= Finite-range partial-wave threshold law
{title2=$\delta_\ell(k)=O(k^{2\ell+1})$}
For regular low-energy elastic scattering by a finite-range <central potential>, higher <partial waves> are suppressed by their <centrifugal barriers>. Matching the regular interior radial solution to small-argument exterior spherical Bessel functions gives the displayed scaling of <scattering phase shift>, in the generic absence of a threshold singularity. Since the channel amplitude is proportional to $\sin\delta_\ell/k$, it scales as $k^{2\ell}$. The $\ell=0$ term is isotropic and normally leads. A tuned vanishing leading amplitude or a threshold resonance requires examining the next terms rather than applying the generic statement blindly.
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