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{{Short description|Subatomic particle having no substructure}}
{{Standard model of particle physics}}

In [[particle physics]], an '''elementary particle''' or '''fundamental particle''' is a  [[<!-- wiktionary:particle|-->subatomic particle]] that is not composed of other particles.<ref name=PFI /> The [[Standard Model]] recognizes seventeen distinct particles—twelve [[fermion]]s and five [[boson]]s. As a consequence of [[Flavour (particle physics)|flavor]] and [[Quantum chromodynamics|color]] combinations and [[antimatter]], the fermions and bosons are known to have 48 and 13 variations, respectively.<ref name="braibant">{{cite book |last1=Braibant |first1=S. |url=https://books.google.com/books?id=0Pp-f0G9_9sC&q=61+fundamental+particles&pg=PA314 |title=Particles and Fundamental Interactions: An Introduction to Particle Physics |last2=Giacomelli |first2=G. |last3=Spurio |first3=M. |publisher=[[Springer Science+Business Media|Springer]] |year=2009 |isbn=978-94-007-2463-1 |pages=313–314 |access-date=19 October 2020 |archive-url=https://web.archive.org/web/20210415025723/https://books.google.com/books?id=0Pp-f0G9_9sC&q=61+fundamental+particles&pg=PA314 |archive-date=15 April 2021 |url-status=live}}</ref> These include [[electron]]s and other [[lepton]]s, [[quark]]s, and the fundamental [[boson]]s. [[Subatomic particle]]s such as [[proton]]s or [[neutron]]s, which [[Quark|contain]] two or more elementary particles, are known as [[composite particle]]s.

Ordinary matter is composed of [[atom]]s, themselves once thought to be indivisible elementary particles. The name ''atom'' comes from the Ancient Greek word ''ἄτομος'' ([[wikt:átomo#:~:text=Learned borrowing from Latin atomus,, "to cut").|atomos]]) which means ''indivisible'' or ''uncuttable''. Despite the [[Democritus|theories about atoms]] that had existed for [[De rerum natura|thousands of years]], the factual existence of atoms remained controversial until 1905. In that year, [[Albert Einstein]] published [[Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen|his paper]] on [[Brownian motion]], putting to rest theories that had regarded [[molecule]]s as mathematical illusions. Einstein subsequently identified matter as ultimately composed of various concentrations of [[Mass–energy equivalence|energy]].<ref name=PFI /><ref>
{{cite journal
 |first1=Ronald |last1=Newburgh
 |first2=Joseph |last2=Peidle
 |first3=Wolfgang |last3=Rueckner
 |year=2006
 |title=Einstein, Perrin, and the reality of atoms: 1905 revisited
 |url=http://physlab.lums.edu.pk/images/f/fe/Ref1.pdf
 |journal=[[American Journal of Physics]]
 |volume=74
 |issue=6
 |pages=478–481
 |bibcode=2006AmJPh..74..478N
 |doi=10.1119/1.2188962
 |access-date=2013-08-17
 |archive-url=https://web.archive.org/web/20170803105918/https://physlab.lums.edu.pk/images/f/fe/Ref1.pdf
 |archive-date=2017-08-03 |df=dmy-all
 }}</ref>

Subatomic constituents of the atom were first identified toward the end of the [[19th century in science|19th century]], beginning with the [[J. J. Thomson#Discovery of the electron|electron]], followed by the [[Ernest Rutherford#Discovery of the proton|proton]] in 1919, the [[photon]] in the 1920s, and the [[Discovery of the neutron|neutron]] in 1932.<ref name="PFI" /> By that time, the advent of [[quantum mechanics]] had [[History of quantum mechanics|radically altered]] the definition of a "particle" by putting forward an understanding in which they carried out a simultaneous existence as [[matter wave]]s.<ref>
{{cite book
 |first=Friedel |last=Weinert
 |year=2004
 |title=The Scientist as Philosopher: Philosophical consequences of great scientific discoveries
 |publisher=[[Springer (publisher)|Springer]]
 |pages=43, 57–59
 |url=https://books.google.com/books?id=E0NRcFEjvU4C&pg=PA43
 |isbn=978-3-540-20580-7
 |bibcode=2004sapp.book.....W
}}</ref><ref name="Kuhlmann">
{{cite magazine
 |first=Meinard |last=Kuhlmann
 |date=24 July 2013
 |url=http://www.scientificamerican.com/article.cfm?id=physicists-debate-whether-world-made-of-particles-fields-or-something-else
 |title=Physicists debate whether the world is made of particles or fields – or something else entirely
 |magazine=[[Scientific American]]
}}</ref>

