Observation of the super Efimov tower in two-dimensional p-wave fermions
In plain words
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.
Precise statement
Spin-polarized fermions in 2D at a p-wave Feshbach resonance in a single orbital channel $l = +1$ or $l = -1$. Nishida, Moroz and Son (PRL 2013) predict trimers with $l = +1$ or $-1$ and binding energies $E_n \sim \exp(-2 \exp(3 \pi n/4 + \theta))$, proven rigorously by Gridnev (2014). Answer: observation of at least two trimer features (three-body loss or spectroscopic) whose positions follow the $3 \pi/4$ double-exponential law, or a quantified demonstration that p-wave two-body losses preclude it.
What would settle it
Three-body loss or radio-frequency spectroscopy in a quasi-2D single-component Fermi gas near a channel-resolved p-wave resonance showing two trimer features at the predicted ratio.
Status in the literature
Unverified note
Theoretical follow-ups continued through 2025 and a related dynamical super-Efimovian expansion of a unitary gas was observed in 2017 (arXiv:1707.06732), but the super Efimov trimers have not been observed.