Precise Determination of Λ12C Level Structure Byγ-Ray Spectroscopy

The level structure of the |${}^{12}_{\Lambda }{\rm C}$| hypernucleus was precisely determined by means of |$\gamma$|-ray spectroscopy. We identified four |$\gamma$|-ray transitions via the |${}^{12}{\rm C}(\pi ^{+ },K^{+ }\gamma )$| reaction using a germanium detector array, Hyperball2. The spacing of the ground-state doublet |$(2^{-},1^{-}_{1})$| was measured to be |$161.5 \pm 0.3{\text {(stat)}}\pm 0.3(\hbox {syst})$|keV from the direct |$M1$| transition. Excitation energies of the |$1^{-}_{2}$| and |$1^{-}_{3}$| states were measured to be |$2832\pm 3\pm 4$|keV and |$6050\pm 8\pm 7$|keV, respectively. The level energies obtained provide definitive references for the reaction spectroscopy of |$\Lambda$| hypernuclei.