Precise determination of $^{12}_{\Lambda}$C level structure by $\gamma$-ray spectroscopy

The level structure of the C-12(Lambda) hypernucleus was precisely determined by means of gamma-ray spectroscopy. We identified four gamma-ray transitions via the C-12(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 +/- 0.3(stat) +/- 0.3(syst) keV from the direct M1 transition. Excitation energies of the 1(2)(-) and 1(3)(-) states were measured to be 2832 +/- 3 +/- 4 keV and 6050 +/- 8 +/- 7 keV, respectively. The level energies obtained provide definitive references for the reaction spectroscopy of Lambda hypernuclei.