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{{Short description|Type of particle accelerator that performs particle collisions}}
{{About|the particle accelerator|other uses|Collider (disambiguation)}}

A '''collider''' is a type of [[particle accelerator]] that brings two opposing [[particle beam]]s together such that the particles [[Collision|collide]].<ref>{{Cite web|url = https://news.fnal.gov/2013/08/fixed-target-vs-collider/|title = Fixed-target vs. Collider|date = 2 August 2013|access-date = 17 December 2019|archive-date = 21 January 2022|archive-url = https://web.archive.org/web/20220121054434/https://news.fnal.gov/2013/08/fixed-target-vs-collider/|url-status = dead}}</ref> Compared to other particle accelerators in which the moving particles collide with a stationary matter target, colliders can achieve higher collision energies.  Colliders may either be [[ring accelerator]]s or [[linear accelerator]]s.  

Colliders are used as a research tool in [[particle physics]] by accelerating particles to very high [[kinetic energy]] and letting them impact other particles. Analysis of the byproducts of these collisions gives scientists good evidence of the structure of the subatomic world and the laws of nature governing it. These may become apparent only at high energies and for extremely short periods of time, and therefore may be hard or impossible to study in other ways.

==Explanation==
In [[particle physics]] one gains knowledge about [[elementary particle]]s by accelerating particles to very high [[kinetic energy]] and guiding them to colide with other particles. For sufficiently high energy, a [[Nuclear reaction|reaction]] occurs that transforms the particles into other particles. Detecting these products gives insight into the [[physics]] involved.

To do such experiments there are two possible setups:

* [[Fixed-target experiment|Fixed target setup:]] A beam of particles (the ''projectiles'') is accelerated with a [[particle accelerator]], and as collision partner, one puts a stationary target into the path of the beam.
* Collider: ''Two'' beams of particles are accelerated and the beams are directed against each other, so that the particles collide while flying in opposite directions.

The collider setup is harder to construct but has the great advantage that according to [[special relativity]] the energy of an [[inelastic collision]] between two particles approaching each other with a given velocity is not just 4 times as high as in the case of one particle resting (as it would be in non-relativistic physics); it can be orders of magnitude higher if the collision velocity is near the speed of light.

In the case of a collider where the collision point is at rest in the laboratory frame (i.e. <math> \vec p_1 = -\vec p_2 </math>), the center of mass energy <math>E_\mathrm{cm}</math> (the energy available for producing new particles in the collision) is simply <math>E_\mathrm{cm} = E_1 + E_2</math>, where <math>E_1</math> and <math>E_2</math> is the total energy of a particle from each beam.
For a fixed target experiment where particle 2 is at rest, <math>E_\mathrm{cm}^2 = m_1^2 + m_2^2 + 2 m_2 E_1 </math>.<ref>{{cite journal|last1=Herr|first1=Werner|last2=Muratori|first2=Bruno|title=Concept of Luminosity|journal=CERN Accelerator School|date=2003|pages=361–378|url=https://cds.cern.ch/record/941318/|access-date=2 November 2016}}</ref>

== History ==
The first serious proposal for a collider originated with a group at the [[Midwestern Universities Research Association]] (MURA).  This group proposed building two tangent radial-sector [[FFAG accelerator]] rings.<ref>{{cite journal | last1 = Kerst | first1 = D. W. | author-link1 = Donald William Kerst | last2 = Cole | first2 = F. T. | last3 = Crane | first3 = H. R. | last4 = Jones | first4 = L. W. | year = 1956 | title = Attainment of Very High Energy by Means of Intersecting Beams of Particles | journal = [[Physical Review]] | volume = 102 | issue = 2 | pages = 590–591 | doi = 10.1103/PhysRev.102.590 |display-authors=etal|bibcode = 1956PhRv..102..590K }}</ref> [[Tihiro Ohkawa]], one of the authors of the first paper, went on to develop a radial-sector FFAG accelerator design that could accelerate two counterrotating particle beams within a single ring of magnets.<ref>{{US patent reference
 | number = 2890348
 | y = 1959
 | m = 06
 | d = 09
 | inventor = Tihiro Ohkawa
 | title = [https://patents.google.com/patent/US2890348 Particle Accelerator]
}}</ref><ref>Science: Physics & Fantasy, [https://web.archive.org/web/20121106173215/http://www.time.com/time/magazine/article/0,9171,809067-1,00.html Time], Monday, Feb. 11, 1957.</ref>  The third FFAG prototype built by the MURA group was a 50&nbsp;MeV electron machine built in 1961 to demonstrate the feasibility of this concept.

