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{{Short description|CERN LHC experiment}}
{{about|the LHC detector at CERN|other experiments|Atlas (disambiguation)}}
{{LHC}}
'''ATLAS'''<ref name=fact_sheets>{{cite web |title= ATLAS Fact Sheets |url= https://atlas.cern/resources/fact-sheets |publisher= ATLAS |access-date=27 January 2022}}</ref><ref name=the_bible>{{cite journal |last1=Aad |first1=G. |collaboration= ATLAS Collaboration |year=2008 |title= The ATLAS Experiment at the CERN Large Hadron Collider |journal=[[Journal of Instrumentation]] |volume=3 |issue=8 |pages=S08003 |bibcode=2008JInst...3S8003A |doi=10.1088/1748-0221/3/08/S08003 |hdl=2027.42/64167
|s2cid=250683252 |display-authors=etal |doi-access=free |hdl-access=free }}</ref><ref name="TPoveralldetector">{{cite book| year=1994| title=ATLAS Technical Proposal| chapter=Overall detector concept| publisher=CERN| chapter-url=http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node6.html#SECTION00420000000000000000| archive-date=2021-01-07| access-date=2007-03-03| archive-url=https://web.archive.org/web/20210107205456/http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node6.html#SECTION00420000000000000000| url-status=dead}}</ref> is the largest general-purpose [[particle detector]] experiment at the [[Large Hadron Collider]] (LHC), a [[particle accelerator]] at [[CERN]] (the European Organization for Nuclear Research) in Switzerland.<ref name="CERNATLAS">{{cite web|url=https://home.cern/science/experiments/atlas|title=ATLAS Experiment|publisher=CERN|access-date=24 October 2019}}</ref> The experiment is designed to take advantage of the unprecedented energy available at the LHC and observe phenomena that involve highly massive [[elementary particle|particles]] which were not observable using earlier lower-[[energy]] accelerators. ATLAS was one of the two LHC experiments involved in the discovery of the [[Higgs boson]] in July 2012.<ref name="Higgs2012">{{cite web|url=http://press.cern/press-releases/2012/07/cern-experiments-observe-particle-consistent-long-sought-higgs-boson|title=CERN experiments observe particle consistent with long-sought Higgs boson|date=4 July 2012|publisher=[[CERN]]|access-date=2016-11-23}}</ref><ref name="Higgs2013">{{cite web|url=http://press.cern/backgrounders/cern-and-higgs-boson|title=CERN and the Higgs boson|publisher=CERN|access-date=23 November 2016|archive-url=https://web.archive.org/web/20161123201553/http://press.cern/backgrounders/cern-and-higgs-boson|archive-date=23 November 2016|url-status=dead}}</ref> It was also designed to search for evidence of [[theories]] of [[particle physics]] beyond the [[Standard Model]].
The experiment is a collaboration involving 6,003 members, out of which 3,822 are [[physicist]]s (last update: June 26, 2022) from 243 institutions in 40 countries.<ref name=fact_sheets/><ref name=collaboration>{{cite web |title=The Collaboration |url= https://atlas.cern/discover/collaboration |publisher= ATLAS |access-date=15 March 2025}}</ref>
<!-- [[File:CERN ATLAS Detector.jpg|thumb|[https://tools.wmflabs.org/panoviewer/#CERN_ATLAS_Detector.jpg 360° Panorama ATLAS detector]]] -->
==History==
===Particle accelerator growth===
[[File:CERN Atlas Caverne.jpg|thumb|ATLAS detector under construction in October 2004 in the experiment pit. Construction was completed in 2008 and ATLAS has been successfully collecting data since November 2009, when colliding beam operation at the LHC started. Note the people in the background, for size comparison.]]
The first [[cyclotron]], an early type of particle accelerator, was built by [[Ernest O. Lawrence]] in 1931, with a radius of just a few centimetres and a particle [[energy]] of 1 [[electron volt#MeV|megaelectronvolt (MeV)]]. Since then, accelerators have grown enormously in the quest to produce new particles of greater and greater [[mass]]. As accelerators have grown, so too has the [[list of particles|list of known particles]] that they might be used to investigate.
===ATLAS Collaboration===
The ATLAS Collaboration, the international group of physicists belonging to different universities and research centres who built and run the detector, was formed in 1992 when the proposed EAGLE (Experiment for Accurate Gamma, Lepton and Energy Measurements) and ASCOT (Apparatus with Super Conducting Toroids) collaborations merged their efforts to build a single, general-purpose particle detector for a new [[particle accelerator]], the [[Large Hadron Collider]].<ref>{{cite web |url=http://library.cern.ch/archives/isad/isaatlas.html |title=ATLAS Collaboration records |access-date=2007-02-25 |publisher=CERN Archive |archive-date=2007-01-01 |archive-url=https://web.archive.org/web/20070101230746/http://library.cern.ch/archives/isad/isaatlas.html |url-status=dead }}</ref> At present, the ATLAS Collaboration involves 6,003 members, out of which 3,822 are [[physicist]]s (last update: June 26, 2022) from 257 institutions in 42 countries.<ref name=fact_sheets/><ref name=collaboration/>
===Detector design and construction===
The design was a combination of two previous projects for LHC, EAGLE and ASCOT, and also benefitted from the detector research and development that had been done for the [[Superconducting Super Collider]], a [[United States|US]] project interrupted in 1993. The ATLAS experiment was proposed in its current form in 1994, and officially funded by the CERN member countries in 1995. Additional countries, [[universities]], and [[laboratories]] have joined in subsequent years. Construction work began at individual institutions, with detector components then being shipped to CERN and assembled in the ATLAS experiment pit starting in 2003.
