Identification of a universal three-body s-wave resonance in ¹⁰He

Universality in physics describes the emergence of common features in vastly different systems. One fascinating example is the Efimov phenomenon where three-body resonantly interacting systems display universal properties. Efimov states have been observed in ultra-cold-atom systems, but their manifestation in nuclei remains elusive due to the long-range repulsive Coulomb interaction and the stringent requirement for at least two s-wave resonances in its subsystems. Recent theories propose another universality in three-body halos with only one s-wave resonance. Here, we report the identification of a precursor of this phenomenon in a neutron-rich ¹⁰He nucleus. With higher statistics and better sensitivities than previous experiments, we identify two low-lying 0⁺ states of ¹⁰He at  ~ 1 MeV and  ~ 2 MeV above its two-neutron decay threshold, and determine an s-wave scattering length of  ~ - 3.5 fm between ⁸He and neutron. It is revealed that the lower energy state, the ground state of ¹⁰He, is a three-body state with only s-wave interactions among its subsystems. This state manifests as a resonance structure, which is a direct consequence of a universal long-range three-body repulsion. Our work sheds new light on quantum halos with finite lifetimes, providing a path toward their unified understanding across various scales and fields.