First observation of a nuclear s-state of a Ξ hypernucleus, ¹⁵Ξ C

Bound systems of Ξ⁻–¹⁴_ N are studied via Ξ⁻ capture at rest followed by emission of a twin single-Λ hypernucleus in the emulsion detectors. Two events forming extremely deep Ξ⁻ 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 Ξ⁻+¹⁴\!\,\!\! N→⁵Λ\!\!\! He + ⁵Λ\!\! He + ⁴\!\! He + n. Since there are no excited states for daughter particles, the binding energy of the Ξ⁻ hyperon, BΞ⁻, in the ¹⁴_ N nucleus was uniquely determined to be 6.27 ± 0.27 MeV. Another Ξ⁻–¹⁴_ N system via the decay ⁹Λ Be + ⁵Λ\!\!\! He + n brings a BΞ⁻ value, 8.00 ± 0.77 MeV or 4.96 ± 0.77 MeV, where the two possible values of BΞ⁻ correspond to the ground and the excited states of the daughter ⁹Λ Be nucleus, respectively. Because the BΞ⁻ values are larger than those of the previously reported events (KISO and IBUKI), which are both interpreted as the nuclear $1p$ state of the Ξ⁻–¹⁴_ N system, these new events give the first indication of the nuclear $1s$ state of the Ξ hypernucleus, ¹⁵Ξ C.