The schematic energy landscape for the Efimov scenario with four identical bosons.The energy thresholds for the different configurations — B+B+B+B, B2+B+B, and B2+B2 — 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 a 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.