14.2: S-wave scattering

Although exact at all energies, the partial wave method is most useful for dealing with scattering of low energy particles. This is because for slow moving particles to have large angular momentum $(\hbar kb)$ they must have large impact parameters $b$. Classically, particles with impact parameter larger than the range of the potential miss the potential. Thus for scattering of slow-moving particles we need only consider a few partial waves, all the others are unaffected by the potential $(\delta_l \approx 0)$. Thus partial waves and the Born approximation are complementary methods, good for slow and fast particles respectively.

For very low energy we need consider only the first term in the partial wave expansion. This is known as S-wave scattering. In this case it is possible to solve for the differential cross section, since only the first term in the series for $f(\theta )$ is involved: Since the angular variation is $P_0(\cos \theta ) = 1$ the scattering is isotropic.

$$\frac{d\sigma}{d\Omega} = |f(\theta )|^2 = k^{−2} \sin^2 \delta_0 \nonumber$$

At higher energies, other angular momentum components come into play. For a given $l$ component, scattering is maximised for $\delta_l = \pi /2$.