The schematic energy landscape for the Efimov scenario with four identical bosons.The energy thresholds for the different configurations — +++, ++, and + — are indicated with different shading. The green lines follow the energy trajectory of the Efimov trimers, and each green curve is a copy of the previous one scaled by a factor in each direction. Only three of the infinity of such curves are shown. For each Efimov trimer, there are two universal four-body bound states indicated by red lines. The experiment in Ref.  was carried out at essentially zero energy by fixing the scattering length to some negative value and measuring the four-body recombination rate (the rate at which the system recombined into a lower-energy configuration). Where the red or green lines intersect zero energy, recombination is resonant, giving the signal interpreted as an Efimov feature.