Editing Barn (unit)

{{short description|Unit for cross sectional area used in high-energy physics}}
{{Infobox unit
| bgcolor      = 
| name         = Barn
| image        = 
| caption      = 
| standard     = [[particle physics]]
| quantity     = area
| symbol       = b
| symbol2      = 
| namedafter   = ''the broad side of a [[barn]]''
| units1       = [[SI base unit]]s
| inunits1     = {{val|e=−28|u=m2}}
| units2       = equivalent
| inunits2     = 100&nbsp;[[femtometer|fm]]<sup>2</sup>
}}
A '''barn''' (symbol: '''b''') is a [[Metric units#Area|metric unit]] of [[area]] equal to {{val|e=−28|u=m2}} (100&nbsp;[[femtometer|fm]]<sup>2</sup>). This is equivalent to a square that is {{val|e=−14|u=m}} (10&nbsp;[[femtometer|fm]]) each side, or a circle of diameter approximately {{val|1.128|e=−14|u=m}} (11.28&nbsp;fm). 

Originally used in [[nuclear physics]] for expressing the [[Cross section (physics)|cross sectional]] area of [[Atomic nucleus|nuclei]] and [[nuclear reaction]]s, today it is also used in all fields of [[Particle physics|high-energy physics]] to express the cross sections of any [[Scattering|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 never was an [[SI]] unit, the [[International Bureau of Weights and Measures|SI standards body]] acknowledged it in the 8th SI Brochure (superseded in 2019) due to its use in [[particle physics]].<ref name="bipm-chapter4">
{{cite web
  | date = May 2006
  | url = http://www.bipm.org/en/publications/si-brochure/table8.html
  | title = Non-SI units accepted for use with the SI, and units based on fundamental constants
  | work = SI brochure (8th edition)
  | publisher = [[International Bureau of Weights and Measures|BIPM]]
  | access-date = 2009-03-13
}}</ref>

==Etymology==
[[File:Helium atom QM DE.svg|thumb|Image of a [[helium-4]] nucleus; <sup>4</sup>He 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 section (physics)|cross section]]s 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 [[eponym]]s, 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 "[[J. Robert Oppenheimer|Oppenheimer]]" too long (in retrospect, they considered an "Oppy" to perhaps have been allowable), and considered "[[Hans Bethe|Bethe]]" to be too easily confused with the commonly-used Greek letter [[beta]]. They then considered naming it after [[John Henry Manley|John Manley]], another scientist associated with their work, but considered "Manley" too long and "John" too closely associated with [[toilet]]s. 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.<ref>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.</ref>

==Commonly used prefixed versions==
The unit symbol for the barn (b) is also the IEEE standard symbol for [[Bit#Multiple_bits|bit]]. In other words, 1 Mb can mean one megabarn or one megabit.
{|class="wikitable"
|+Multiples and sub-multiples<ref>{{cite web
 | author = Russ Rowlett
 | date = September 1, 2004
 | title = Units: S
 | url = http://www.unc.edu/~rowlett/units/dictS.html
 | work = How Many? A Dictionary of Units of Measurement
 | publisher = [[University of North Carolina at Chapel Hill]]
 | access-date = 2009-12-14
}}</ref><ref>{{cite book|last=Green|first=Jonathon|title=Dictionary of Jargon|url=https://books.google.com/books?id=P0QOAAAAQAAJ&pg=PA487|access-date=2009-12-14|date=December 1987|publisher=Routledge Kegan & Paul|isbn=0-7100-9919-3|page=487}}</ref>
|-
! Unit
! Symbol
! m<sup>2</sup>
! cm<sup>2</sup>
|-
| megabarn
| Mb
| 10<sup>−22</sup>
| 10<sup>−18</sup>
|-
| kilobarn
| kb
| 10<sup>−25</sup>
| 10<sup>−21</sup>
|-
| barn
| b
| 10<sup>−28</sup>
| 10<sup>−24</sup>
|-
| millibarn 
| mb
| 10<sup>−31</sup>
| 10<sup>−27</sup>
|-
| microbarn
| μb
| 10<sup>−34</sup>
| 10<sup>−30</sup>
|-
| nanobarn
| nb
| 10<sup>−37</sup>
| 10<sup>−33</sup>
|-
| picobarn
| pb
| 10<sup>−40</sup>
| 10<sup>−36</sup>
|-
| femtobarn
| fb
| 10<sup>−43</sup>
| 10<sup>−39</sup>
|-
| attobarn 
| ab
| 10<sup>−46</sup>
| 10<sup>−42</sup>
|-
| zeptobarn 
| zb
| 10<sup>−49</sup>
| 10<sup>−45</sup>
|-
| yoctobarn
| yb
| 10<sup>−52</sup>
| 10<sup>−48</sup>
|}

==Conversions==
Calculated cross sections are often given in terms of inverse squared gigaelectronvolts ([[Electronvolt|GeV<sup>−2</sup>]]), via the conversion [[reduced Planck constant|''ħ'']]<sup>2</sup>[[Speed of light|''c'']]<sup>2</sup>/[[Electronvolt|GeV]]<sup>2</sup> = {{val|0.3894|u=mb}} = {{val|38940|u=am<sup>2</sup>}}.

In [[Natural units#Natural units (particle and atomic physics)|natural units]] (where [[reduced Planck constant|''ħ'']] = [[Speed of light|''c'']] = 1), this simplifies to [[Electronvolt|GeV]]<sup>−2</sup> = {{val|0.3894|u=mb}} = {{val|38940|u=am<sup>2</sup>}}.