==
Construction was completed in 2008 and the experiment detected its first single [[proton]] beam events on 10 September of that year.<ref>{{cite web|url=http://atlas.cern/updates/atlas-news/first-beam-and-first-events-atlas|title=First beam and first events in ATLAS|date=2008-09-10|publisher=Atlas.ch|access-date=2016-08-16}}</ref>
<!-- old cites that might be useful: <ref>{{cite web| url = http://atlasexperiment.org/news/2007/progress-on-magnets.html | title = Progress on Toroid Magnets | publisher = Cern - Atlas Experiment |author1=Colin Barras |author2=Michael Barnett |date=December 2007}}</ref> -->
Data-taking was then interrupted for over a year due to an LHC [[Large Hadron Collider#Quench incident|magnet quench incident]]. On 23 November 2009, the first proton–proton collisions occurred at the LHC and were recorded by ATLAS, at a relatively low injection energy of 900 GeV in the [[center of mass]] of the collision. Since then, the LHC energy has been increasing: 1.8 TeV at the end of 2009, 7 TeV for the whole of 2010 and 2011, then 8 TeV in 2012.
The first data-taking period performed between 2010 and 2012 is referred to as Run I. After a long shutdown (LS1) in 2013 and 2014, in 2015 ATLAS saw 13 TeV collisions.<ref>[https://www.forbes.com/sites/chadorzel/2015/05/21/things-to-know-as-the-large-hadron-collider-breaks-energy-records/#2715e4857a0b4588cb75b112 "Eight Things To Know As The Large Hadron Collider Breaks Energy Records"].</ref><ref>[http://atlas.ch/news/2015/atlas-completes-first-year-at-13-tev.html "ATLAS Completes First Year at 13 TeV"] {{Webarchive|url=https://web.archive.org/web/20160117090052/http://atlas.ch/news/2015/atlas-completes-first-year-at-13-tev.html |date=2016-01-17 }}.</ref><ref>[http://www.atlas.ch/news/2015/atlas-begins-recording-physics-data-at-13-tev.html "ATLAS Begins Recording Physics Data at 13 TeV"] {{Webarchive|url=https://web.archive.org/web/20160305002833/http://www.atlas.ch/news/2015/atlas-begins-recording-physics-data-at-13-tev.html |date=2016-03-05 }}.</ref>
The second data-taking period, Run II, was completed, always at 13 TeV energy, at the end of 2018 with a recorded integrated [[Luminosity (scattering theory)|luminosity]] of nearly 140 fb<sup>−1</sup> (inverse [[femtobarn]]).<ref>{{Cite web |url=https://twiki.cern.ch/twiki/bin/view/AtlasPublic/LuminosityPublicResultsRun2|title=LuminosityPublicResultsRun2 < AtlasPublic < TWiki|website=twiki.cern.ch|access-date=2020-03-10}}</ref>
A second long shutdown (LS2) in 2019–22 with upgrades to the ATLAS detector<ref>{{Cite book|title=ATLAS|date=2018-05-05|publisher=World Scientific|isbn=978-981-327-179-1|series=Advanced Series on Directions in High Energy Physics|volume=30|language=en|doi=10.1142/11030}}</ref> was followed by Run III, which started in July 2022.<ref>{{cite web|url=https://home.cern/news/news/physics/atlas-moves-top-gear-run-3|title=ATLAS moves into top gear for Run 3|date=16 December 2022|access-date=24 January 2022}}</ref>
[[File:Dr Fabiola Gianotti.jpg|thumb|right|upright=1.00|[[Fabiola Gianotti]], ATLAS spokesperson (2009 - 2013) and CERN director-general (2016 - 2025).]]
{|
|-
| '''Periods of LHC''' || '''Operation'''
|-
| Apr 2010 – Jan 2013 || Run I
|-
| Feb 2013 – Jan 2015 || LS1
|-
| Feb 2015 – Nov 2018 || Run II
|-
| Dec 2018 – Jun 2022 || LS2
|-
| Jul 2022 – Jun 2026 || Run III
|}
=== Leadership ===
The ATLAS Collaboration is currently led by Spokesperson [[Stephane Willocq]] and Deputy Spokespersons [[Anna Sfyrla]] and [[Guillaume Unal]].<ref>{{Cite web |title=ATLAS: now under new management |url=https://atlas.cern/Updates/News/New-Management-Steers-Forward |access-date=2025-03-15 |website=ATLAS |language=en}}</ref> Former Spokespersons have been:
{|
|-
| [[Friedrich Dydak]] and [[Peter Jenni]] || (1992 – 1995)
|-
| [[Peter Jenni]] || (1995 – 2009)
|-
| [[Fabiola Gianotti]] || (2009 – 2013)
|-
| [[David Charlton]] || (2013 – 2017)
|-
| [[Karl Jakobs]]|| (2017 – 2021)
|-
| [[Andreas Hoecker]]|| (2021 - 2025)
|-
| [[Stephane Willocq]]|| (2025 – Present)
|}
==Experimental program==
In the field of [[particle physics]], ATLAS studies different types of processes detected or detectable in [[energy|energetic]] collisions at the [[Large Hadron Collider]] (LHC). For the processes already known, it is a matter of measuring more and more accurately the properties of known [[particle (physics)|particles]] or finding quantitative confirmations of the [[Standard Model]]. Processes not observed so far would allow, if detected, to discover new [[Elementary particle|particles]] or to have confirmation of physical theories that go beyond the [[Standard Model]].
===Standard Model===
{{Standard model of particle physics}}
The [[Standard Model]] of [[particle physics]] is the [[theory]] describing three of the four known [[fundamental force]]s (the [[Electromagnetism|electromagnetic]], [[Weak interaction|weak]], and [[Strong interaction|strong]] interactions, while omitting [[gravity]]) in the [[universe]], as well as classifying all known [[elementary particle]]s. It was developed in stages throughout the latter half of the 20th century, through the work of many scientists around the world,<ref>
{{cite book
|author = R. Oerter
|year=2006
|title=The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics
|url = https://archive.org/details/theoryofalmostev0000oert
|url-access = registration
|page=[https://archive.org/details/theoryofalmostev0000oert/page/2 2]
|publisher=Penguin Group
|edition=Kindle
|isbn=978-0-13-236678-6
}}</ref> with the current formulation being finalized in the mid-1970s upon [[experimental confirmation]] of the existence of [[quark]]s. Since then, confirmation of the [[top quark]] (1995), the [[tau neutrino]] (2000), and the [[Higgs boson]] (2012) have added further credence to the [[Standard Model]]. In addition, the Standard Model has predicted various properties of [[weak neutral current]]s and the [[W and Z bosons]] with great accuracy.
