A step beyond the Efimov scenario: evidence of a universal four-body state

Ultracold cesium atoms and Feshbach molecules provide unique model systems to explore universal few-body phenomena. In previous experiments, we have investigated atomic three-body recombination and atom-dimer relaxtion in ultracold atomic and molecular samples at large scattering lengths and obtained clear evidence for Efimov three-body states. Here, we report on a step beyond the Efimov scenario by investigating a four-body phenomenon that was recently predicted by von Stecher et al. (arXiv:0810.3876). Our recombination measurements in an atomic sample at negative scattering lengths show a resonant enhancement of losses and provide strong evidence for the existence of the predicted four-body bound states.