Editing Standard Model (section)

== Particle content ==
The Standard Model includes members of several classes of elementary particles, which in turn can be distinguished by other characteristics, such as [[color charge]].

All particles can be summarized as follows:
{{Elementary particles|SM=yes}}

=== Fermions ===
<!--
[[File:Standard Model.svg|upright=1.5|right|thumb|The pattern of [[weak isospin]], T<sub>3</sub>, [[weak hypercharge]], Y<sub>W</sub>, and [[color charge]] of all known elementary particles, rotated by the [[Weinberg angle|weak mixing angle]] to show electric charge, Q, roughly along the vertical. The neutral [[Higgs field]] (gray square) breaks the [[electroweak symmetry]] and interacts with other particles to give them mass.]]
-->

The Standard Model includes 12 [[elementary particle]]s of [[Spin (physics)|spin]] {{1/2}}, known as [[fermion]]s.<ref name=":0">{{Cite web |title=The Standard Model |url=https://www-project.slac.stanford.edu/e158/StandardModel.html |url-status=live |archive-url=https://web.archive.org/web/20060620190613/http://www-project.slac.stanford.edu/e158/StandardModel.html |archive-date=June 20, 2006 |access-date=January 18, 2024 |website=[[SLAC National Accelerator Laboratory]]}}</ref> Fermions respect the [[Pauli exclusion principle]], meaning that two [[Indistinguishable particles|identical fermions]] cannot simultaneously occupy the same quantum state in the same atom.<ref>{{Cite journal |last=Eisert |first=Jens |date=January 22, 2013 |title=Pauli Principle, Reloaded |url=https://physics.aps.org/articles/v6/8 |journal=Physics |language=en |volume=6 |issue=4 |pages=8 |doi=10.1103/PhysRevLett.110.040404|pmid=25166142 |arxiv=1210.5531 }}</ref> Each fermion has a corresponding [[antiparticle]], which are particles that have corresponding properties with the exception of [[Additive inverse|opposite]] [[Charge (physics)|charges]].<ref>{{Cite web |date=January 24, 2002 |title=What is antimatter? |url=https://www.scientificamerican.com/article/what-is-antimatter-2002-01-24/ |url-status=live |archive-url=https://web.archive.org/web/20140331153524/http://www.scientificamerican.com/article/what-is-antimatter-2002-01-24 |archive-date=March 31, 2014 |access-date=January 19, 2024 |website=[[Scientific American]]}}</ref> Fermions are classified based on how they interact, which is determined by the charges they carry, into two groups: [[Quark|quarks]] and [[Lepton|leptons]]. Within each group, pairs of particles that exhibit similar physical behaviors are then grouped into [[Generation (particle physics)|generations]] (see the table). Each member of a generation has a greater mass than the corresponding particle of generations prior. Thus, there are three generations of quarks and leptons.<ref name=":1">{{cite web |title=Standard Model - ATLAS Physics Cheat Sheet |url=https://cds.cern.ch/record/2759492/files/Standard%20Model%20-%20ATLAS%20Physics%20Cheat%20Sheet.pdf |access-date=2024-01-19 |website=[[ATLAS experiment|ATLAS]] |publisher=[[CERN]]}}</ref>  As first-generation particles do not decay, they comprise all of ordinary ([[baryon]]ic) matter. Specifically, all atoms consist of electrons orbiting around the [[atomic nucleus]], ultimately constituted of up and down quarks. On the other hand, second- and third-generation charged particles decay with very short [[Half-life|half-lives]] and can only be observed in high-energy environments. Neutrinos of all generations also do not decay, and pervade the universe, but rarely interact with baryonic matter.

