First observation of a nuclear <i>s</i>-state of a $\Xi$ hypernucleus, $^{15}_{\Xi}{\rm C}$

Abstract Bound systems of $\Xi^-$–$^{14}_{}{\rm N}$ are studied via $\Xi^-$ capture at rest followed by emission of a twin single-$\Lambda$ hypernucleus in the emulsion detectors. Two events forming extremely deep $\Xi^-$ bound states were obtained by analysis of a hybrid method in the E07 experiment at J-PARC and reanalysis of the E373 experiment at KEK-PS. The decay mode of one event was assigned as $\Xi^-+^{14}\!\,\!\!{\rm N}\to^{5}_{\Lambda}\!\!\!{\rm He} + ^{5}_{\Lambda}\!\!{\rm He} + ^{4}\!\!{\rm He} + \mathrm{n}$. Since there are no excited states for daughter particles, the binding energy of the $\Xi^-$ hyperon, $B_{\Xi^-}$, in the $^{14}_{}{\rm N}$ nucleus was uniquely determined to be $6.27 \pm 0.27$ MeV. Another $\Xi^-$–$^{14}_{}{\rm N}$ system via the decay $^{9}_{\Lambda}{\rm Be} + ^{5}_{\Lambda}\!\!\!{\rm He} + \mathrm{n}$ brings a $B_{\Xi^-}$ value, $8.00 \pm 0.77$ MeV or $4.96 \pm 0.77$ MeV, where the two possible values of $B_{\Xi^-}$ correspond to the ground and the excited states of the daughter $^{9}_{\Lambda}{\rm Be}$ nucleus, respectively. Because the $B_{\Xi^-}$ values are larger than those of the previously reported events (KISO and IBUKI), which are both interpreted as the nuclear $1p$ state of the $\Xi^-$–$^{14}_{}{\rm N}$ system, these new events give the first indication of the nuclear $1s$ state of the $\Xi$ hypernucleus, $^{15}_{\Xi}{\rm C}$.