Although the [[Standard Model]] is believed to be theoretically self-consistent<ref>{{cite book
|author = R. Mann
|year=2010
|title=An Introduction to Particle Physics and the Standard Model
|publisher=[[CRC Press]]
|isbn=978-1-4200-8298-2
}}</ref> and has demonstrated huge successes in providing [[experimental prediction]]s, it leaves some [[Physics beyond the standard model|phenomena unexplained]] and falls short of being a [[theory of everything|complete theory of fundamental interactions]]. It does not fully explain [[baryon asymmetry]], incorporate the full [[theory of gravitation]]<ref name = DarkMatter>Sean Carroll, PhD, Caltech, 2007, The Teaching Company, ''Dark Matter, Dark Energy: The Dark Side of the Universe'', Guidebook Part 2 page 59, Accessed 7 Oct. 2013, "...Standard Model of Particle Physics: The modern theory of elementary particles and their interactions ... It does not, strictly speaking, include gravity, although it's often convenient to include gravitons among the known particles of nature..."</ref> as described by [[general relativity]], or account for the [[accelerating expansion of the universe]] as possibly described by [[dark energy]]. The model does not contain any viable [[dark matter]] particle that possesses all of the required properties deduced from observational [[Physical cosmology|cosmology]]. It also does not incorporate [[neutrino oscillation]]s and their non-zero masses.
====Precision measurements====
With the important exception of the [[Higgs boson]], detected by the ATLAS and the [[Compact Muon Solenoid|CMS]] experiments in 2012,<ref name="Higgs2015" /> all of the particles predicted by the [[Standard Model]] had been observed by previous experiments. In this field, in addition to the discovery of the [[Higgs boson]], the experimental work of ATLAS has focused on precision measurements, aimed at determining with ever greater accuracy the many physical parameters of theory.
In particular for
* the [[Higgs boson]];
* [[W and Z bosons]];
* the [[top quark|top]] and [[bottom quark|bottom]] quarks
ATLAS measures:
* [[mass]]es;
* channels of production, decay and [[Exponential decay#Mean lifetime|mean lifetimes]];
* interaction mechanisms and [[coupling constant]]s for [[Electroweak interaction|electroweak]] and [[strong interaction]]s.
For example, the data collected by ATLAS made it possible in 2018 to measure the mass [(80,370±19) [[Electronvolt|MeV]]] of the [[W boson]], one of the two mediators of the [[electroweak interaction|weak interaction]], with a [[measurement uncertainty]] of ±2.4[[Per mille|‰]].
====Higgs boson====
[[File:Higgs production gg qq.png|thumb|Schematics, called [[Feynman diagram]]s show the main ways that the Standard Model Higgs boson can be produced from colliding protons at the LHC.]]
One of the most important goals of ATLAS was to investigate a missing piece of the Standard Model, the [[Higgs boson]].<ref name=fact_sheets/><ref name="TPintro">{{cite book| year=1994| title=ATLAS Technical Proposal| chapter=Introduction and Overview| publisher=CERN| chapter-url=http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node4.html#SECTION00400000000000000000| archive-date=2021-01-07| access-date=2007-03-03| archive-url=https://web.archive.org/web/20210107205455/http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node4.html#SECTION00400000000000000000| url-status=dead}}</ref> The [[Higgs mechanism]], which includes the Higgs boson, gives mass to elementary particles, leading to differences between the [[weak force]] and [[electromagnetism]] by giving the [[W and Z bosons]] mass while leaving the [[photon]] massless.
On July 4, 2012, ATLAS — together with CMS, its sister experiment at the LHC — reported evidence for the existence of a particle consistent with the Higgs boson at a confidence level of 5 [[Standard deviation|sigma]],<ref name="Higgs2012" /> with a mass around 125 GeV, or 133 times the proton mass. This new "Higgs-like" particle was detected by its decay into two [[photon]]s (<math>H\rightarrow\gamma\gamma </math>) and its decay to four [[lepton]]s (<math>H\rightarrow ZZ^*\rightarrow 4l</math> and <math>H\rightarrow WW^*\rightarrow e\nu\mu\nu</math>).
In March 2013, following the updated results from ATLAS and CMS, CERN announced that the newly discovered particle was indeed a Higgs boson. The experiments were also able to show that the properties of the particle as well as the ways it interacts with other particles were well-matched with those of a Higgs boson, which is expected to have [[Spin (physics)|spin]] 0 and positive [[Parity (physics)|parity]]. Analysis of more properties of the particle and data collected in 2015 and 2016 confirmed this further.<ref name="Higgs2015">{{cite web|url=http://press.cern/press-releases/2015/09/atlas-and-cms-experiments-shed-light-higgs-properties|title=ATLAS and CMS experiments shed light on Higgs properties|access-date=2016-11-23}}</ref>
In October 2013, two of the theoretical physicists who predicted the existence of the Standard Model Higgs boson, [[Peter Higgs]] and [[François Englert]], were awarded the [[Nobel Prize in Physics]].
====Top quark properties====
The properties of the [[top quark]], discovered at [[Fermilab]] in 1995, had been measured approximately. With much greater energy and greater collision rates, the LHC produces a tremendous number of top quarks, allowing ATLAS to make much more precise measurements of its mass and interactions with other particles.<ref>{{cite book| year=1994| title=ATLAS Technical Proposal| chapter=Top-Quark Physics| publisher=CERN| chapter-url=http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node416.html#SECTION0024100000000000000000| archive-date=2004-03-02| access-date=2007-04-02| archive-url=https://web.archive.org/web/20040302135347/http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node416.html#SECTION0024100000000000000000| url-status=dead}}</ref> These measurements provide indirect information on the details of the Standard Model, with the possibility of revealing inconsistencies that point to new physics.
