Efimov states in excited nuclear halos

Universality -- an essential concept in physics -- implies that different systems show the same phenomenon and can be described by a unified theory. A prime example of the universal quantum phenomena is the Efimov effect, which is the appearance of multiples of low-energy three-body bound states with progressively large sizes dictated by the discrete scale invariance. The Efimov effect, originally proposed in the nuclear physics context, has been observed in cold atoms and $^4\mathrm{He}$ molecules. The search for the Efimov effect in nuclear physics, however, has been a long-standing challenge owing to the difficulty in identifying ideal nuclides with a sufficiently large $s$-wave scattering length; such nuclides can be unambiguously considered as Efimov states. Here, we propose a systematic method to identify nuclides that exhibit Efimov states in their excited states in the vicinity of the neutron separation threshold. These nuclei are characterised by their enormous low-energy neutron capture cross-sections. Using our protocol, we identified $^{90}\mathrm{Zr}$ and $^{159}\mathrm{Gd}$ as novel candidate nuclides that show the Efimov states. They are well inside the stable region in the nuclear chart, and are particularly suited for experimental realization of the long-sought Efimov states in nuclear physics.