During Manhattan Project research on the atomic bomb during World War II, American physicists at Purdue University needed a secretive unit to describe the approximate cross-sectional area presented by the typical nucleus and decided on "barn". They considered this a large target for particle accelerators that needed to have direct strikes on nuclei, and the American idiom "couldn't hit the broad side of a barn" refers to someone whose aim is very bad. Initially they hoped the name would obscure any reference to the study of nuclear structure; eventually, the word became a standard unit in nuclear and particle physics.
Commonly used prefixed versions
Other related units are the outhouse and the shed, although these are rarely used in practice.
Conversions
Calculated cross sections are often given in terms of gigaelectronvolts, via the conversion ħ2c2/GeV2 = =. In natural units, this simplifies to GeV−2 = =.
barn
GeV-2
1 mb
1 pb
0.389379 mb
0.389379 pb
SI units with prefix
In SI, one can use units such as square femtometers.
The inverse femtobarn is the unit typically used to measure the number of particle collision events per femtobarn of target cross-section, and is the conventional unit for time-integrated luminosity. Thus if a detector has accumulated of integrated luminosity, one expects to find 100 events per femtobarn of cross-section within these data. Consider a particle accelerator where two streams of particles, with cross-sectional areas measured in femtobarns, are directed to collide over a period of time. The total number of collisions will be directly proportional to the luminosity of the collisions measured over this time. Therefore, the collision count can be calculated by multiplying the integrated luminosity by the sum of the cross-section for those collision processes. This count is then expressed as inverse femtobarns for the time period. Inverse femtobarns are often quoted as an indication of particle collider productivity. Fermilab produced in the first decade of the 21st century. Fermilab's Tevatron took about 4 years to reach in 2005, while two of CERN's LHC experiments, ATLAS and CMS, reached over of proton–proton data in 2011 alone. In April 2012 the LHC achieved the collision energy of with a luminosity peak of 6760 inverse microbarns per second; by May 2012 the LHC delivered 1 inverse femtobarn of data per week to each detector collaboration. A record of over 23 fb−1 was achieved during 2012. As of November 2016, the LHC had achieved over that year, significantly exceeding the stated goal of. In total, the second run of the LHC has delivered around to both ATLAS and CMS in 2015–2018.
Usage example
As a simplified example, if a beamline runs for 8 hours at an instantaneous luminosity of , then it will gather data totaling an integrated luminosity of = = during this period. If this is multiplied by the cross-section, then a dimensionless number is obtained which would be simply the number of expected scattering events.
