) and the Lambda-Lambda interaction energy DeltaB(Lambda Lambda) have been measured for the first time devoid of the ambiguities due to the possibilities 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 interaction is weakly attractive.