{| class="wikitable"
|-
! barn !! GeV<sup>−2</sup>
|-
| 1 mb || {{val|2.56819|u=GeV<sup>−2</sup>}}
|-
| 1 pb || {{val|2.56819|e=-9|u=GeV<sup>−2</sup>}}
|-
| 0.389379 mb || {{val|1|u=GeV<sup>−2</sup>}}
|-
| 0.389379 pb || {{val|1|e=-9|u=GeV<sup>−2</sup>}}
|}

===SI units with prefix===
In SI, one can use units such as square femtometers (fm<sup>2</sup>). 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.

{|
| style="vertical-align:top" | 
{|class="wikitable"
|+Conversion from SI units
|-
! SI
! style="width:2in" | barns
|-
| 1&nbsp;pm<sup>2</sup>
| 10&nbsp;kb
|-
| 1&nbsp;fm<sup>2</sup> 
| 10&nbsp;mb
|-
| 1&nbsp;am<sup>2</sup>
| 10&nbsp;nb
|-
| 1&nbsp;zm<sup>2</sup> 
| 10&nbsp;fb
|-
| 1&nbsp;ym<sup>2</sup> 
| 10&nbsp;zb
|-
| 1&nbsp;rm<sup>2</sup> 
| 10&nbsp;rb
|}
|
{|class="wikitable"
|+Conversion to SI units
|-
! Barns
! SI
! Other names
|-
| 1&nbsp;b
| 100&nbsp;fm<sup>2</sup>
|-
| 1&nbsp;cb
| 1&nbsp;fm<sup>2</sup>
|-
| 1&nbsp;mb
| 0.1&nbsp;fm<sup>2</sup> = 100000&nbsp;am<sup>2</sup>
|-
| 1&nbsp;μb
| 100&nbsp;am<sup>2</sup>
| Outhouse <ref>
{{cite web
 | author = Craig A. Stone, Ph.D.
 | title = Glossary
 | url = http://www.nuclearglossary.com/abcs/nuclearglossary_B.html
 | work = The Language of the Nucleus
 | publisher = Scientific Digital Visions, Inc
 | archive-url = https://web.archive.org/web/20070630125216/http://www.nuclearglossary.com/abcs/nuclearglossary_B.html
 | access-date = 2022-11-22
| archive-date = 2007-06-30
 }} </ref>
|-
| 1&nbsp;nb
| 0.1&nbsp;am<sup>2</sup> = 100000&nbsp;zm<sup>2</sup>
|-
| 1&nbsp;pb
| 100&nbsp;zm<sup>2</sup>
|-
| 1&nbsp;fb
| 0.1&nbsp;zm<sup>2</sup> = 100000&nbsp;ym<sup>2</sup>
|-
| 1&nbsp;ab
| 100&nbsp;ym<sup>2</sup>
|-
| 1&nbsp;zb
| 0.1&nbsp;ym<sup>2</sup> = 100000&nbsp;rm<sup>2</sup>
|-
| 1&nbsp;yb
| 100&nbsp;rm<sup>2</sup>
| Shed
|}
|}

==Inverse femtobarn==
The inverse femtobarn (fb<sup>−1</sup>) is the unit typically used to measure the number of [[Event (particle physics)|particle collision events]] per femtobarn of [[Cross section (physics)|target cross-section]], and is the conventional unit for time-integrated [[Luminosity (scattering theory)|luminosity]]. Thus if a detector has accumulated {{val|100|u=fb<sup>−1</sup>}} 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&nbsp;fb<sup>−1</sup> in nine months). Inverse femtobarns are often quoted as an indication of [[particle collider]] productivity.<ref name="Metropolis">{{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 University|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
 }}</ref><ref name="Inman">{{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 National Accelerator Laboratory|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
}}</ref>

[[Fermilab]] produced {{val|10|u=fb<sup>−1</sup>}} in the first decade of the 21st century.<ref>{{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}}</ref>  Fermilab's [[Tevatron]] took about 4 years to reach {{val|1|u=fb<sup>−1</sup>}} in 2005, while two of [[CERN]]'s [[Large Hadron Collider|LHC]] experiments, [[ATLAS experiment|ATLAS]] and [[Compact Muon Solenoid|CMS]], reached over {{val|5|u=fb<sup>−1</sup>}} of proton–proton data in 2011 alone.<ref>{{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}}</ref><ref>{{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}}</ref><ref>{{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}}</ref><ref>{{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}}</ref><ref>{{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}}</ref><ref>{{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}}</ref> 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&nbsp;fb<sup>−1</sup> was achieved during 2012.<ref>{{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 }}</ref> As of November 2016, the LHC had achieved {{val|40|u=fb<sup>−1</sup>}} over that year, significantly exceeding the stated goal of {{val|25|u=fb<sup>−1</sup>}}.<ref>{{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}}</ref> In total, the second run of the LHC has delivered around {{val|150|u=fb<sup>−1</sup>}} to both ATLAS and CMS in 2015–2018.<ref>{{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}}</ref>

===Usage example ===
As a simplified example, if a [[beamline]] runs for 8 hours (28&nbsp;800 seconds) at an instantaneous luminosity of {{val|300|e=30|u=cm<sup>−2</sup>⋅s<sup>−1</sup>}}&nbsp;{{=}} {{val|300|u=μb<sup>−1</sup>⋅s<sup>−1</sup>}}, then it will gather data totaling an integrated luminosity of {{val|8640000|u=μb<sup>−1</sup>}}&nbsp;= {{val|8.64|u=pb<sup>−1</sup>}}&nbsp;= {{val|0.00864|u=fb<sup>−1</sup>}} 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.

==See also==
* "[[Shake (unit)|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]]

==References==
{{reflist|30em}}

==External links==
*[[IUPAC]] [http://goldbook.iupac.org/B00598.html citation for this usage of "barn"]

{{DEFAULTSORT:Barn (Unit)}}
[[Category:Units of area]]
[[Category:Non-SI metric units]]
[[Category:Particle physics]]