Observation of a ${}_{\Lambda\Lambda}\mathrm{Be}$ double-$\Lambda$ hypernucleus in the J-PARC E07 experiment.

A double-$\ Lambda$ hypernucleus, ${}_{\ Lambda\Lambda}\mathrm{Be}$, was observed by the J-PARCE07 collaboration in nuclear emulsionstagged by the $(K^{-},K^{+})$ reaction. This event was interpreted as a production and decay of $ {}_{\ Lambda\Lambda}^{\;10}\mathrm{Be}$, ${}_{\ Lambda\Lambda}^{\;11}\mathrm{Be}$, or ${}_{\ Lambda\Lambda}^{\;12}\mathrm{Be}^{*}$ via $\Xi^{-}$ capture in ${}^{16}\mathrm{O}$. By assuming the capture in the atomic 3D state, the binding energy of two $\ Lambda$ hyperons$\,$($B_{\ Lambda\Lambda}$) of these double-$\ Lambda$ hypernuclei are obtained to be $15.05 \pm 0.11\,\mathrm{MeV}$, $19.07 \pm 0.11\,\mathrm{MeV}$, and $13.68 \pm 0.11\,\mathrm{MeV}$, respectively. Based on the kinematic fitting, ${}_{\ Lambda\Lambda}^{\;11}\mathrm{Be}$ is the most likely explanation for the observed event.