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 . In a hypothetical experiment in which a flux of particles (incident particles per unit area per unit time) scatter at a rate (scattered particles per unit time), the cross section is .
Cross sections are measured in units of area, where the special unit barn (b) is used with prefixes. A barn is 10−28 m2 (100 fm2). Cross sections at the LHC can range from hundreds of millibarns (the total cross section of proton-proton collisions) down to fb or smaller for rare processes that are still considered to be measurable.