Many theoretical elaborations upon, and [[Physics beyond the Standard Model|beyond]], the Standard Model have been made since its [[Standard Model#Historical background|codification]] in the 1970s. These include notions of [[supersymmetry]], which double the number of elementary particles by hypothesizing that each known particle associates with a "shadow" partner far more massive.<ref>
{{cite web
 |collaboration=Particle Data Group
 |publisher=[[Berkeley Lab]]
 |url=http://www.particleadventure.org/supersymmetry.html
 |title=Unsolved mysteries: Supersymmetry
 |work=The Particle Adventure
 |access-date=2013-08-28 |df=dmy-all
}}</ref><ref>
{{cite book
 |collaboration=National Research Council
 |year=2006
 |title=Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics
 |page=68
 |publisher=[[National Academies Press]]
 |url=https://books.google.com/books?id=zXoZjZFZF-kC&pg=PA68
 |isbn=978-0-309-66039-6
 |bibcode=2006rhns.book......
}}</ref> However, like an [[Graviton|additional elementary boson]] mediating gravitation, such [[superpartner]]s remain undiscovered as of 2025.<ref name="ONeill">{{cite news |last=O'Neill |first=Ian |date=24 Jul 2013 |title=LHC discovery maims supersymmetry, again |website=[[Discovery News]] |url=http://news.discovery.com/space/lhc-discovery-maims-supersymmetry-again-130724.htm |access-date=2013-08-28 |archive-url=https://web.archive.org/web/20160313000505/http://news.discovery.com/space/lhc-discovery-maims-supersymmetry-again-130724.htm |archive-date=2016-03-13 |df=dmy-all}}</ref><ref>
{{cite web
 |url=http://phys.org/news/2013-07-cern-latest-supersymmetry.html
 |title=CERN latest data shows no sign of supersymmetry – yet
 |work=[[Phys.Org]]
 |date=25 Jul 2013
 |access-date=2013-08-28 |df=dmy-all
}}</ref><ref name=PFI />{{update inline|date=January 2025}}

== Overview ==
{{Main|Standard Model}}
{{See also|Physics beyond the Standard Model}}
<!--[[File:Particle overview.svg|thumb|400px|An overview of the various families of elementary and composite particles, and the theories describing their interactions]]
-->
All elementary particles are either [[fermion]]s or [[boson]]s. These classes are distinguished by their [[quantum statistics]]: fermions obey [[Fermi–Dirac statistics]] and bosons obey [[Bose–Einstein statistics]].<ref name=PFI>
{{cite book
 |first1=Sylvie |last1=Braibant
 |first2=Giorgio |last2=Giacomelli
 |first3=Maurizio |last3=Spurio
 |year=2012
 |title=Particles and Fundamental Interactions: An introduction to particle physics
 |url=https://books.google.com/books?id=e8YUUG2pGeIC&pg=PA384
 |edition=2nd
 |publisher=[[Springer (publisher)|Springer]]
 |isbn=978-94-007-2463-1
 |pages=1–3
}}</ref> Their [[Spin (physics)|spin]] is differentiated via the [[spin–statistics theorem]]: it is [[half-integer]] for fermions, and [[integer]] for bosons.
{{Elementary particles}}
<!--
;'''Elementary fermions:'''
* [[Matter]] particles
** [[Quark]]s:
*** [[up quark|up]], [[down quark|down]]
*** [[charm quark|charm]], [[strange quark|strange]]
*** [[top quark|top]], [[bottom quark|bottom]]
** [[Lepton]]s:
*** [[electron]], [[electron neutrino]] ([[Pseudonym|a.k.a.]], "neutrino")
*** [[muon]], [[muon neutrino]]
*** [[tau (particle)|tau]], [[tau neutrino]]
* [[Antimatter]] particles
** [[Antiquark]]s
** [[Antilepton]]s

