γ-ray spectroscopy ofΛ16O andΛ15N hypernuclei via theO16(K−,π−γ) reaction

The bound-state level structures of the ${}_{\ensuremath{\Lambda}}^{16}$O and ${}_{\ensuremath{\Lambda}}^{15}$N hypernuclei were studied by \ensuremath{\gamma}-ray spectroscopy using a germanium detector array (Hyperball) via the $^{16}\mathrm{O}$ (${K}^{\ensuremath{-}},{\ensuremath{\pi}}^{\ensuremath{-}}\ensuremath{\gamma}$) reaction. A level scheme for ${}_{\ensuremath{\Lambda}}^{16}$O was determined from the observation of three \ensuremath{\gamma}-ray transitions from the doublet of states (${2}^{\ensuremath{-}},{1}^{\ensuremath{-}}$) at $~6.7$ MeV to the ground-state doublet (${1}^{\ensuremath{-}},{0}^{\ensuremath{-}}$). The ${}_{\ensuremath{\Lambda}}^{15}$N hypernuclei were produced via proton emission from unbound states in ${}_{\ensuremath{\Lambda}}^{16}$O. Three \ensuremath{\gamma} rays were observed, and the lifetime of the $1/{2}^{+};1$ state in ${}_{\ensuremath{\Lambda}}^{15}$N was measured by the Doppler shift attenuation method. By comparing the experimental results with shell-model calculations, the spin dependence of the $\ensuremath{\Lambda}N$ interaction is discussed. In particular, the measured ${}_{\ensuremath{\Lambda}}^{16}$O ground-state doublet spacing of $26.4\ifmmode\pm\else\textpm\fi{}1.6\ifmmode\pm\else\textpm\fi{}0.5$ keV determines a small but nonzero strength of the $\ensuremath{\Lambda}N$ tensor interaction.