Multichannel nature of the lithium few-body puzzle

In the past two decades, quantum simulators based on cold atomic gases have proven to be ideal platforms for studying strongly interacting few-body quantum systems. By exploiting magnetic Feshbach resonances, a rich data set of three-body scattering observables has now been collected for the majority of the alkali metals. Generally, these experiments have been well described by universal theories, which neglect much of the detail of the interactions in favor of simplicity. Experiments with lithium-7 however, the lightest bosonic alkali, have found peculiar behavior which deviates significantly from the heavier elements, and defies all theoretical predictions. This challenge to few-body physics is now usually referred to as the lithium few-body puzzle. Here, we perform a sophisticated numerical analysis of the lithium three-body system, focusing specifically on the effect of spin-exchange coupling between different hyperfine states. We find that three-body observables in lithium show strong sensitivity to the multichannel nature of the underlying model. By going far beyond traditional approaches and expanding our model to the full three-body hyperfine space, we obtain unprecedented match with the available experimental data. These findings strongly suggest that the solution to the lithium few-body puzzle can be found in multichannel physics.