Double- Λ hypernuclei observed in a hybrid emulsion experiment

A hybrid experiment with nuclear emulsion and scintillating-fiber detectors (KEK-E373) has been performed to search for double-strangeness systems. Among about 10³ events of Ξ^- hyperons captured at rest by emulsion nuclei, we have observed four events which clearly show the topology of cascade weak decays of double-Λ hypernuclei including the ``Nagara'' event. Regarding the Nagara event, values of the two-Λ binding energy (B_ΛΛ) and the Λ-Λ interaction energy (ΔB_ΛΛ) of _Λ^_Λ⁶He have been revised to 6.91 ± 0.16 and 0.67 ± 0.17 MeV, respectively, due to the recent change of the Ξ^- mass value (Particle Data Group). For another three events, we have determined possible species of double-Λ hypernuclei together with their binding energies.