Cross section (physics)

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In physics, the cross section is a measure related to the probability that a specific process will take place in a collision of two particles. For example, the Rutherford cross section is a measure of probability that an alpha particle will be deflected by a given angle during an interaction with an atomic nucleus. Cross section is typically denoted σ (sigma) and has dimension of area, with units of square meter or more often in barns. In a way, it can be thought of as the size of the object that the excitation must hit in order for the process to occur, but more exactly, it is a parameter of a stochastic process. When two discrete particles interact in classical physics, their mutual cross section is the area transverse to their relative motion within which they must meet in order to scatter from each other. If the particles are hard hard spheres that interact only upon contact, their scattering cross section is related to their geometric size. If the particles interact through some action...

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