Illustration: Alan Stonebraker

Figure 1: 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. [7] 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.