Mechanisms of anomalous three-body loss in a population-imbalanced three-component Fermi gas

Abstract

Achieving precise control of ultracold atomic gases requires a detailed understanding of atom loss mechanisms. Motivated by the anomalous three-body decay in a three-component Fermi gas reported in Ref. [1], this work investigates mechanisms that possibly contribute to the observed loss. The three-body Schr\"odinger equation is solved in the hyperspherical adiabatic representation with pairwise van der Waals interactions, and the $S$-matrix is obtained via the eigenchannel $R$-matrix method to compute recombination rate coefficients $K_3$ and two-body cross sections. At the magnetic field strength where the anomalous decay occurs, $K_3$ is unitary limited, exhibiting the threshold energy scaling $K_3(E)\propto E^{-1}$. Consequently, the thermally averaged $\langle K_3 \rangle$ acquires a temperature dependence. Because the experiment is performed in the degenerate regime, $\langle K_3 \rangle$ also explicitly depends on the per-spin densities through the per-spin Fermi energies $E_{F}^{(i)}\propto n_i^{2/3}$. As the gas is diluted and degeneracy is reduced, $\langle K_3 \rangle$ approaches the non-degenerate value and becomes a function of temperature only. Channel-resolved branching ratios and cross sections are folded into a Monte Carlo cascade simulation of secondary collisions and trap escape. The analysis indicates that typical three-body recombination events remove fewer than three atoms on average, and that the atom losses are primarily due to the ejection of secondary collision products, rather than the initial three-body recombination products. Therefore, a significant fraction of the released binding energy remains in the trapped ensemble as kinetic energy. Retained energy drives evaporative loss, offering a plausible, partial explanation for the anomalous decay.