;'''Elementary bosons:'''
* [[Force carrier|Force particles]] ([[gauge boson]]s):
** [[photon]]
** [[gluon]] (numbering eight)<ref name=PFI />
** [[W and Z bosons|''W''<sup>+</sup>, ''W''<sup>−</sup>, and ''Z''<sup>0</sup> bosons]]
** [[graviton]] (hypothetical)<ref name=PFI />
* [[Scalar boson]]
** [[Higgs boson]]
-->

In the [[Standard Model]], elementary particles are represented for [[scientific formalism|predictive utility]] as [[point particle]]s. Though extremely successful, the Standard Model is limited by its omission of [[gravitation]] and has some parameters arbitrarily added but unexplained.<ref>{{harvnb|Braibant|Giacomelli|Spurio|2012|p=384}}</ref>

== Cosmic abundance of elementary particles ==
{{Main|Cosmic abundance of elements }}

According to the current models of [[Big Bang nucleosynthesis]], the primordial composition of visible matter of the universe should be about 75% hydrogen and 25% helium-4 (in mass). Neutrons are made of one up and two down quarks, while protons are made of two up and one down quark. Since the other common elementary particles (such as electrons, neutrinos, or weak bosons) are so light or so rare when compared to atomic nuclei, we can neglect their mass contribution to the observable universe's total mass. Therefore, one can conclude that most of the visible mass of the universe consists of protons and neutrons, which, like all [[baryon]]s, in turn consist of up quarks and down quarks. {{cn|date=June 2025}}

Some estimates imply that there are roughly {{10^|80}} baryons (almost entirely protons and neutrons) in the observable universe.<ref>{{Cite journal |last=Padilla |first=Antonio |date=2022-08-13 |title=The universe by numbers |url=https://linkinghub.elsevier.com/retrieve/pii/S0262407922014476 |journal=New Scientist |volume=255 |issue=3399 |pages=42–45 |doi=10.1016/S0262-4079(22)01447-6 |bibcode=2022NewSc.255...42P |issn=0262-4079|url-access=subscription }}</ref>

The number of protons in the observable universe is called the [[Eddington number]]. {{cn|date=June 2025}}

In terms of number of particles, some estimates imply that nearly all the matter, excluding [[dark matter]], occurs in neutrinos, which constitute the majority of the roughly {{10^|86}} elementary particles of matter that exist in the visible universe.<ref name=mrob>
{{cite web
 |first=Robert |last=Munafo
 |date=24 Jul 2013
 |title=Notable Properties of Specific Numbers
 |url=http://mrob.com/pub/math/numbers-19.html
 |access-date=2013-08-28 |df=dmy-all
}}</ref> Other estimates imply that roughly {{10^|97}} elementary particles exist in the visible universe (not including [[dark matter]]), mostly photons and other massless force carriers.<ref name=mrob />

== Standard Model ==
{{Main|Standard Model}}
The Standard Model of particle physics contains 12 flavors of elementary [[fermion]]s, plus their corresponding [[antiparticle]]s, as well as elementary bosons that mediate the forces and the [[Higgs boson]], which was reported on July 4, 2012, as having been likely detected by the two main experiments at the [[Large Hadron Collider]] ([[ATLAS experiment|ATLAS]] and [[Compact Muon Solenoid|CMS]]).<ref name=PFI /> The Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, however, since it is not known if it is compatible with [[Albert Einstein|Einstein]]'s [[general relativity]]. There may be hypothetical elementary particles not described by the Standard Model, such as the [[graviton]], the particle that would carry the [[gravity|gravitational force]], and [[superpartner|sparticles]], [[supersymmetry|supersymmetric]] partners of the ordinary particles.<ref>{{cite journal |last=Holstein |first=Barry R. |date=November 2006 |title=Graviton physics |journal=[[American Journal of Physics]] |volume=74 |issue=11 |pages=1002–1011 |doi=10.1119/1.2338547 |arxiv=gr-qc/0607045 |bibcode=2006AmJPh..74.1002H |s2cid=15972735 }}</ref>

=== Fundamental fermions ===
{{Main|Fermion}}

The 12&nbsp;fundamental fermions are divided into 3&nbsp;[[generation (particle physics)|generations]] of 4&nbsp;particles each. Half of the fermions are [[lepton]]s, three of which have an electric charge of −1&nbsp;''e'', called the electron ({{Subatomic particle|electron-}}), the [[muon]] ({{Subatomic particle|muon-}}), and the [[tau (particle)|tau]] ({{Subatomic particle|tau-}}); the other three leptons are [[neutrino]]s ({{Subatomic particle|electron neutrino}}, {{Subatomic particle|muon neutrino}}, {{Subatomic particle|tau neutrino}}), which are the only elementary fermions with neither electric nor [[color charge]]. The remaining six particles are [[quark]]s (discussed below).

