Universality of the three-body parameter in heavy-heavy-light atom mixtures
In plain words
Mixtures of heavy and light atoms, such as cesium with lithium, have Efimov ladders with steps of about 5 instead of 22.7. Where these ladders start depends on how the two heavy atoms interact with each other, and a full predictive theory is missing.
Precise statement
Heavy-heavy-light systems (Cs-Cs-Li-6 and other mass-imbalanced mixtures) near an interspecies Feshbach resonance, with interspecies van der Waals length r_vdW,HL and heavy-heavy scattering length $a_{\mathrm{HH}}$. Predict the first Efimov resonance position a_-^(1)/r_vdW,HL as a function of $a_{\mathrm{HH}}$ (including its sign) and resonance width, and the ratios between successive resonances. Answer: a model reproducing all measured values within error and predicting a new species pair.
What would settle it
A three-body calculation with realistic potentials matching measured Efimov spectra for at least two heteronuclear mixtures and both signs of $a_{HH}$.
Status in the literature
Li-Cs experiments showed that the Efimov spectrum changes strongly with the sign of the Cs-Cs scattering length (Ulmanis et al., PRL 117, 153201, 2016; Hafner et al., PRA 95, 062708, 2017).