= Scattering cross-section
{title2=$\sigma=R_{\rm sc}/j_{\rm in}$}
A scattering cross-section divides the scattered rate per target by the incident <particle flux>. Its differential form resolves that rate by outgoing direction. It has units of area for three-dimensional scattering and length for two-dimensional scattering. In two dimensions, the convention $\psi\sim e^{ikr\cos\theta}+e^{i\pi/4}f(\theta)e^{ikr}/\sqrt{kr}$ gives $d\sigma/d\theta=|f(\theta)|^2/k$ by comparing outgoing and incident <probability currents>.
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