Finite-range corrections to the Efimov scaling factor 22.7
In plain words
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.
Precise statement
For identical bosons, the zero-range ratio a_-^(2)/a_-^(1) = exp(pi/s_0) = 22.7 with $s_0 = 1.00624$. Measure the ratio to 2% in a species other than Cs and compute the finite-range correction from realistic van der Waals potentials. Answer: measured and predicted ratios agreeing within errors.
What would settle it
Observation of the second Efimov resonance in a second bosonic species with percent-level precision, compared with a calculation using the measured interatomic potential.
Status in the literature
A 2014 cesium measurement gave a ratio of about 21.0(1.3) (Huang et al., PRL); no second homonuclear species has been measured at comparable precision.