Cross section

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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.

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.

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.

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