Recombination of Three Atoms in the Ultracold Limit

We identify two qualitatively different mechanisms that control three-body recombination in a spin-polarized gas near zero temperature. A universal curve describes the recombination rate versus the two-body scattering length a. It grows as a⁴ for large |a|, with different mechanisms for a<0 and a>0. Our calculations document a previously established mechanism that causes K₃ to grow rapidly as the two-body scattering length a increases toward +∞, and a new tunneling mechanism that produces an even stronger enhancement of K₃ as a→-∞. The expectations based on these two mechanisms can be modified by quantum mechanical interference or resonance effects.