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

Bound systems of Ξ- -14N 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- +14N → 5λHe +5λHe +4He + n. Since there are no excited states for daughter particles, the binding energy of the Ξ- hyperon, BΞ-, in the 14N nucleus was uniquely determined to be 6.27 ± 0.27MeV. Another Ξ- -14N system via the decay 9 λBe +5λHe + n brings a BΞ- value, 8.00 ± 0.77MeV or 4.96 ± 0.77MeV, 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-14Nsystem, these newevents give the first indication of the nuclear 1s state of the Ξ hypernucleus, 15ΞC.