High-Precision Spectroscopy of O 20 Benchmarking Ab Initio Calculations in Light Nuclei

The excited states of unstable O20 were investigated via γ-ray spectroscopy following the O19(d,p)O20 reaction at 8 AMeV. By exploiting the Doppler shift attenuation method, the lifetimes of the 22+ and 31+ states were firmly established. From the γ-ray branching and E2/M1 mixing ratios for transitions deexciting the 22+ and 31+ states, the B(E2) and B(M1) were determined. Various chiral effective field theory Hamiltonians, describing the nuclear properties beyond ground states, along with a standard USDB interaction, were compared with the experimentally obtained data. Such a comparison for a large set of γ-ray transition probabilities with the valence space in medium similarity renormalization group ab initio calculations was performed for the first time in a nucleus far from stability. It was shown that the ab initio approaches using chiral effective field theory forces are challenged by detailed high-precision spectroscopic properties of nuclei. The reduced transition probabilities...