==== Generations ====
{| class="wikitable"  style="text-align:center;"
|+ Particle generations
|-
!colspan="6"| [[Lepton]]s
|-
|colspan="2"| ''First generation''
|colspan="2"| ''Second generation''
|colspan="2"| ''Third generation''
|-
|''Name'' || ''Symbol'' || ''Name'' || ''Symbol'' || ''Name'' || ''Symbol''
|-
| [[electron]] || {{Subatomic particle|electron-}} || [[muon]] || {{math|{{Subatomic particle|muon-}}}} || [[tau (particle)|tau]] || {{math|{{Subatomic particle|tau-}}}}
|-
| [[electron neutrino]] || {{math|{{Subatomic particle|electron neutrino}}}} || [[muon neutrino]]|| {{math|{{Subatomic particle|Muon neutrino}}}} || [[tau neutrino]] || {{math|{{Subatomic particle|Tau neutrino}}}}
|-
!colspan="6"| [[Quark]]s
|-
|colspan="2"| ''First generation''
|colspan="2"| ''Second generation''
|colspan="2"| ''Third generation''
|-
| [[up quark]] || {{Subatomic particle|Up quark}} || [[charm quark]] || c || [[top quark]] || {{Subatomic particle|Top quark}}
|-
| [[down quark]] || {{Subatomic particle|Down quark}} || [[strange quark]] || {{Subatomic particle|Strange quark}} || [[bottom quark]]|| {{Subatomic particle|Bottom quark}}
|}

==== Mass ====
The following table lists current measured masses and mass estimates for all the fermions, using the same scale of measure: [[Electronvolt|millions of electron-volts]] relative to square of light speed (MeV/''c''<sup>2</sup>). For example, the most accurately known quark mass is of the top quark ({{Subatomic particle|top quark}}) at {{val|172.7|ul=GeV/c2}}, estimated using the [[on-shell scheme]].

{| class="wikitable" style="margin:0 0 1em 1em;"
|+ Current values for elementary fermion masses
|-
! Particle symbol
! Particle name
! Mass value
! Quark mass estimation scheme (point)
|-
| {{math|{{Subatomic particle|electron neutrino}}}}, {{math|{{Subatomic particle|muon neutrino}}}}, {{math|{{Subatomic particle|tauon neutrino}}}}
| [[Neutrino]]<br />(any&nbsp;type)
| style="text-align:right;" | < {{val|0.8|ul=eV/c2}}<ref>{{cite journal |last1=Navas |first1=S. |collaboration=Particle Data Group |title=Review of Particle Physics |journal=[[Physical Review D]] |volume=110 |issue=3 |date=2024-08-01 |article-number=030001 |df=dmy-all |doi=10.1103/PhysRevD.110.030001 |bibcode=2024PhRvD.110c0001N |hdl=11384/149923 |hdl-access=free }}</ref>
|
|-
| {{Subatomic particle|electron}}
| [[electron]]
| style="text-align:right;" | {{val|0.511|ul=MeV/c2}}
|
|-
| {{Subatomic particle|up quark}}
| [[up quark]]
| style="text-align:right;" | {{val|1.9|ul=MeV/c2}}
| [[MSbar scheme]] ({{mvar|μ}}<sub>{{overline|MS}}</sub> = {{val|2|u=GeV}})
|-
| {{Subatomic particle|down quark}}
| [[down quark]]
| style="text-align:right;" | {{val|4.4|ul=MeV/c2}}
| [[MSbar scheme]] ({{mvar|μ}}<sub>{{overline|MS}}</sub> = {{val|2|u=GeV}})
|-
| {{Subatomic particle|strange quark}}
| [[strange quark]]
| style="text-align:right;" | {{val|87|u=MeV/c2}}
| [[MSbar scheme]] ({{mvar|μ}}<sub>{{overline|MS}}</sub> = {{val|2|u=GeV}})
|-
| {{math|{{Subatomic particle|muon}}}}
| [[muon]]<br />([[mu lepton]])
| style="text-align:right;" | {{val|105.7|ul=MeV/c2}}
|
|-
| {{Subatomic particle|charm quark}}
| [[charm quark]]
| style="text-align:right;" | {{val|1320|ul=MeV/c2}}
| [[MSbar scheme]] ({{mvar|μ}}<sub>{{overline|MS}}</sub> = {{mvar|m}}<sub>c</sub>)
|-
| {{math|{{Subatomic particle|tau}}}}
| [[tauon]] ([[tau&nbsp;lepton]])
| style="text-align:right;" | {{val|1780|ul=MeV/c2}}
|
|-
| {{Subatomic particle|bottom quark}}
| [[bottom quark]]
| style="text-align:right;" | {{val|4240|ul=MeV/c2}}
| [[MSbar scheme]] ({{mvar|μ}}<sub>{{overline|MS}}</sub> = {{mvar|m}}<sub>b</sub>)
|-
| {{Subatomic particle|top quark}}
| [[top quark]]
| style="text-align:right;" | {{val|172700|ul=MeV/c2}}
| [[On-shell scheme]]
|}

