Physics:Barn (unit)

Short description: Unit for cross sectional area used in high-energy physics

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 ...
SI base units	10−28 m2
non standard	100 fm2

A barn (symbol: b) is a metric 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 While the barn never was an SI unit, the SI standards body acknowledged it in the 8th SI Brochure (superseded in 2019) due to its use in particle physics.

Etymology

During Manhattan Project research on the atomic bomb during World War II, American physicists at Purdue University needed a secretive name for a unit with which to quantify the cross-sectional area presented by the typical nucleus (~10⁻²⁸ m²) and decided on "barn". They considered this a large target for particle accelerators that needed to have direct strikes on nuclei, and the proposers, physicists Marshall Holloway and Richard Baker, said that the constant "for nuclear purposes was really as big as a barn". 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

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.

Multiples and sub-multiples

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 (GeV⁻²), via the conversion ħ²c²/GeV² = 0.3894 mb = 38940 am².

In natural units (where ħ = c = 1), this simplifies to GeV⁻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.

Conversion from SI units SI barns 1 pm2 10 kb 1 fm2 10 mb 1 am2 10 nb 1 zm2 10 fb 1 ym2 10 zb 1 rm2 10 rb

Conversion to SI units Barns SI Other names 1 b 100 fm2 1 cb 1 fm2 1 mb 0.1 fm2 = 100000 am2 1 μb 100 am2 Outhouse 1 nb 0.1 am2 = 100000 zm2 1 pb 100 zm2 1 fb 0.1 zm2 = 100000 ym2 1 ab 100 ym2 1 zb 0.1 ym2 = 100000 rm2 1 yb 100 rm2 Shed

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 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.

Fermilab produced 10 fb⁻¹ in the first decade of the 21st century. 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. 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. As of November 2016, the LHC had achieved 40 fb⁻¹ over that year, significantly exceeding the stated goal of 25 fb⁻¹. In total, the second run of the LHC has delivered around 150 fb⁻¹ to both ATLAS and CMS in 2015–2018.

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

External links

• IUPAC citation for this usage of "barn"