Barn (unit)#Commonly used prefixed versions

File:Helium atom QM DE.svg nucleus; ⁴He has a very small cross-section, less than 0.01 barn.]]

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 an account of theirs from a couple years later, they were dining in a cafeteria in December 1942 and discussing their work. They "lamented" that there was no name for the unit of cross section and challenged themselves to develop one. They initially tried eponyms, names of "some great men closely associated with the field" that they could name the unit after, but struggled to find one that was appropriate. They considered "Oppenheimer" too long (in retrospect, they considered an "Oppy" to perhaps have been allowable), and considered "Bethe" to be too easily confused with the commonly-used Greek letter beta. They then considered naming it after John Manley, another scientist associated with their work, but considered "Manley" too long and "John" too closely associated with toilets. But this latter association, combined with the "rural background" of one of the scientists, suggested to them the term "barn", which also worked because the unit was "really as big as a barn." 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.M.G. Holloway and C.P. Baker, "Note on the origin of the term 'barn'"], [https://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-00523-MS LAMS-523] (submitted 13 September 1944, issued 5 March 1947). Reprinted as {{cite journal|last1=Holloway|first1=M.G.|last2=Baker|first2=C.P.|title=How the barn was born|journal=Physics Today|volume=25|number=7|page=9|year=1972|doi=10.1063/1.3070918}} The initial published report to feature the unit was C.P. Baker, M.G. Holloway, L.D.P. King, and R.E. Schreiber, "Cross section for the reaction 20(230, 240)10," [LAMS-2](https://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-00002-MS) (June 28, 1943), which also defines the unit in its abstract.

The unit symbol for the barn (b) is also the IEEE standard symbol for bit. In other words, 1 Mb can mean one megabarn or one megabit.

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

In natural units (where ħ = c = 1), this simplifies to GeV⁻² = {{val|0.3894|u=mb}} = {{val|38940|u=am²}}.

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

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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 {{val|100|u=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.{{cite news

| author = Kate Metropolis

| date = July 21, 2004

| title = Understanding luminosity through 'barn', a unit that helps physicists count particle events

| url = http://news-service.stanford.edu/news/2004/july21/femtobarn-721.html

| publisher = Stanford News Service

| access-date = 2009-03-13

| archive-date = 2009-05-10

| archive-url = https://web.archive.org/web/20090510020557/http://news-service.stanford.edu/news/2004/july21/femtobarn-721.html

| url-status = dead

}}{{cite web

|author = Mason Inman, Emily Ball

|date = April 16, 2004

|title = PEP-II's luminous life

|url = http://www2.slac.stanford.edu/tip/special/PEP-II-04-16-04.htm

|publisher = SLAC

|access-date = 2009-03-13

|url-status = dead

|archive-url = https://web.archive.org/web/20090602222519/http://www2.slac.stanford.edu/tip/special/PEP-II-04-16-04.htm

|archive-date = June 2, 2009

}}

Fermilab produced {{val|10|u=fb⁻¹}} in the first decade of the 21st century.{{cite web|url=http://www.science20.com/quantum_diaries_survivor/10_inverse_femtobarns_celebration_time_fermilab |title=10 Inverse Femtobarns: Celebration Time At Fermilab! |publisher=Science20.com |date=December 12, 2010 |access-date=2011-04-08}} Fermilab's Tevatron took about 4 years to reach {{val|1|u=fb⁻¹}} in 2005, while two of CERN's LHC experiments, ATLAS and CMS, reached over {{val|5|u=fb⁻¹}} of proton–proton data in 2011 alone.{{cite web|url=http://www.quantumdiaries.org/2011/10/17/what-will-we-do-with-all-this-data/ |title=What will we do with all this data? |publisher=Pauline Gagnon, Quantum Diaries |date=October 17, 2011}}{{cite web|url=http://www.fnal.gov/pub/today/archive_2008/today08-09-26.html |title=CDF, DZero reach 5 inverse femtobarns of luminosity |publisher=Fermilab Today |date=September 26, 2008 |access-date=2011-07-23}}{{cite web|url=http://www.symmetrymagazine.org/breaking/2011/06/17/lhc-experiments-reach-record-data-milestone/ |title=LHC experiments reach record data milestone |publisher=Symmetry magazine |date=June 17, 2011 |access-date=2011-07-23}}{{cite web|url=https://twitter.com/CERN/status/99405126694993920|title=Thanks to recent fine tuning, the LHC has delivered 2 inverse femtobarns of data already this year; peak luminosity is now over 2x10^33.|publisher=CERN |date=August 5, 2011 |access-date=2011-08-05}}{{cite web|url=http://cdsweb.cern.ch/record/1372204 |title=LHC Report: 2 inverse femtobarns! |publisher=CERN Bulletin |date=2011-08-05 |access-date=2011-08-06}}{{cite web|url=http://www.interactions.org/cms/?pid=1031152|title=LHC proton run for 2011 reaches successful conclusion|publisher=CERN Press Office|date=2011-10-31|access-date=2011-10-31}} In April 2012, the LHC achieved the collision energy of {{val|8|u=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.{{cite web|url=http://lpc.web.cern.ch/lpc/lumiplots_2012.htm |title=LHC luminosity plots |publisher=CERN |date=2012-12-17 |url-status=dead |archive-url=https://archive.today/20130219192316/http://lpc.web.cern.ch/lpc/lumiplots_2012.htm |archive-date=2013-02-19 }} As of November 2016, the LHC had achieved {{val|40|u=fb⁻¹}} over that year, significantly exceeding the stated goal of {{val|25|u=fb⁻¹}}.{{Cite web|url=http://home.cern/about/opinion/2016/11/lhc-smashes-targets-2016-run-0|title=LHC smashes targets for 2016 run {{!}} CERN|website=home.cern|access-date=2016-11-02}} In total, the second run of the LHC has delivered around {{val|150|u=fb⁻¹}} to both ATLAS and CMS in 2015–2018.{{Cite web|url=https://home.cern/news/news/accelerators/lhc-report-final-days-run-2|title=LHC Report: The final days of Run 2 {{!}} CERN|website=home.cern|access-date=2018-12-07}}

As a simplified example, if a beamline runs for 8 hours (28 800 seconds) at an instantaneous luminosity of {{val|300|e=30|u=cm⁻²⋅s⁻¹}} {{=}} {{val|300|u=μb⁻¹⋅s⁻¹}}, then it will gather data totaling an integrated luminosity of {{val|8640000|u=μb⁻¹}} = {{val|8.64|u=pb⁻¹}} = {{val|0.00864|u=fb⁻¹}} during this period. If this is multiplied by the cross-section, then a dimensionless number is obtained equal to the number of expected scattering events.

• "Shake", a unit of time created by the same people at the same time as the barn
• Orders of magnitude (area)
• List of unusual units of measurement
• List of humorous units of measurement

{{reflist|30em}}

• IUPAC [http://goldbook.iupac.org/B00598.html citation for this usage of "barn"]

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Category:Units of area

Category:Non-SI metric units

Category:Particle physics