===Beyond the Standard Model===
While the [[Standard Model]] predicts that [[quark]]s, [[lepton]]s and [[neutrino]]s should exist, it does not explain why the [[mass]]es of these particles are so different (they differ by [[order of magnitude|orders of magnitude]]). Furthermore, the mass of the [[neutrino]]s should be, according to the [[Standard Model]], exactly zero as that of the [[photon]]. Instead, neutrinos have [[mass]]. In 1998 research results at [[particle detector|detector]] [[Super-Kamiokande]] determined that neutrinos can oscillate from one [[Flavor (physics)|flavor]] to another, which dictates that they have a mass other than zero. For these and other reasons, many [[particle physics|particle physicists]] believe it is possible that the [[Standard Model]] will break down at energies at the [[electron volt#TeV|teraelectronvolt (TeV)]] scale or higher. Most alternative theories, the [[Grand Unified Theories]] (GUTs) including [[Supersymmetry]] (SUSY), predicts the existence of new particles with [[mass]]es greater than those of [[Standard Model]].
====Supersymmetry====
Most of the currently proposed theories predict new higher-mass particles, some of which may be light enough to be observed by ATLAS. Models of [[supersymmetry]] involve new, highly massive particles. In many cases these decay into high-energy [[quark]]s and stable heavy particles that are very unlikely to interact with ordinary matter. The stable particles would escape the detector, leaving as a signal one or more high-energy [[jet (particle physics)|quark jets]] and a large amount of [[missing energy|"missing"]] [[momentum]]. Other hypothetical massive particles, like those in the [[Kaluza–Klein theory]], might leave a similar signature.
The data collected up to the end of LHC Run II do not show evidence of supersymmetric or unexpected particles, the research of which will continue in the data that will be collected from Run III onwards.
====CP violation====
The asymmetry between the behavior of matter and [[antimatter]], known as [[CP violation]], is also being investigated.<ref name="TPintro"/> Recent experiments dedicated to measurements of CP violation, such as [[BaBar]] and [[Belle experiment|Belle]], have not detected sufficient CP violation in the Standard Model to explain the lack of detectable antimatter in the universe. It is possible that new models of physics will introduce additional CP violation, shedding light on this problem. Evidence supporting these models might either be detected directly by the production of new particles, or indirectly by measurements of the properties of B- and D-[[meson]]s. [[LHCb]], an LHC experiment dedicated to B-mesons, is likely to be better suited to the latter.<ref name="PhysicsatLHC">{{cite journal |author1=N. V. Krasnikov |author2=V. A. Matveev |date=September 1997 |title = Physics at LHC |journal= Physics of Particles and Nuclei| volume= 28 |issue= 5 | pages= 441–470 |arxiv = hep-ph/9703204 |doi = 10.1134/1.953049 |bibcode = 1997PPN....28..441K |s2cid=118907038 }}</ref>
====Microscopic black holes====
Some hypotheses, based on the [[ADD model]], involve large extra dimensions and predict that [[micro black holes]] could be formed by the LHC.<ref>{{cite journal|title= Exploring higher dimensional black holes at the Large Hadron Collider |doi-access=free |first1=C.M. |last1=Harris |first2=M.J. |last2=Palmer |first3=M.A. |last3=Parker |first4=P. |last4=Richardson |first5=A. |last5=Sabetfakhri |first6=B.R. |last6=Webber |journal=Journal of High Energy Physics|volume=2005|year=2005|pages=053|doi= 10.1088/1126-6708/2005/05/053|arxiv = hep-ph/0411022 |bibcode = 2005JHEP...05..053H|issue= 5 |s2cid=15199183 }}</ref> These would decay immediately by means of [[Hawking radiation]], producing all particles in the Standard Model in equal numbers and leaving an unequivocal signature in the ATLAS detector.<ref>{{cite journal|title=Study of Black Holes with the ATLAS detector at the LHC |first1=J. |last1=Tanaka |first2=T. |last2=Yamamura |first3=S. |last3=Asai |first4=J. |last4=Kanzaki |journal=European Physical Journal C|volume=41|issue=s2|year=2005|pages=19–33|doi=10.1140/epjcd/s2005-02-008-x|arxiv = hep-ph/0411095 |bibcode = 2005EPJC...41...19T |s2cid=119444406 }}</ref>
==ATLAS detector==
The ATLAS detector is 46 metres long, 25 metres in diameter, and weighs about 7,000 tonnes; it contains some 3,000 km of cable.<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/>
At 27 km in [[circumference]], the [[Large Hadron Collider]] (LHC) at [[CERN]] [[collider|collides]] two beams of protons together, with each proton carrying up to 6.8 [[electron volt#TeV|TeV]] of energy – enough to produce particles with masses significantly greater than any particles currently known, if these particles exist. When the proton [[particle beam|beams]] produced by the Large Hadron Collider interact in the center of the detector, a variety of different particles with a broad range of energies are produced.
===General-purpose requirements===
The ATLAS detector is designed to be general-purpose. Rather than focusing on a particular physical process, ATLAS is designed to measure the broadest possible range of signals. This is intended to ensure that whatever form any new physical processes or particles might take, ATLAS will be able to detect them and measure their properties. ATLAS is designed to detect these particles, namely their masses, [[momentum]], [[energy|energies]], lifetime, charges, and [[nuclear spin]]s.
Experiments at earlier colliders, such as the [[Tevatron]] and [[Large Electron–Positron Collider]], were also designed for general-purpose detection. However, the beam energy and extremely high rate of collisions require ATLAS to be significantly larger and more complex than previous experiments, presenting unique challenges of the Large Hadron Collider.