[[Gerard K. O'Neill]] proposed using a single accelerator to inject particles into a pair of tangent [[storage ring]]s.  As in the original MURA proposal, collisions would occur in the tangent section.  The benefit of storage rings is that the storage ring can accumulate a high beam flux from an injection accelerator that achieves a much lower flux.<ref>{{Cite journal | last1 = O'Neill | first1 = G. | author-link1 = Gerard K. O'Neill | title = Storage-Ring Synchrotron: Device for High-Energy Physics Research | doi = 10.1103/PhysRev.102.1418 | journal = [[Physical Review]] | volume = 102 | issue = 5 | pages = 1418–1419 | year = 1956 | bibcode = 1956PhRv..102.1418O | url = http://www.feynman.princeton.edu/mumu/physics/oneill_pr_102_1418_56.pdf | url-status = dead | archive-url = https://web.archive.org/web/20120306163409/http://www.feynman.princeton.edu/mumu/physics/oneill_pr_102_1418_56.pdf | archive-date = 2012-03-06 }}</ref>

The first [[electron]]-[[positron]] colliders were built in late 1950s-early 1960s in Italy, at the [[Istituto Nazionale di Fisica Nucleare]] in [[Frascati]] near Rome, by the Austrian-Italian physicist [[Bruno Touschek]] and in the US, by the Stanford-Princeton team that included William C.Barber, Bernard Gittelman, Gerry O’Neill, and [[Burton Richter]]. Around the same time, the ''VEP-1'' electron-electron collider was independently developed and built under supervision of [[Gersh Budker]] in the [[Budker Institute of Nuclear Physics|Institute of Nuclear Physics]] in [[Novosibirsk]], [[USSR]]. The first observations of particle reactions in the colliding beams were reported almost simultaneously by the three teams in mid-1964 - early 1965. <ref>{{cite arXiv | last1 = Shiltsev | first1 = V. | title = The first colliders: AdA, VEP-1 and Princeton-Stanford | eprint = 1307.3116 | class = physics.hist-ph | year = 2013 }}</ref>

In 1966, work began on the [[Intersecting Storage Rings]] at [[CERN]], and in 1971, this collider was operational.<ref>Kjell Johnsen, The ISR in the time of Jentschke, [http://cerncourier.com/cws/article/cern/28876 CERN Courier], June 1, 2003.</ref>  The ISR was a pair of storage rings that accumulated and collided protons injected by the CERN [[Proton Synchrotron]].  This was the first [[hadron]] collider, as all of the earlier efforts had worked with [[electrons]] or with electrons and [[positrons]].

In 1968 construction began on the highest energy proton accelerator complex at [[Fermilab]]. It was eventually upgraded to become the [[Tevatron]] collider and in October 1985 the first [[proton]]-[[antiproton]] collisions were recorded at a center of mass energy of 1.6 TeV, making it the highest energy collider in the world, at the time. The energy had later reached 1.96 TeV and at the end of the operation in 2011 the collider luminosity exceeded 430 times its original design goal. <ref>{{cite journal | last1 = Holmes | first1 = Stephen D. | last2 = Shiltsev | first2 = Vladimir D. | year = 2013 | title = The Legacy of the Tevatron in the Area of Accelerator Science | journal = [[Annual Review of Nuclear and Particle Science]] | volume = 63 | pages = 435–465 | doi = 10.1146/annurev-nucl-102212-170615 | arxiv = 1302.2587 | bibcode = 2013ARNPS..63..435H | s2cid = 118385635 }}</ref>

Since 2009, the most high-energetic collider in the world is the [[Large Hadron Collider]] (LHC) at CERN. It currently operates at 13 TeV center of mass energy in proton-proton collisions. More than a dozen future particle collider projects of various types - circular and linear, colliding hadrons (proton-proton or ion-ion), leptons (electron-positron or muon-muon), or electrons and ions/protons - are currently under consideration for detail exploration of the Higgs/electroweak physics and discoveries at the post-LHC energy frontier. <ref>{{cite journal | last1 = Shiltsev | first1 = Vladimir | last2 = Zimmermann | first2 = Frank | year = 2021 | title = Modern and future colliders | journal = [[Reviews of Modern Physics]] | volume = 93 | issue = 1 | pages = 015006  | doi = 10.1103/RevModPhys.93.015006| arxiv = 2003.09084 | bibcode = 2021RvMP...93a5006S | s2cid = 214605600 }}</ref>

