When a slow particle scatters off a short ranged scatterer it cannot resolve the structure of the object since its de Broglie wavelength is very long. The idea is that then it should not be important what precise potential one scatters off, but only how the potential looks at long length scales. The formal way to solve this problem is to do a partial wave expansion, where one expands in the angular momentum components of the outgoing wave. At very low energy the incoming particle does not see any structure, therefore to lowest order one has only a spherical outgoing wave, called the s-wave in analogy with the atomic orbital at angular momentum quantum numberl=0. At higher energies one also needs to consider p and d-wave scattering and so on. The idea of describing low energy properties in terms of a few parameters and symmetries is very powerful, and is also behind the concept of renormalization. The concept of the scattering length can also be extended to potentials that decay slower than as. A famous example, relevant for proton-proton scattering, is the Coulomb-modified scattering length.
Example
As an example on how to compute the s-wave scattering length for a given potential we look at the infinitely repulsive spherical potential well of radius in 3 dimensions. The radial Schrödinger equation outside of the well is just the same as for a free particle: where the hard core potential requires that the wave function vanishes at,. The solution is readily found: Here and is the s-wave phase shift, which is fixed by the boundary condition ; is an arbitrary normalization constant. One can show that in general for small . The parameter of dimension length is defined as the scattering length. For our potential we have therefore, in other words the scattering length for a hard sphere is just the radius. To relate the scattering length to physical observables that can be measured in a scattering experiment we need to compute the cross section. In scattering theory one writes the asymptotic wavefunction as : where is the scattering amplitude. According to the probability interpretation of quantum mechanics the differential cross section is given by . If we consider only s-wave scattering the differential cross section does not depend on the angle, and the total scattering cross section is just. The s-wave part of the wavefunction is projected out by using the standard expansion of a plane wave in terms of spherical waves and Legendre polynomials : By matching the component of to the s-wave solution one has: This gives:
