© David Kaplan.

A scattering cross section is an amount of area that is computed to understand the chance of two objects scattering off one another. In the case of the two billiard balls on the left—i.e., classical physics—it is simply the area within which the incoming ball must aim to hit the target ball. At the particle level—i.e., in quantum mechanics—it is a mathematical quantity that represents the probability of one particle scattering off another, or the fraction of particles in an incoming beam—the figure on the right—that scatter off the target. In the quantum case, the area can be different for different particles in the beam. For example electrons impinging on a nucleus (Rutherford's experiment, described in Unit 1) will "see" a much bigger area than neutrinos impinging on the same nucleus (at Rutherford's beam energy). (Unit: 2)