There are six quarks: [[up quark|up]], [[down quark|down]], [[charm quark|charm]], [[strange quark|strange]], [[top quark|top]], and [[bottom quark|bottom]].<ref name=":0" /><ref name=":1" /> Quarks carry [[color charge]], and hence interact via the [[strong interaction]]. The [[color confinement]] phenomenon results in quarks being strongly bound together such that they form color-neutral composite particles called [[hadron]]s; quarks cannot individually exist and must always bind with other quarks. Hadrons can contain either a quark-antiquark pair ([[meson]]s) or three quarks ([[baryon]]s).<ref>{{cite web |title=Color Charge and Confinement |url=https://fafnir.phyast.pitt.edu/particles/color.html |url-status=live |archive-url=https://web.archive.org/web/20020322100232/http://fafnir.phyast.pitt.edu/particles/color.html |archive-date=March 22, 2002 |access-date=January 8, 2024 |website=[[University of Pittsburgh]]}}</ref> The lightest baryons are the [[Nucleon|nucleons]]: the [[proton]] and [[neutron]]. Quarks also carry [[electric charge]] and [[weak isospin]], and thus interact with other fermions through [[electromagnetism]] and [[weak interaction]]. The six leptons consist of the [[electron]], [[electron neutrino]], [[muon]], [[muon neutrino]], [[tau (particle)|tau]], and [[tau neutrino]]. The leptons do not carry color charge, and do not respond to strong interaction. The charged leptons carry an [[electric charge]] of −1&nbsp;''[[Elementary charge|e]]'', while the three [[neutrino]]s carry zero electric charge. Thus, the neutrinos' motions are influenced by only the [[weak interaction]] and [[gravity]], making them difficult to observe.

=== Gauge bosons ===
[[File:Standard Model – All Feynman diagram vertices.svg|upright=1.5|thumb|right|class=skin-invert-image|Interactions in the Standard Model. All Feynman diagrams in the model are built from combinations of these vertices. ''q'' is any quark, ''g'' is a gluon, 

''X'' is any charged particle, γ is a photon, ''f'' is any fermion, ''m'' is any particle with mass (with the possible exception of the neutrinos), ''m''<sub>B</sub> is any boson with mass. 

In diagrams with multiple particle labels separated by '/', one particle label is chosen. In diagrams with particle labels separated by '<nowiki>|</nowiki>', the labels must be chosen in the same order. For example, in the four boson electroweak case the valid diagrams are WWWW, WWZZ, WWγγ, WWZγ. The conjugate of each listed vertex (reversing the direction of arrows) is also allowed.<ref>{{cite thesis |type=PhD |last=Lindon |first=Jack |date=2020 |title=Particle Collider Probes of Dark Energy, Dark Matter and Generic Beyond Standard Model Signatures in Events With an Energetic Jet and Large Missing Transverse Momentum Using the ATLAS Detector at the LHC |publisher=CERN |url=https://cds.cern.ch/record/2746537/ }}</ref>]]

