Barn (unit)

Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is also used in all fields of high-energy physics to express the cross sections of any scattering process, and is best understood as a measure of the probability of interaction between small particles.

[1] During Manhattan Project research on the atomic bomb during World War II, American physicists Marshall Holloway and Charles P. Baker were working at Purdue University on a project using a particle accelerator to measure the cross sections of certain nuclear reactions.

According to the authors, the first published use of the term was in a (secret) Los Alamos report from late June 1943, on which the two originators were co-authors.

Calculated cross sections are often given in terms of inverse squared gigaelectronvolts (GeV−2), via the conversion ħ2c2/GeV2 = 0.3894 mb = 38940 am2.

The most common SI prefixed unit for the barn is the femtobarn, which is equal to a tenth of a square zeptometer.

Many scientific papers discussing high-energy physics mention quantities of fractions of femtobarn level.

Thus if a detector has accumulated 100 fb−1 of integrated luminosity, one expects to find 100 events per femtobarn of cross-section within these data.