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 $a^4$ for large $|a|,$ with different mechanisms for $a<\mathrm{}0\mathrm{}$ and $a>\mathrm{}0\mathrm{}$. Our calculations document a previously established mechanism that causes $K_3$ to grow rapidly as the two-body scattering length $a$ increases toward $+\infty$, and a new tunneling mechanism that produces an even stronger enhancement of $K_3$ as $a\to -\infty$. The expectations based on these two mechanisms can be modified by quantum mechanical interference or resonance effects.