Editing ATLAS experiment (section)

==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}}</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}}</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>