Efimov states and three-body universality

6 problems

• Finite-range corrections to the Efimov scaling factor 22.7

  Partially resolved

  Ideal theory says each Efimov molecule is 22.7 times larger than the previous one. Measurements in cesium gave a smaller ratio, and the size of the correction is not pinned down.

• Is a separate four-body parameter needed for Efimov-linked tetramers?

  Partially resolved

  Four atoms can also bind into states attached to each Efimov molecule. The question is whether their positions follow from the three-atom number alone, or need extra information.

• Universality of the three-body parameter in heavy-heavy-light atom mixtures

  Partially resolved

  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.

• Observation of the super Efimov tower in two-dimensional p-wave fermions

  Open

  Theory predicts that identical fermions confined to a plane and tuned to a resonance with one unit of rotation bind in threes, with an infinite ladder of trimers whose energies shrink double-exponentially. No experiment has seen this ladder.

• Predicting the three-body parameter across Feshbach resonances of any width

  Partially resolved

  The position of the first Efimov molecule is measured by a number (the three-body parameter), found to be close to 9.7 times a basic atomic length. The task is a theory that predicts it for every atom and resonance, including narrow ones.

• Equation of state and contacts of the unitary Bose gas

  Partially resolved

  A Bose gas tuned to infinitely strong pair interaction should behave in a universal way, but three-atom collisions destroy it quickly. The question is what state the gas reaches before it decays and what its energy is.