===Layered design===
In order to identify all particles produced at the [[interaction point]] where the particle beams collide, the detector is designed in layers made up of detectors of different types, each of which is designed to observe specific types of particles. The different traces that particles leave in each layer of the detector allow for effective [[particle identification]] and accurate measurements of energy and momentum. (The role of each layer in the detector is discussed [[#Detector systems|below]].) As the energy of the particles produced by the accelerator increases, the detectors attached to it must grow to effectively measure and stop higher-energy particles. As of 2022, the ATLAS detector is the largest ever built at a particle collider.<ref name="CERNpr">{{cite press release| publisher=CERN| date=2006-11-20| title= World's largest superconducting magnet switches on| url=http://press.cern/press-releases/2006/11/worlds-largest-superconducting-magnet-switches| access-date=2016-11-23}}</ref>
===Detector systems===
[[File:ATLAS Drawing with Labels.svg|thumb|upright=1.5|Computer generated cut-away view of the ATLAS detector showing its various components. <br>
[[#Muon Spectrometer|Muon Spectrometer]]: <br>
(1) Forward regions (End-caps) <br>
(1) Barrel region <br>
[[#Magnet System|Magnet System]]: <br>
(2) Toroid Magnets <br>
(3) Solenoid Magnet <br>
[[#Inner Detector|Inner Detector]]: <br>
(4) Transition Radiation Tracker <br>
(5) Semi-Conductor Tracker <br>
(6) Pixel Detector <br>
[[#Calorimeters|Calorimeters]]: <br>
(7) Liquid Argon Calorimeter <br>
(8) Tile Calorimeter <br>
]]
The
# Inner Detector;
# Calorimeters;
# [[Muon]] Spectrometer;
# Magnet system.
Each of these is in turn made of multiple layers. The detectors are complementary: the Inner Detector tracks particles precisely, the calorimeters measure the energy of easily stopped particles, and the muon system makes additional measurements of highly penetrating muons. The two magnet systems bend [[electric charge|charged]] particles in the Inner Detector and the Muon Spectrometer, allowing their [[electric charge]]s and [[momentum|momenta]] to be measured.
The only established stable particles that cannot be detected directly are [[neutrino]]s; their presence is inferred by measuring a momentum imbalance among detected particles. For this to work, the detector must be "[[Hermetic detector|hermetic]]", meaning it must detect all non-neutrinos produced, with no blind spots.
The installation of all the above detector systems was finished in August 2008. The detectors collected millions of cosmic rays during the magnet repairs which took place between fall 2008 and fall 2009, prior to the first proton collisions. The detector operated with close to 100% efficiency and provided performance characteristics very close to its design values.<ref>{{cite journal|title= Performance of the ATLAS Detector using First Collision Data|journal=JHEP|volume=1009|year=2010|pages=056|doi= 10.1007/JHEP09(2010)056|arxiv = 1005.5254 |bibcode = 2010JHEP...09..056A|last1= Aad|first1= G.|author2= (ATLAS Collaboration)|issue= 9|s2cid=118543167|display-authors=etal}}</ref>
===Inner Detector===
[[
The Inner Detector<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/><ref>{{cite journal | title=Alignment of the ATLAS inner detector tracking system | author=Regina Moles-Valls | journal=Nuclear Instruments and Methods in Physics Research Section A | year=2010 | volume=617 | issue=1–3 | pages=568–570 | doi=10.1016/j.nima.2009.09.101|bibcode = 2010NIMPA.617..568M | arxiv=0910.5156 }}</ref> begins a few centimetres from the proton beam axis, extends to a radius of 1.2 metres, and is 6.2 metres in length along the beam pipe. Its basic function is to track charged particles by detecting their interaction with material at discrete points, revealing detailed information about the types of particles and their momentum.<ref name="TPinnerdetector">{{cite book| year=1994| title=ATLAS Technical Proposal| chapter=Inner detector| publisher=CERN| chapter-url=http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node10.html#SECTION00433000000000000000| archive-date=2007-05-26| access-date=2007-03-03| archive-url=https://web.archive.org/web/20070526154601/http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node10.html#SECTION00433000000000000000| url-status=dead}}</ref>
The Inner Detector has three parts, which are explained below.
The [[magnetic field]] surrounding the entire inner detector causes charged particles to curve; the direction of the curve reveals a particle's charge and the degree of curvature reveals its momentum. The starting points of the tracks yield useful information for [[particle identification|identifying particles]]; for example, if a group of tracks seem to originate from a point other than the original proton–proton collision, this may be a sign that the particles came from the decay of a hadron with a [[bottom quark]] (see [[b-tagging]]).
====Pixel Detector====
The Pixel Detector,<ref>{{cite journal|title=The ATLAS pixel detector|author=Hugging, F.|journal=IEEE Transactions on Nuclear Science|year=2006|volume=53|issue=6|doi=10.1109/TNS.2006.871506|pages=1732–1736|arxiv = physics/0412138 |bibcode = 2006ITNS...53.1732H |s2cid=47545925}}</ref> the innermost part of the detector, contains four concentric layers and three disks on each end-cap, with a total of 1,744 ''modules'', each measuring 2 centimetres by 6 centimetres. The detecting material is 250 μm thick [[silicon]]. Each module contains 16 readout [[computer chip|chips]] and other electronic components. The smallest unit that can be read out is a pixel (50 by 400 micrometres); there are roughly 47,000 pixels per module.
The minute pixel size is designed for extremely precise tracking very close to the interaction point. In total, the Pixel Detector has over 92 million readout channels, which is about 50% of the total readout channels of the whole detector. Having such a large count created a considerable design and engineering challenge. Another challenge was the [[radiation]] to which the Pixel Detector is exposed because of its proximity to the interaction point, requiring that all components be [[radiation hardened]] in order to continue operating after significant exposures.
