  In the E07 experiment at J-PARC, we observed an extremely deep
$\Xi^-$-$^{14}_{}{\rm N}$ bound-system via $\Xi^-$ capture at rest followed by
a decay into a twin single-$\Lambda$ hypernucleus in nuclear emulsion
detectors. The decay mode of the system was determined as $^{5}_{\Lambda}{\rm
He}$ + $^{5}_{\Lambda}{\rm He}$ + $^{4}_{}{\rm He}$ + n. Since there are no
excited state for daughter particles, the binding energy of $\Xi^-$ hyperon,
$B_{\Xi^-}$, in $^{14}_{}{\rm N}$ nucleus was uniquely determined to be 6.27
$\pm$ 0.27 MeV for the one event. By reanalysis of the past E373 experiment at
KEK-PS, we obtained another $\Xi^-$-$^{14}_{}{\rm N}$ system having a
$B_{\Xi^-}$ value, 8.00 $\pm$ 0.77 MeV or 4.96 $\pm$ 0.77 MeV, via the decay
$^{9}_{\Lambda}{\rm Be}$ + $^{5}_{\Lambda}{\rm He}$ + n, where the two possible
values of $B_{\Xi^-}$ correspond to the ground and the excited state of the
daughter $^{9}_{\Lambda}{\rm Be}$ nucleus, respectively. Since the $B_{\Xi^-}$
values are larger than those of the previously reported events (KISO and IBUKI)
interpreted as the nuclear $1p$ state of $\Xi^-$-$^{14}_{}{\rm N}$ system,
these two events give the first indication of the nuclear $1s$ state of $\Xi$
hypernucleus, $^{15}_{\Xi}{\rm C}$.