The Standard Model includes 4 kinds of [[gauge boson]]s of [[Spin (physics)|spin]] 1,<ref name=":0" /> with bosons being quantum particles containing an integer spin. The gauge bosons are defined as [[force carrier]]s, as they are responsible for mediating the [[fundamental interaction]]s. The Standard Model explains the four fundamental forces as arising from the interactions, with fermions [[Static forces and virtual-particle exchange|exchanging]] [[Virtual particle|virtual]] force carrier particles, thus mediating the forces. At a macroscopic scale, this manifests as a [[force]].<ref>{{cite journal |last1=Jaeger |first1=Gregg |year=2021 |title=Exchange Forces in Particle Physics |journal=Foundations of Physics |volume=51 |issue=1 |page=13 |bibcode=2021FoPh...51...13J |doi=10.1007/s10701-021-00425-0 |s2cid=231811425}}</ref> As a result, they do not follow the Pauli exclusion principle that constrains fermions; bosons do not have a theoretical limit on their [[volume number density|spatial density]]. The types of gauge bosons are described below.
* [[Electromagnetism]]: [[Photon]]s mediate the electromagnetic force, responsible for interactions between electrically charged particles. The photon is massless and is described by the theory of [[quantum electrodynamics]] (QED).
* [[Strong interaction|Strong Interactions]]: [[Gluon]]s mediate the strong interactions, which binds quarks to each other by influencing the [[color charge]], with the interactions being described in the theory of [[quantum chromodynamics]] (QCD). They have no mass, and there are eight distinct gluons, with each being denoted through a color-anticolor charge combination (e.g. red–antigreen).{{NoteTag|Although nine color–anticolor combinations mathematically exist, gluons form color octet particles. As one color-symmetric combination is linear and forms a color singlet particles, there are eight possible gluons.<ref>{{Cite book |last1=Cahn |first1=Robert N. |title=The Experimental Foundations of Particle Physics |last2=Goldbaher |first2=Gerson |publisher=[[Cambridge University Press]] |year=2010 |isbn=978-0521521475 |edition=2nd |publication-date=August 31, 2009 |pages=306 |chapter=Quarks, gluons, and jets |chapter-url=http://hitoshi.berkeley.edu/129A/Cahn-Goldhaber/chapter10.pdf |chapter-format=[[PDF]] |archive-url=https://web.archive.org/web/20120714015451/http://hitoshi.berkeley.edu/129A/Cahn-Goldhaber/chapter10.pdf |archive-date=July 14, 2012 |url-status=live}}</ref>}} As gluons have an effective color charge, they can also interact amongst themselves.
* [[Weak interaction|Weak Interactions]]: The [[W and Z bosons|{{SubatomicParticle|W boson+}}, {{SubatomicParticle|W boson-}}, and {{SubatomicParticle|Z boson}}]] gauge bosons mediate the weak interactions between all fermions, being responsible for [[Radioactive decay|radioactivity]]. They contain mass, with the {{SubatomicParticle|Z boson}} having more mass than the {{SubatomicParticle|W boson+-}}. The weak interactions involving the {{SubatomicParticle|W boson+-}} act only on [[Chirality (physics)|''left-handed'' particles and ''right-handed'' antiparticles]] respectively. The {{SubatomicParticle|W boson+-}} carries an electric charge of +1 and −1 and couples to the electromagnetic interaction. The electrically neutral {{SubatomicParticle|Z boson}} boson interacts with both left-handed particles and right-handed antiparticles. These three gauge bosons along with the photons are grouped together, as collectively mediating the [[electroweak]] interaction.
* [[Gravity]]: It is currently unexplained in the Standard Model, as the hypothetical mediating particle [[graviton]] has been proposed, but not observed.<ref>{{Cite web |last=Hooper |first=Dan |date=2022-05-19 |title=What is the Standard Model of particle physics, and why are scientists looking beyond it? |url=https://www.astronomy.com/science/what-is-the-standard-model-of-particle-physics-and-why-are-scientists-looking-beyond-it/ |access-date=2024-01-20 |website=[[Astronomy Magazine]] |language=en-US}}</ref> This is due to the incompatibility of quantum mechanics and [[General relativity|Einstein's theory of general relativity]], regarded as being the best explanation for gravity. In general relativity, gravity is explained as being the geometric curving of spacetime.<ref>{{Cite news |last=Butterworth |first=Jon |date=2014-06-01 |title=Gravity versus the Standard Model |url=https://www.theguardian.com/science/life-and-physics/2014/jun/01/gravity-versus-the-standard-model |access-date=2024-01-20 |work=[[The Guardian]] |language=en-GB |issn=0261-3077}}</ref>
The [[Feynman diagram]] calculations, which are a graphical representation of the [[perturbation theory (quantum mechanics)|perturbation theory]] approximation, invoke "force mediating particles", and when applied to analyze [[particle accelerator|high-energy scattering experiments]] are in reasonable agreement with the data. However, perturbation theory (and with it the concept of a "force-mediating particle") fails in other situations. These include low-energy quantum chromodynamics, [[bound state]]s, and [[soliton]]s. The interactions between all the particles described by the Standard Model are summarized by the diagrams on the right of this section.

=== Higgs boson ===
{{Main|Higgs boson}}

The Higgs particle is a massive [[Scalar field theory|scalar]] elementary particle theorized by [[Peter Higgs]] ([[1964 PRL symmetry breaking papers|and others]]) in 1964, when he showed that Goldstone's 1962 theorem (generic continuous symmetry, which is spontaneously broken) provides a third polarisation of a massive vector field. Hence, Goldstone's original scalar doublet, the massive spin-zero particle, was [[1964 PRL symmetry breaking papers|proposed as the Higgs boson]], and is a key building block in the Standard Model.<ref>
{{cite journal
 |author=G.S. Guralnik
 |year=2009
 |title=The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles
 |journal=[[International Journal of Modern Physics A]]
 |volume=24 |issue=14 |pages=2601–2627
 |arxiv=0907.3466
 |bibcode=2009IJMPA..24.2601G
 |doi=10.1142/S0217751X09045431
 |s2cid=16298371
}}</ref> It has no intrinsic [[Spin (physics)|spin]], and for that reason is classified as a [[boson]] with spin-0.<ref name=":0" />