== Operating colliders ==
Sources: Information was taken from the website [[Particle Data Group]].<ref>{{cite web|url=http://pdg.lbl.gov/2020/reviews/rpp2020-rev-hep-collider-params.pdf|title=High Energy Collider Parameters|accessdate=2021-06-03}}</ref>
{| class="wide wikitable sortable"
 !Accelerator
 !Centre, city, country
 !First operation
 !Accelerated particles
 !Max energy per beam, [[GeV]]
 ![[Luminosity (scattering theory)|Luminosity]], 10<sup>30</sup> cm<sup>−2 </sup>s<sup>−1</sup>
 !Perimeter (length), [[km]]
 |-
 |[[VEPP-2000]]
 |[[Budker Institute of Nuclear Physics|INP]], [[Novosibirsk]], [[Russia]]
 |2006
 |{{Subatomic Particle|positron}}{{Subatomic Particle|electron}}
 |1.0
 |100
 |0.024
 |-
 |[[VEPP-4|VEPP-4М]]
 |INP, Novosibirsk, Russia
 |1994
 |{{Subatomic Particle|positron}}{{Subatomic Particle|electron}}
 |6
 |20
 |0.366
 |-
 |[[Beijing Electron–Positron Collider II|BEPC II]]
 |[[Institute of High Energy Physics|IHEP]], [[Beijing]], [[China]]
 |2008
 |{{Subatomic Particle|positron}}{{Subatomic Particle|electron}}
 |2.45<ref>{{cite book |last1=Ye |first1=Minghan |last2=Yuan |first2=Changzheng |title=30 Years of Bes Physics: Proceedings of the Symposium |date=2020 |publisher=[[World Scientific]] |isbn=978-981-121-772-2 |page=319 |url=https://books.google.com/books?id=OGjvDwAAQBAJ&pg=PA319}}</ref>
 |1000
 |0.240
 |-
 |[[DAFNE]]
 |[[Laboratori Nazionali di Frascati|LNF]], [[Frascati]], [[Italy]]
 |1999
 |{{Subatomic Particle|positron}}{{Subatomic Particle|electron}}
 |0.510
 |453<ref>{{Cite journal | last1 = Zobov | first1 = M.| title = Test of crab-waist collisions at DAΦNE Φ factory | doi = 10.1103/PhysRevLett.104.174801 | journal = [[Physical Review Letters]] | volume = 104 | issue = 17 | pages = 174801 | year = 2010 | pmid = 20482112| bibcode = 2010PhRvL.104q4801Z| url = https://inspirehep.net/files/647309ee2751bcad193abb4508a51194 }}</ref>
 |0.098
 |-
 |[[SuperKEKB]]
 |[[KEK]], [[Tsukuba]], [[Japan]]
 |2018
 |{{Subatomic Particle|positron}}{{Subatomic Particle|electron}}
 |7 ({{Subatomic Particle|electron}}), 4 ({{Subatomic Particle|positron}})
 |24000<ref>{{cite news|url=https://www.kek.jp/en/newsroom/2020/06/26/1400/|title=SuperKEKB collider achieves the world's highest luminosity|date=2020-06-26|access-date=2020-06-26}}</ref>
 |3.016
 |-
 |[[RHIC]]
 |[[Brookhaven National Laboratory|BNL]], [[New York (state)|New York]], [[United States]]
 |2000
 |{{Subatomic Particle|proton}}{{Subatomic Particle|proton}},<br /> Au-Au, Cu-Cu, [[Deuterium|d]]-Au
 |255,<br />100/[[nucleon|n]]
 |245,<br /> 0.0155, 0.17, 0.85
 |3.834
 |-
 |[[Large Hadron Collider|LHC]]
 |[[CERN]], [[Geneva]], [[Switzerland]]/[[France]]
 |2008
 |pp,<br /> [[lead|Pb]]-Pb, p-Pb, Xe-Xe
 |6500 (planned 7000),<br />2560/[[nucleon|n]] (planned 2760/[[nucleon|n]])
 |21000,<ref>{{Cite journal | author=ATLAS Collaboration | title = Performance of electron and photon triggers in ATLAS during LHC Run 2 | doi = 10.1140/epjc/s10052-019-7500-2 | journal = [[The European Physical Journal C]] | volume = 80| issue = 1| pages = 47| year = 2020 | arxiv = 1909.00761 | bibcode = 2020EPJC...80...47A| s2cid = 202538006 | url = https://inspirehep.net/files/647309ee2751bcad193abb4508a51194 }}</ref><br />0.0061, 0.9, 0.0004
 |26.659
 |-
 |}

==See also==
{{cols|colwidth=20em}}
*[[List of accelerators in particle physics#Colliders|List of colliders]]
*[[Fixed-target experiment]]
*[[Large Electron–Positron Collider]]
*[[Large Hadron Collider]]
*[[Very Large Hadron Collider]]
*[[Relativistic Heavy Ion Collider]]
*[[International Linear Collider]]
*[[Storage ring]]
*[[Tevatron]]
*[[International Conference on Photonic, Electronic and Atomic Collisions]]
*[[Future Circular Collider]]
{{colend}}

==References==
{{reflist}}

==External links==
{{wikiquote}}
*[http://lhc.web.cern.ch/lhc LHC - The Large Hadron Collider on the web]
*[http://www.bnl.gov/rhic The Relativistic Heavy Ion Collider (RHIC)]

[[Category:Accelerator physics]]