High-Precision Spectroscopy of ${}^{20}\mathrm{O}$ Benchmarking Ab Initio Calculations in Light Nuclei

The excited states of unstable ${}^{20}\mathrm{O}$ were investigated via $\gamma$-ray spectroscopy following the ${}^{19}\mathrm{O}(d,p){}^{20}\mathrm{O}$ reaction at $8A\mathrm{MeV}$. By exploiting the Doppler shift attenuation method, the lifetimes of the $2_2^{+}$ and $3_1^{+}$ states were firmly established. From the $\gamma$-ray branching and $E2/M1$ mixing ratios for transitions deexciting the $2_2^{+}$ and $3_1^{+}$ 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 $\gamma$-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 were found to be a very constraining test of the performance of the ab initio models.

Figure 1

Two-dimensional correlation between the ${}^{20}\mathrm{O}$ excitation energy on the $y$ axis and the corresponding $\gamma$-ray decays on the $x$ axis. The transitions from which the lifetimes are extracted are highlighted in red. The weakest transitions depopulating the $2_{2,3,4}^{+}$ states are marked with black boxes.

Figure 2

Prompt $\gamma$-ray spectrum following the ${}^{19}\mathrm{O}(d,p){}^{20}\mathrm{O}$ reaction measured in AGATA in coincidence with MUGAST and VAMOS++.

Figure 3

Comparison between experimental data and simulation for the lifetime of the $2_2^{+}$ state in ${}^{20}\mathrm{O}$.

Figure 4

Experimental ${}^{20}\mathrm{O}$ excited states compared to theoretical USDB shell-model calculations and VS-IMSRG results obtained with three different Hamiltonians. For theoretical states, the leading configurations are reported with a color code for each level: $(d_{5/2}{)}^4$ (black), $(d_{5/2}{)}^3(s_{1/2}{)}^1$ (green), and $(d_{5/2}{)}^2(s_{1/2}{)}^2$ (violet). Other configurations are shown in gray. The colored bar length is proportional to each contribution in the wave function. The measured and calculated $B(E2)\mathrm{s}$ (blue) and $B(M1)\mathrm{s}$ (red) are also reported proportionally to the arrow thickness.