Electromagnetic properties of $^{21}$O for benchmarking nuclear Hamiltonians

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.