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.