Estimates of the values of quark masses depend on the version of [[quantum chromodynamics]] used to describe quark interactions. Quarks are always confined in an envelope of [[gluon]]s that confer vastly greater mass to the [[meson]]s and [[baryon]]s where quarks occur, so values for quark masses cannot be measured directly. Since their masses are so small compared to the effective mass of the surrounding gluons, slight differences in the calculation make large differences in the masses. {{cn|date=June 2025}}

==== Antiparticles ====
{{Main|Antimatter}}

There are also 12&nbsp;fundamental fermionic antiparticles that correspond to these 12&nbsp;particles. For example, the [[antielectron]] (positron) {{Subatomic particle|antielectron}} is the electron's antiparticle and has an electric charge of +1&nbsp;''e''.

{| class="wikitable"  style="text-align:center;"
|+ Particle generations
|-
!colspan="6"| [[Lepton|Antileptons]]
|-
|colspan="2"| ''First generation''
|colspan="2"| ''Second generation''
|colspan="2"| ''Third generation''
|-
|''Name'' || ''Symbol'' || ''Name'' || ''Symbol'' || ''Name'' || ''Symbol''
|-
| [[positron]] || {{Subatomic particle|antielectron}} || [[antimuon]] || {{math|{{Subatomic particle|antimuon}}}} || [[antitau]] || {{math|{{Subatomic particle|antitau}}}}
|-
| [[electron antineutrino]] || {{math|{{Subatomic particle|electron antineutrino}}}} || [[muon antineutrino]]|| {{math|{{Subatomic particle|Muon antineutrino}}}} || [[tau antineutrino]] || {{math|{{Subatomic particle|Tau antineutrino}}}}
|-
!colspan="6"| [[Quark|Antiquarks]]
|-
|colspan="2"| ''First generation''
|colspan="2"| ''Second generation''
|colspan="2"| ''Third generation''
|-
| [[up antiquark]] || {{Subatomic particle|Up antiquark}} || [[charm antiquark]] || {{Subatomic particle|Charm antiquark}} || [[top antiquark]] || {{Subatomic particle|Top antiquark}}
|-
| [[down antiquark]] || {{Subatomic particle|Down antiquark}} || [[strange antiquark]] || {{Subatomic particle|Strange antiquark}}  ||  [[bottom antiquark]]|| {{Subatomic particle|Bottom antiquark}}
|}

==== Quarks ====
{{Main|Quark}}
Isolated quarks and antiquarks have never been detected, a fact explained by [[Colour confinement|confinement]]. Every quark carries one of three [[color charge]]s of the [[strong interaction]]; antiquarks similarly carry anticolor. Color-charged particles interact via [[gluon]] exchange in the same way that charged particles interact via [[photon]] exchange. Gluons are themselves color-charged, however, resulting in an amplification of the strong force as color-charged particles are separated. Unlike the [[electromagnetism|electromagnetic force]], which diminishes as charged particles separate, color-charged particles feel increasing force.<ref>{{cite web|url=https://www.britannica.com/science/strong-force|title=Strong force|author=Christine Sutton|publisher=Britannica}}</ref>

Nonetheless, color-charged particles may combine to form color neutral [[composite particle]]s called [[hadron]]s. A quark may pair up with an antiquark: the quark has a color and the antiquark has the corresponding anticolor. The color and anticolor cancel out, forming a color neutral [[meson]]. Alternatively, three quarks can exist together, one quark being "red", another "blue", another "green". These three colored quarks together form a color-neutral [[baryon]]. Symmetrically, three antiquarks with the colors "antired", "antiblue" and "antigreen" can form a color-neutral [[antibaryon]].

