The structure of exotic nuclei provides valuable tests for state-of-the-art nuclear theory. In particular electromagnetic transition rates are more sensitive to aspects of nuclear forces and many-body physics than excitation energies alone. We report the first lifetime measurement of excited states in $^{21}$O, finding $\tau_{1/2^+}=420^{+35}_{-32}\text{(stat)}^{+34}_{-12}\text{(sys)}$\,ps. This result together with the deduced level scheme and branching ratio of several $\gamma$-ray decays are compared to both phenomenological shell-model and ab initio calculations based on two- and three-nucleon forces derived from chiral effective field theory. We find that the electric quadrupole reduced transition probability of $\rm B(E2;1/2^+ \rightarrow 5/2^+_{g.s.}) = 0.71^{+0.07\ +0.02}_{-0.06\ -0.06}$~e$^2$fm$^4$, derived from the lifetime of the $1/2^+$ state, is smaller than the phenomenological result where standard effective charges are employed, suggesting the need for modifications of the latter in neutron-rich oxygen isotopes. We compare this result to both large-space and valence-space ab initio calculations, and by using multiple input interactions we explore the sensitivity of this observable to underlying details of nuclear forces.

中文翻译：

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用于基准核哈密顿量的 21O 的电磁特性

奇异核的结构为最先进的核理论提供了有价值的测试。特别是电磁跃迁率对核力和多体物理学的各个方面比单独的激发能更敏感。我们报告了 $^{21}$O 中激发态的第一次生命周期测量，发现 $\tau_{1/2^+}=420^{+35}_{-32}\text{(stat)}^{ +34}_{-12}\text{(sys)}$\,ps。将此结果连同推导出的几个 $\gamma$ 射线衰变的能级方案和分支比与现象学壳模型和基于从手性有效场理论导出的两核和三核力的从头计算进行比较。我们发现电四极减少了$\rm B(E2;1/2^+ \rightarrow 5/2^+_{gs}) = 0.71^{+0.07\ +0.02}_{-0.06\ - 0.06}$~e$^2$fm$^4$，源自 $1/2^+$ 状态的寿命，小于使用标准有效电荷的现象学结果，表明需要在富中子氧同位素中对后者进行修改。我们将这个结果与大空间和价空间的从头算计算进行比较，并通过使用多个输入相互作用来探索这个可观察到的核力的潜在细节的敏感性。