Observation of universal four-body states tied to an Efimov Trimer F. Ferlaino, M. Berninger, S. Knoop, W. Harm, J. P. D' Incao, H.-C.Naegerl and R. Grimm
Ultracold cesium atoms and Feshbach molecules provide unique model systems to explore universal few-body phenomena. In previous experiments, we have obtained clear evidence for Efimov three-body states by studying atomic three-body recombination and atom-dimer relaxation in ultracold atomic and molecular samples. Here, we report on a fundamental step beyond the Efimov scenario by investigating universal four-body processes. We measure the four-body recombination rate coefficients in an atomic gas at large negative scattering lengths and we observe two resonant enhancements of losses. This provides a strong evidence for the existence of a pair of four-body states, which is strictly connected to Efimov trimers via universal relations. Our findings confirm recent theoretical predictions and demonstrate the enrichment of the Efimov scenario when a fourth particle is added to the generic three-body problem.