# Feshbach -Resonance why are hyperfine structures important?

The hyperfine structure of energy levels around the ground state seem to enable Feshbach and be intrinsic to it. Why do we need hyperfine levels?

I.e. why is Feshbach specific to ultracold atoms in ground stae, why would a magnetic field in any gas of atoms not achieve the similar effect of inducing atom-atom coupling?

The threshold energy for a partial wave with angular momentum $l$ goes as $E_{th}(l)\sim A (l(l+1))^{3/2}$, where $A$ depends on the inter-atomic potential and masses and might range from 10 $\mu$K - 1 mK. So, at the K scale and above, it would presumably be difficult to see the effects of any one collision channel. However, I do not know if anyone has tried this experiment, or if it would be feasible with some very careful measurement.
There are probably other considerations too. For example, remember that a typical ultracold atomic gas has something like $10^{-6}$ the density of air, which is important for two-body collisions to dominate the physics.