====Semi-Conductor Tracker====
The Semi-Conductor Tracker (SCT) is the middle component of the inner detector. It is similar in concept and function to the Pixel Detector but with long, narrow strips rather than small pixels, making coverage of a larger area practical. Each strip measures 80 micrometres by 12 centimetres. The SCT is the most critical part of the inner detector for basic tracking in the plane perpendicular to the beam, since it measures particles over a much larger area than the Pixel Detector, with more sampled points and roughly equal (albeit one-dimensional) accuracy. It is composed of four double layers of silicon strips, and has 6.3 million readout channels and a total area of 61 square meters.
====Transition Radiation Tracker====
The Transition Radiation Tracker (TRT), the outermost component of the inner detector, is a combination of a [[straw tracker]] and a [[transition radiation detector]]. The detecting elements are drift tubes (straws), each four millimetres in diameter and up to 144 centimetres long. The uncertainty of track position measurements (position resolution) is about 200 micrometres. This is not as precise as those for the other two detectors, but it was necessary to reduce the cost of covering a larger volume and to have transition radiation detection capability. Each straw is filled with gas that becomes [[ion]]ized when a charged particle passes through. The straws are held at about −1,500 V, driving the negative ions to a fine wire down the centre of each straw, producing a current pulse (signal) in the wire. The wires with signals create a pattern of 'hit' straws that allow the path of the particle to be determined. Between the straws, materials with widely varying [[index of refraction|indices of refraction]] cause ultra-relativistic charged particles to produce [[transition radiation]] and leave much stronger signals in some straws. [[Xenon]] and [[argon]] gas is used to increase the number of straws with strong signals. Since the amount of transition radiation is greatest for highly [[special relativity|relativistic]] particles (those with a speed very near the [[speed of light]]), and because particles of a particular energy have a higher speed the lighter they are, particle paths with many very strong signals can be identified as belonging to the lightest charged particles: [[electron]]s and their antiparticles, [[positron]]s. The TRT has about 298,000 straws in total.
===Calorimeters===
[[
[[
[[File:ATLAS Tile Calorimeter.png|thumb|The extended barrel section of the hadronic calorimeter.]]
The [[calorimeter (particle physics)|calorimeters]]<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/> are situated outside the solenoidal [[magnet]] that surrounds the Inner Detector. Their purpose is to measure the energy from particles by absorbing it. There are two basic calorimeter systems: an inner electromagnetic calorimeter and an outer [[hadronic]] calorimeter.<ref name="TPcalorimetry">{{cite book| year=1994| title=ATLAS Technical Proposal| chapter=Calorimetry| publisher=CERN| chapter-url=http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node9.html#SECTION00432000000000000000| archive-date=2021-01-07| access-date=2007-03-03| archive-url=https://web.archive.org/web/20210107205536/http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node9.html#SECTION00432000000000000000| url-status=dead}}</ref> Both are ''sampling calorimeters''; that is, they absorb energy in high-density metal and periodically sample the shape of the resulting [[particle shower]], inferring the energy of the original particle from this measurement.
====Electromagnetic calorimeter====
The electromagnetic (EM) calorimeter absorbs energy from particles that interact [[Electromagnetism|electromagnetically]], which include charged particles and photons. It has high precision, both in the amount of energy absorbed and in the precise ___location of the energy deposited. The angle between the particle's trajectory and the detector's beam axis (or more precisely the [[pseudorapidity]]) and its angle within the perpendicular plane are both measured to within roughly 0.025 [[radian]]s. The barrel EM calorimeter has accordion shaped electrodes and the energy-absorbing materials are [[lead]] and [[stainless steel]], with liquid [[argon]] as the sampling material, and a [[cryostat]] is required around the EM calorimeter to keep it sufficiently cool.
====Hadron calorimeter====
The [[hadron]] calorimeter absorbs energy from particles that pass through the EM calorimeter, but do interact via the [[strong force]]; these particles are primarily hadrons. It is less precise, both in energy magnitude and in the localization (within about 0.1 radians only).<ref name="PhysicsatLHC"/> The energy-absorbing material is steel, with scintillating tiles that sample the energy deposited. Many of the features of the calorimeter are chosen for their cost-effectiveness; the instrument is large and comprises a huge amount of construction material: the main part of the calorimeter – the tile calorimeter – is 8 metres in diameter and covers 12 metres along the beam axis. The far-forward sections of the hadronic calorimeter are contained within the forward EM calorimeter's cryostat, and use liquid argon as well, while copper and tungsten are used as absorbers.
===Muon Spectrometer===
The [[Muon]] [[Spectrometer]]<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/> is an extremely large tracking system, consisting of three parts:
# A magnetic field provided by three toroidal magnets;
# A set of 1200 chambers measuring with high spatial precision the tracks of the outgoing muons;
# A set of triggering chambers with accurate time-resolution.
The extent of this sub-detector starts at a radius of 4.25 m close to the calorimeters out to the full radius of the detector (11 m). Its tremendous size is required to accurately measure the momentum of muons, which first go through all the other elements of the detector before reaching the muon spectrometer. It was designed to measure, standalone, the momentum of 100 GeV muons with 3% accuracy and of 1 TeV muons with 10% accuracy. It was vital to go to the lengths of putting together such a large piece of equipment because a number of interesting physical processes can only be observed if one or more muons are detected, and because the total energy of particles in an event could not be measured if the muons were ignored. It functions similarly to the Inner Detector, with muons curving so that their momentum can be measured, albeit with a different [[magnetic field]] configuration, lower spatial precision, and a much larger volume. It also serves the function of simply identifying muons – very few particles of other types are expected to pass through the calorimeters and subsequently leave signals in the Muon Spectrometer. It has roughly one million readout channels, and its layers of detectors have a total area of 12,000 square meters.
