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