Observation of an Efimov-like trimer resonance in ultracold atom-dimer scattering

The field of few-body physics has originally been motivated by understanding nuclear matter, but in the past few years ultracold gases with tunable interactions have emerged as model systems to experimentally explore few-body quantum systems(1-3). Even though the energy scales involved are vastly different for ultracold and nuclear matter (picoelectronvolt as compared with megaelectronvolt), few-body phenomena acquire universal properties for near-resonant two-body interactions(2). So-called Efimov states represent a paradigm for universal quantum states in the three-body sector(4). After decades of theoretical work, a first experimental signature of such a weakly bound trimer state was recently found under conditions where a weakly bound dimer state is absent(5-7). Here, we report on a trimer state in the opposite regime, where such a dimer state exists. The trimer state manifests itself in a resonant enhancement of inelastic collisions in a mixture of atoms and dimers. Our observation is closely related to an atom-dimer resonance as predicted by Efimov(8-10), but occurs in the theoretically challenging regime where the trimer spectrum reveals effects beyond the universal limit.