Dfyjkt

A barn (symbol: b) is a unit of area equal to 10⁻²⁸ m² (100 fm²). 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. A barn is approximately the cross-sectional area of a uranium nucleus. The barn is also the unit of area used in nuclear quadrupole resonance and nuclear magnetic resonance to quantify the interaction of a nucleus with an electric field gradient. While the barn is not an SI unit, the SI standards body acknowledges its existence due to its continued use in particle physics.^[1]

Etymology

The etymology of the unit barn is whimsical: 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 (10⁻²⁸ m²) and decided on "barn". This was particularly applicable because 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"^[2] refers to someone whose aim is terrible. 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.^[3][4]

Commonly used prefixed versions

Other related units are the outhouse (1 μb, or 10⁻³⁴ m²) and the shed (10⁻²⁴ b (1 yb), or 10⁻⁵² m²), although these are rarely used in practice.^[7]

Conversions

Calculated cross sections are often given in terms of gigaelectronvolts (GeV), via the conversion ħ²c²/GeV² = 6996389400000000000♠0.3894 mb = 7004389400000000000♠38940 am².

In natural units (where ħ = c = 1), this simplifies to GeV⁻² = 6996389400000000000♠0.3894 mb = 7004389400000000000♠38940 am².

SI units with prefix

In SI, one can use units such as square femtometers (fm²).

Inverse femtobarn

The inverse femtobarn (fb⁻¹) 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 7002100000000000000♠100 fb⁻¹ 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 (e.g., 100 fb⁻¹ in nine months). Inverse femtobarns are often quoted as an indication of particle collider productivity.^[8][9]

Fermilab produced 7001100000000000000♠10 fb⁻¹ in the first decade of the 21st century.^[10] Fermilab's Tevatron took about 4 years to reach 7000100000000000000♠1 fb⁻¹ in 2005, while two of CERN's LHC experiments, ATLAS and CMS, reached over 7000500000000000000♠5 fb⁻¹ of proton–proton data in 2011 alone.^{[11][12][13][14][15][16]} In April 2012 the LHC achieved the collision energy of 6994128174118960000♠8 TeV 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.^[17] As of November 2016, the LHC had achieved 7001400000000000000♠40 fb⁻¹ over that year, significantly exceeding the stated goal of 7001250000000000000♠25 fb⁻¹.^[18] In total, the second run of the LHC has delivered around 7002150000000000000♠150 fb⁻¹ to both ATLAS and CMS in 2015–2018.^[19]

Usage example

As a simplified example, if a beamline runs for 8 hours (28 800 seconds) at an instantaneous luminosity of 7032300000000000000♠300×10³⁰ cm⁻²⋅s⁻¹ = 7002300000000000000♠300 μb⁻¹⋅s⁻¹, then it will gather data totaling an integrated luminosity of 7006864000000000000♠8640000 μb⁻¹ = 7000864000000000000♠8.64 pb⁻¹ = 6997864000000000000♠0.00864 fb⁻¹ 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.

See also

• Orders of magnitude (area)


• List of unusual units of measurement

References

External links

• 
  IUPAC citation for this usage of "barn"