Direct Observation of Sequential Weak Decay of a Double Hypernucleus

We have studied stars in nuclear emulsion due to the capture at rest of the Ξ- hyperons produced in the (K-, K+) reaction. The sequential weak decay of a double hypernucleus (nucleus with S = -2) has been directly observed. The double hypernucleus is assigned as either 10ΛΛBe or 13ΛΛB. This assignment excludes the existence of the H dibaryon lighter than 2203.7 ±0.7 MeV/c2.