# Cross section

The cross section for a given process is a measure for the rate of interactions that originate from the process that can be quoted independently of the experimental set-up of the scattering experiment itself. It is denoted $\sigma$. In a hypothetical experiment in which a flux of particles $F$ (incident particles per unit area per unit time) scatter at a rate $R$ (scattered particles per unit time), the cross section is $\sigma = R/F$.
In calculations using natural units (with $\hbar = c = 1$), area has dimensions of 1/energy-squared, so units of GeV-2 may be used. The conversion is 1 GeV-2 = 0.389 mb.