Gamma-Ray Spectroscopy of ${}_{\Lambda }^{16}$O and ${}_{\Lambda }^{15}$N Hypernuclei via the ${}^{16}$O$(K^{-},{\pi }^{-})$ reaction

Abstract

he bound-state level structures of the ${}_{\Lambda }^{16}$O and ${}_{\Lambda }^{15}$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 ${}_{\Lambda }^{16}$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 ${}_{\Lambda }^{15}$N hypernuclei were produced via proton emission from unbound states in ${}_{\Lambda }^{16}$O . Three $\gamma$ -rays were observed and the lifetime of the $1/2^{+};1$ state in ${}_{\Lambda }^{15}$N was measured by the Doppler shift attenuation method. By comparing the experimental results with shell-model calculations, the spin-dependence of the $\Lambda N$ interaction is discussed. In particular, the measured ${}_{\Lambda }^{16}$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.

22 pages, 17 figures