Barn (unit)
Barn | |
---|---|
Unit system | particle physics |
Unit of | area |
Symbol | b |
Named after | the broad side of a barn |
Conversions | |
1 b in ... | ... is equal to ... |
fm 2 |
A barn (symbol: b) is a
Etymology
During
Commonly used prefixed versions
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.
Unit | Symbol | m2 | cm2 |
---|---|---|---|
megabarn | Mb | 10−22 | 10−18 |
kilobarn | kb | 10−25 | 10−21 |
barn | b | 10−28 | 10−24 |
millibarn | mb | 10−31 | 10−27 |
microbarn | μb | 10−34 | 10−30 |
nanobarn | nb | 10−37 | 10−33 |
picobarn | pb | 10−40 | 10−36 |
femtobarn | fb | 10−43 | 10−39 |
attobarn | ab | 10−46 | 10−42 |
zeptobarn | zb | 10−49 | 10−45 |
yoctobarn | yb | 10−52 | 10−48 |
Conversions
Calculated cross sections are often given in terms of inverse squared gigaelectronvolts (
2 = 0.3894 mb = 38940 am2.In
−2 = 0.3894 mb = 38940 am2.barn | GeV−2 |
---|---|
1 mb | 2.56819 GeV−2 |
1 pb | 2.56819×10−9 GeV−2 |
0.389379 mb | 1 GeV−2 |
0.389379 pb | 1×10−9 GeV−2 |
SI units with prefix
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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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 100 fb−1 of integrated luminosity, one expects to find 100 events per femtobarn of cross-section within these data.
Consider a
Fermilab produced 10 fb−1 in the first decade of the 21st century.[8] Fermilab's Tevatron took about 4 years to reach 1 fb−1 in 2005, while two of CERN's LHC experiments, ATLAS and CMS, reached over 5 fb−1 of proton–proton data in 2011 alone.[9][10][11][12][13][14] In April 2012 the LHC achieved the collision energy of 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.[15] As of November 2016, the LHC had achieved 40 fb−1 over that year, significantly exceeding the stated goal of 25 fb−1.[16] In total, the second run of the LHC has delivered around 150 fb−1 to both ATLAS and CMS in 2015–2018.[17]
Usage example
As a simplified example, if a beamline runs for 8 hours (28 800 seconds) at an instantaneous luminosity of 300×1030 cm−2⋅s−1 = 300 μb−1⋅s−1, then it will gather data totaling an integrated luminosity of 8640000 μb−1 = 8.64 pb−1 = 0.00864 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.
See also
- "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
- ^ "Non-SI units accepted for use with the SI, and units based on fundamental constants". SI brochure (8th edition). BIPM. May 2006. Retrieved 2009-03-13.
- 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.
- ^ Russ Rowlett (September 1, 2004). "Units: S". How Many? A Dictionary of Units of Measurement. University of North Carolina at Chapel Hill. Retrieved 2009-12-14.
- ISBN 0-7100-9919-3. Retrieved 2009-12-14.
- ^ Craig A. Stone, Ph.D. "Glossary". The Language of the Nucleus. Scientific Digital Visions, Inc. Archived from the original on 2007-06-30. Retrieved 2022-11-22.
- ^ Kate Metropolis (July 21, 2004). "Understanding luminosity through 'barn', a unit that helps physicists count particle events". Stanford News Service. Retrieved 2009-03-13.
- ^ Mason Inman, Emily Ball (April 16, 2004). "PEP-II's luminous life". SLAC. Archived from the original on June 2, 2009. Retrieved 2009-03-13.
- ^ "10 Inverse Femtobarns: Celebration Time At Fermilab!". Science20.com. December 12, 2010. Retrieved 2011-04-08.
- ^ "What will we do with all this data?". Pauline Gagnon, Quantum Diaries. October 17, 2011.
- ^ "CDF, DZero reach 5 inverse femtobarns of luminosity". Fermilab Today. September 26, 2008. Retrieved 2011-07-23.
- ^ "LHC experiments reach record data milestone". Symmetry magazine. June 17, 2011. Retrieved 2011-07-23.
- ^ "Thanks to recent fine tuning, the LHC has delivered 2 inverse femtobarns of data already this year; peak luminosity is now over 2x10^33". CERN. August 5, 2011. Retrieved 2011-08-05.
- ^ "LHC Report: 2 inverse femtobarns!". CERN Bulletin. 2011-08-05. Retrieved 2011-08-06.
- ^ "LHC proton run for 2011 reaches successful conclusion". CERN Press Office. 2011-10-31. Retrieved 2011-10-31.
- ^ "LHC luminosity plots". CERN. 2012-12-17. Archived from the original on 2013-02-19.
- ^ "LHC smashes targets for 2016 run | CERN". home.cern. Retrieved 2016-11-02.
- ^ "LHC Report: The final days of Run 2 | CERN". home.cern. Retrieved 2018-12-07.