γ-ray spectroscopy of Λ16O and Λ15N hypernuclei via the 16O(K-,π-γ) reaction

The bound-state level structures of the Λ16O and Λ15N hypernuclei were studied by γ-ray spectroscopy using a germanium detector array (Hyperball) via the 16O (K-,π-γ) reaction. A level scheme for Λ16O was determined from the observation of three γ-ray transitions from the doublet of states (2-,1-) at ∼6.7 MeV to the ground-state doublet (1-,0-). The Λ15N hypernuclei were produced via proton emission from unbound states in Λ16O. Three γ rays were observed, and the lifetime of the 1/2+;1 state in Λ15N was measured by the Doppler shift attenuation method. By comparing the experimental results with shell-model calculations, the spin dependence of the ΛN interaction is discussed. In particular, the measured Λ16O ground-state doublet spacing of 26.4±1.6±0.5 keV determines a small but nonzero strength of the ΛN tensor interaction