Editing ATLAS experiment (section)

===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.