The Higgs boson plays a unique role in the Standard Model, by explaining why the other elementary particles, except the [[photon]] and [[gluon]], are massive. In particular, the Higgs boson explains why the photon has no mass, while the [[W and Z bosons]] are very heavy. Elementary-particle masses and the differences between [[electromagnetism]] (mediated by the photon) and the [[weak force]] (mediated by the W and Z bosons) are critical to many aspects of the structure of microscopic (and hence macroscopic) matter. In [[electroweak interaction|electroweak theory]], the Higgs boson generates the masses of the leptons (electron, muon, and tau) and quarks.  As the Higgs boson is massive, it must interact with itself.

Because the Higgs boson is a very massive particle and also decays almost immediately when created, only a very high-energy [[particle accelerator]] can observe and record it. Experiments to confirm and determine the nature of the Higgs boson using the [[Large Hadron Collider]] (LHC) at [[CERN]] began in early 2010 and were performed at [[Fermilab]]'s [[Tevatron]] until its closure in late 2011. Mathematical consistency of the Standard Model requires that any mechanism capable of generating the masses of elementary particles must become visible{{clarify|reason=Isn't "apparent" or "manifest" needed here instead of "visible"?|date=July 2013}} at energies above {{val|1.4|ul=TeV}};<ref>
{{cite journal
 |author1=B.W. Lee |author2=C. Quigg |author3=H.B. Thacker |year=1977
 |title=Weak interactions at very high energies: The role of the Higgs-boson mass
 |journal=[[Physical Review D]]
 |volume=16 |issue=5 |pages=1519–1531
 |bibcode=1977PhRvD..16.1519L
 |doi=10.1103/PhysRevD.16.1519
}}</ref> therefore, the LHC (designed to collide two {{val|7|u=TeV}} proton beams) was built to answer the question of whether the Higgs boson actually exists.<ref>
{{cite news
 |date=11 November 2009
 |title=Huge $10 billion collider resumes hunt for 'God particle'
 |url=http://www.cnn.com/2009/TECH/11/11/lhc.large.hadron.collider.beam/index.html
 |publisher=CNN
 |access-date=2010-05-04
}}</ref>

On 4 July 2012, two of the experiments at the LHC ([[ATLAS experiment|ATLAS]] and [[Compact Muon Solenoid|CMS]]) both reported independently that they had found a new particle with a mass of about {{val|125|ul=GeV/c2}} (about 133 proton masses, on the order of {{val|e=-25|u=kg}}), which is "consistent with the Higgs boson".<ref>
{{cite web
 |date=4 July 2012
 |title=Observation of a New Particle with a Mass of 125&nbsp;GeV
 |url=http://cms.web.cern.ch/news/observation-new-particle-mass-125-gev
 |publisher=CERN
 |access-date=2012-07-05
}}</ref><ref name="NYT-20120704">
{{cite news
 |author=D. Overbye
 |date=4 July 2012
 |title=A New Particle Could Be Physics' Holy Grail
 |url=https://www.nytimes.com/2012/07/05/science/cern-physicists-may-have-discovered-higgs-boson-particle.html
 |newspaper=The New York Times
 |access-date=2012-07-04
}}</ref> On 13 March 2013, it was confirmed to be the searched-for Higgs boson.<ref name="CERN_20130314">
{{cite web
 |url=https://home.cern/news/press-release/cern/new-results-indicate-particle-discovered-cern-higgs-boson
 |title=New results indicate that particle discovered at CERN is a Higgs boson
 |date=14 March 2013
 |publisher=CERN
 |access-date=2020-06-14
}}</ref><ref name="CERN_EPS2017">
{{cite web
 |url=https://press.cern/update/2017/07/lhc-experiments-delve-deeper-precision
 |title=LHC experiments delve deeper into precision
 |date=11 July 2017
 |publisher=CERN
 |access-date=2017-07-23
 |archive-date=14 July 2017
 |archive-url=https://web.archive.org/web/20170714090456/http://press.cern/update/2017/07/lhc-experiments-delve-deeper-precision
 |url-status=dead
}}</ref>