In 1970 Vitaly Efimov derived a bizarre result. If, his analysis showed, one could weaken the interaction between two particles, then just at the point when the two-body bound state vanishes, an infinite series of three-body bound states would emerge out of the quantum blue. 1
The counterintuitive result provoked skepticism among Efimov’s fellow theorists. Vindication by experimenters did not appear because the states, though quite generic, require unusual circumstances: The range of the interatomic potential must be much smaller than the scattering length, a quantum mechanical parameter that loosely corresponds to the radius of the scattering cross section at low energies.
At first, Efimov and others hoped to see a clear example of the three-body states in the weakly bound outskirts of nuclei. Excited helium-4 trimers also seemed a likely venue. Unfortunately, the search in those two systems has so far proved fruitless. Nature, unlike a theorist, can’t guarantee...