===Magnet
[[
[[
The ATLAS detector uses two large superconducting magnet systems to bend the trajectory of charged particles, so that their momenta can be measured.<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/> This bending is due to the [[Lorentz force]], whose modulus is proportional to the [[electric charge]] <math>q</math> of the particle, to its speed <math>v</math> and to the intensity <math>B</math> of the magnetic field:
:<math>F = q \, v \, B.</math>
Since all particles produced in the LHC's [[proton]] collisions are traveling at very close to the speed of light in vacuum <math>(v \simeq c)</math>, the [[Lorentz force]] is about the same for all the particles with same [[electric charge]] <math>q</math>:
:<math>F \simeq q \, c \, B.</math>
The radius of curvature <math>r</math> due to the [[Lorentz force]] is equal to
:<math>r = \frac{p}{q \, B}.</math>
where <math> p = \gamma \, m \, v </math> is the [[mass in special relativity|relativistic]] [[momentum]] of the particle. As a result, high-momentum particles curve very little (large <math>r</math>), while low-momentum particles curve significantly (small <math>r</math>). The amount of [[curvature]] can be quantified and the particle [[momentum]] can be determined from this value.
====Solenoid Magnet====
The inner [[solenoid]] produces a two [[Tesla (unit)|tesla]] magnetic field surrounding the Inner Detector.<ref name="TPmagnet">{{cite book| year=1994| title= ATLAS Technical Proposal| chapter=Magnet system| publisher=CERN| chapter-url= http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node8.html#SECTION00431000000000000000}}</ref> This high magnetic field allows even very energetic particles to curve enough for their momentum to be determined, and its nearly uniform direction and strength allow measurements to be made very precisely. Particles with momenta below roughly 400 [[MeV]] will be curved so strongly that they will loop repeatedly in the field and most likely not be measured; however, this energy is very small compared to the several [[TeV]] of energy released in each proton collision.
===
The outer [[toroid]]al magnetic field is produced by eight very large air-core [[superconducting]] barrel loops and two smaller end-caps air toroidal magnets, for a total of 24 barrel loops all situated outside the calorimeters and within the muon system.<ref name="TPmagnet"/> This magnetic field extends in an area 26 metres long and 20 metres in diameter, and it stores 1.6 [[gigajoule]]s of energy. Its magnetic field is not uniform, because a solenoid magnet of sufficient size would be prohibitively expensive to build. It varies between 2 and 8 Teslameters.
===Forward detectors===
{{further|ATLAS Forward Proton Project}}
The ATLAS detector is complemented by a set of four sub-detectors in the forward region to measure particles at very small angles.<ref>[http://atlas-project-lumi-fphys.web.cern.ch/ The ATLAS Forward Detector project]</ref>
# LUCID (LUminosity Cherenkov Integrating Detector) <br> is the first of these detectors designed to measure luminosity, and located in the ATLAS cavern at 17 m from the interaction point between the two muon endcaps;
# ZDC (Zero Degree Calorimeter) <br> is designed to measure neutral particles on-axis to the beam, and located at 140 m from the IP in the LHC tunnel where the two beams are split back into separate beam pipes;
# AFP (Atlas Forward Proton) <br> is designed to tag diffractive events, and located at 204 m and 217 m;
# ALFA (Absolute Luminosity For ATLAS) <br> is designed to measure elastic proton scattering located at 240 m just before the bending magnets of the LHC arc.
===Data systems
====Data generation====
Earlier particle detector read-out and event detection systems were based on parallel shared [[Bus (computing)|buses]] such as [[VMEbus]] or [[FASTBUS]]. Since such a bus architecture cannot keep up with the data requirements of the LHC detectors, all the ATLAS data acquisition systems rely on high-speed point-to-point links and switching networks. Even with advanced [[electronics]] for data reading and storage, the ATLAS detector generates too much raw data to read out or store everything: about 25 [[megabyte|MB]] per raw event, multiplied by 40 million [[beam crossing]]s per second (40 [[Hertz#SI multiples|MHz]]) in the center of the detector. This produces a total of 1 [[Byte#Multiple-byte units|petabyte]] of raw data per second. By avoiding to write empty segments of each event (zero suppression), which do not contain physical information, the average size of an event is reduced to 1.6 [[megabyte|MB]], for a total of 64 [[terabyte]] of data per second.<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/>
====Trigger system====
The [[trigger (particle physics)|trigger]] system<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/><ref>{{cite journal|title=ATLAS Trigger and Data Acquisition: Capabilities and commissioning|author=D. A. Scannicchio|journal=Nuclear Instruments and Methods in Physics Research Section A|year=2010|volume=617|issue=1/3|doi=10.1016/j.nima.2009.06.114|pages=306–309|bibcode = 2010NIMPA.617..306S}}</ref> uses fast event reconstruction to identify, in real time, the most interesting [[event (particle physics)|events]] to retain for detailed analysis. In the second data-taking period of the LHC, Run-2, there were two distinct trigger levels:<ref>{{cite journal|title=ATLAS Run-2 status and performance|author=ATLAS collaboration|journal=Nuclear and Particle Physics Proceedings|year=2016|volume=270|doi=10.1016/j.nuclphysbps.2016.02.002|pages=3–7|bibcode=2016NPPP..270....3P|url=https://cds.cern.ch/record/2048973}}</ref>
# The Level 1 trigger (L1), implemented in custom hardware at the detector site. The decision to save or reject an event data is made in less than 2.5 μs. It uses reduced granularity information from the calorimeters and the muon spectrometer, and reduces the rate of events in the read-out from 40 [[Hertz#SI multiples|MHz]] to 100 [[Hertz#SI multiples|kHz]]. The L1 rejection factor in therefore equal to 400.
