\Lambda$ binding energy $B_{\Lambda \Lambda}=7.25 \pm 0.19\pm ^{0.18}_{0.11}$ MeV  [@Nagara], which leads to a smaller $\Lambda \Lambda$ binding, $\Delta B_{\Lambda \Lambda}=1.01 \pm 0.20 \pm ^{0.18}_{0.11}$ MeV, than the previous understanding. Sometimes the emulsion events include ambiguities related with serious difficulty of identifying emission of neutral particles such as neutrons and $\gamma$-rays. In the NAGARA event, however, the production of $_\Lambda^{ }$$_\Lambda^6$He has been uniquely identified free from such an ambiguity on the basis of the observation of sequential weak decays.

Historically, in the 1960’s, there appeared two reports on the observation of double-$\Lambda$ hypernuclei, $_\Lambda^{ }$$_\Lambda^{\!\!\!10}$Be [@Danysz63] and $_\Lambda^{ }$$_\Lambda^6$He [@Prowse66], but the reality of the latter case was considered doubtful [@Dalitz89]. Two decades later the modern emulsion-counter hybrid technique has been applied in the KEK-E176 experiment [@Aoki91], in which a new double-$\Lambda$ hypernucleus event was found but no unique identification was given so far: One explanation as $_\Lambda^{ }$$_\Lambda^{\!\!\!10}$Be leads to a repulsive $\Lambda \Lambda$ interaction ($\Delta B_{\Lambda \Lambda}<0$), while the other possibility involving $_\Lambda^{ }$$_\Lambda^{\!\!\!13}$B leads to an attractive $\Lambda \Lambda$ interaction  [@Dover91; @Yamamoto91]. If the latter is the case, the extracted strength of the $\Lambda \Lambda$ interaction is attractive with $\Delta B_{\Lambda \Lambda} \simeq 4$ MeV. Although 