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.