# The High Level Trigger trigger (HLT), implemented in software, uses a computer battery consisting of approximately 40,000 [[Central processing unit|CPUs]]. In order to decide which of the 100,000 events per second coming from L1 to save, specific analyses of each collision are carried out in 200 μs. The HLT uses limited regions of the detector, so-called Regions of Interest (RoI), to be reconstructed with the full detector granularity, including tracking, and allows matching of energy deposits to tracks. The HLT rejection factor is 100: after this step, the rate of events is reduced from 100 to 1 [[Hertz#SI multiples|kHz]]. The remaining data, corresponding to about 1,000 events per second, are stored for further analyses.<ref name="CERN">{{cite news |work=[[ATLAS collaboration]] Research News |title=Trigger and Data Acquisition System|url=https://atlas.cern/discover/detector/trigger-daq |date=October 2019}}</ref>
====Analysis process====
ATLAS permanently records more than 10 [[Byte#Multiple-byte units|petabyte]]s of data per year.<ref name=fact_sheets/>
Offline [[event reconstruction]] is performed on all permanently stored events, turning the pattern of signals from the detector into physics objects, such as [[Particle jet|jets]], [[photon]]s, and [[lepton]]s. [[Grid computing]] is being used extensively for event reconstruction, allowing the parallel use of university and laboratory computer networks throughout the world for the [[central processing unit|CPU]]-intensive task of reducing large quantities of raw data into a form suitable for physics analysis.
The [[software]] for these tasks has been under development for many years, and refinements are ongoing, even after data collection has begun.
Individuals and groups within the collaboration are continuously writing their own [[computational physics|code]] to perform further analyses of these objects, searching the patterns of detected particles for particular physical models or hypothetical particles. This activity requires processing 25 [[Byte#Multiple-byte units|petabyte]]s of data every week.<ref name=fact_sheets/>
==Trivia==
The researcher pictured for scale in the famous [[:File:Installing the ATLAS Calorimeter.jpg|ATLAS detector image]] is Roger Ruber, a researcher from Uppsala University, Sweden. Ruber, one of the researchers responsible for the ATLAS detector's central cryostat magnet, was inspecting the magnets in the LHC tunnel at the same time Maximilien Brice, the photographer, was setting up to photograph the ATLAS detector. Brice asked Ruber to stand at the base of the detector to illustrate the scale of the ATLAS detector. This was revealed by Maximilien Brice, and confirmed by Roger Ruber during interviews in 2020 with [[Becky Smethurst|Rebecca Smethurst]] of the University of Oxford.<ref>{{cite AV media | people = Smethurst, Rebecca (University of Oxford) | date = March 11, 2020 | access-date = August 9, 2021 | title = I found the man in the most famous physics picture ever | medium = Video | url = https://www.youtube.com/watch?v=sB0_ohLM3Kg&t=256s | time = 4:29 and 6:49 | ___location = Geneva, Switzerland | publisher = Smethurst, Rebecca}}</ref>
==References==
{{Reflist}}
==Further reading==
* [http://atlas.web.cern.ch/Atlas/TP/tp.html ATLAS Technical Proposal.] {{Webarchive|url=https://web.archive.org/web/20120802084610/http://atlas.web.cern.ch/Atlas/TP/tp.html |date=2012-08-02 }} CERN: The Atlas Experiment. Retrieved on 2007-04-10
* "[http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/TDR/access.html ATLAS Detector and Physics Performance Technical Design Report] {{Webarchive|url=https://web.archive.org/web/20030516011026/http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/TDR/access.html |date=2003-05-16 }}". [[CERN]]: The Atlas Experiment. Retrieved on 2007-04-10
* {{cite journal
|
| title = Physics at LHC
| journal = Physics of Particles and Nuclei
Line 115 ⟶ 273:
| arxiv = hep-ph/9703204
| doi = 10.1134/1.953049
|bibcode = 1997PPN....28..441K |s2cid=118907038
}} * [http://www.scholarpedia.org/article/The_ATLAS_experiment The Atlas Experiment] Monica Lynn Dunford and Peter Jenni, [[Scholarpedia]] 9(10):32147. [[doi:10.4249/scholarpedia.32147]]
==External links==
{{Commons
* [http://atlas.ch Official ATLAS Public Webpage] at CERN ''(The "award winning ATLAS movie" is a very good general introduction!)''
* [
* [http://atlasexperiment.org/webcams.html ATLAS Cavern Webcams] {{Webarchive|url=https://web.archive.org/web/20170720151518/http://atlasexperiment.org/webcams.html |date=2017-07-20 }}
* [
* [https://web.archive.org/web/20090413155822/http://www.uslhc.us/What_is_the_LHC/Experiments/ATLAS ATLAS section from US/LHC Website]
* [
* [http://www.eurekalert.org/features/doe/2004-03/dnal-tsg032604.php United States Department of Energy article on ATLAS] {{Webarchive|url=https://web.archive.org/web/20210301200956/https://www.eurekalert.org/features/doe/2004-03/dnal-tsg032604.php |date=2021-03-01 }}
* [https://web.archive.org/web/20131002231102/http://www.ingenia.org.uk/ingenia/articles.aspx?
* {{Cite journal |title=The ATLAS Experiment at the CERN Large Hadron Collider |collaboration=The ATLAS Collaboration |last1=Aad |first1=G. |display-authors=etal |journal=Journal of Instrumentation |date=2008-08-14 |volume=3 |issue=S08003 |doi=10.1088/1748-0221/3/08/S08003 |pages=S08003|bibcode = 2008JInst...3S8003A |hdl=2027.42/64167 |s2cid=250683252 |url=http://resolver.sub.uni-goettingen.de/purl?gs-1/12156 |hdl-access=free }} (Full design documentation)
* [http://sascha.mehlhase.info/physics.php?open=atlaslego LEGO model of ATLAS], by an ATLAS-scientist at the [[Niels Bohr Institute]] <!-- ref http://ing.dk/artikel/125038-ku-forsker-bygger-lego-kopi-af-lhcs-atlas-detektor -->
* {{cite web|last=Padilla|first=Antonio (Tony)|title=ATLAS at the Large Hadron Collider|url=http://www.sixtysymbols.com/videos/atlas.htm|work=Sixty Symbols|publisher=[[Brady Haran]] for the [[University of Nottingham]]}}
*{{Cite web|title=ATLAS celebrates results of 1000 collision papers|url=https://atlas.cern/updates/news/1000-collision-papers|access-date=2021-08-03|website=ATLAS|language=en}}
*Record for [https://inspirehep.net/experiments/1108541 ATLAS] experiment on [[INSPIRE-HEP]]
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