  In an emulsion-counter hybrid experiment performed at J-PARC, a $\Xi^-$
absorption event was observed which decayed into twin single-$\Lambda$
hypernuclei. Kinematic calculations enabled a unique identification of the
reaction process as $\Xi^{-} + ^{14}$N$\ \rightarrow\ ^{10}_\Lambda$Be +
$^5_\Lambda$He. For the binding energy of the $\Xi^{-}$ hyperon in the
$\Xi^-$-$^{14}$N system a value of $1.27 \pm 0.21$ MeV was deduced. The energy
level of $\Xi^-$ is likely a nuclear $1p$ state which indicates a weak
${\Xi}N$-$\Lambda\Lambda$ coupling.

Nuclear Experiment