Quarks also carry fractional [[electric charge]]s, but, since they are confined within hadrons whose charges are all integral, fractional charges have never been isolated. Note that quarks have electric charges of either {{small|{{sfrac|+|2|3}}}}&nbsp;''e'' or {{small|{{sfrac|−|1|3}}}}&nbsp;''e'', whereas antiquarks have corresponding electric charges of either {{small|{{sfrac|−|2|3}}}}&nbsp;''e'' or&nbsp;{{small|{{sfrac|+|1|3}}}}&nbsp;''e''.

Evidence for the existence of quarks comes from [[deep inelastic scattering]]: firing [[electron]]s at [[atomic nucleus|nuclei]] to determine the distribution of charge within [[nucleon]]s (which are baryons). If the charge is uniform, the [[electric field]] around the proton should be uniform and the electron should scatter elastically. Low-energy electrons do scatter in this way, but, above a particular energy, the protons deflect some electrons through large angles. The recoiling electron has much less energy and a [[jet (particle physics)|jet of particles]] is emitted. This inelastic scattering suggests that the charge in the proton is not uniform but split among smaller charged particles: quarks.

=== Fundamental bosons ===
{{Main|Boson}}

In the Standard Model, vector ([[Spin (physics)|spin]]-1) bosons ([[gluon]]s, [[photon]]s, and the [[W and Z bosons]]) mediate forces, whereas the [[Higgs boson]] (spin-0) is responsible for the intrinsic [[mass]] of particles. Bosons differ from fermions in the fact that multiple bosons can occupy the same quantum state ([[Pauli exclusion principle]]). Also, bosons can be either elementary, like photons, or a combination, like [[meson]]s. The spin of bosons are integers instead of half integers.

==== Gluons ====
{{Main|Gluon}}

Gluons mediate the [[strong interaction]], which join quarks and thereby form [[hadron]]s, which are either [[baryon]]s (three quarks) or [[meson]]s (one quark and one antiquark). Protons and neutrons are baryons, joined by gluons to form the [[atomic nucleus]]. Like quarks, gluons exhibit [[color charge|color]] and anticolor – unrelated to the concept of visual color and rather the particles' strong interactions – sometimes in combinations, altogether eight variations of gluons.

==== Electroweak bosons ====
{{Main|W and Z bosons|Photon}}

There are three [[weak gauge boson]]s: W<sup>+</sup>, W<sup>−</sup>, and Z<sup>0</sup>; these mediate the [[weak interaction]]. The W&nbsp;bosons are known for their mediation in nuclear decay: The W<sup>−</sup> converts a neutron into a proton then decays into an electron and electron-antineutrino pair.
The Z<sup>0</sup> does not convert particle flavor or charges, but rather changes momentum; it is the only mechanism for elastically scattering neutrinos. The weak gauge bosons were discovered due to momentum change in electrons from neutrino-Z exchange. The massless [[photon]] mediates the [[electromagnetism|electromagnetic interaction]]. These four gauge bosons form the electroweak interaction among elementary particles.

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

Although the weak and electromagnetic forces appear quite different to us at everyday energies, the two forces are theorized to unify as a single [[electroweak force]] at high energies. This prediction was clearly confirmed by measurements of cross-sections for high-energy electron-proton scattering at the [[Hadron Elektron Ring Anlage|HERA]] collider at [[DESY]]. The differences at low energies is a consequence of the high masses of the W and Z bosons, which in turn are a consequence of the [[Higgs mechanism]]. Through the process of [[spontaneous symmetry breaking]], the Higgs selects a special direction in electroweak space that causes three electroweak particles to become very heavy (the weak bosons) and one to remain with an undefined rest mass as it is always in motion (the photon). On 4 July 2012, after many years of experimentally searching for evidence of its existence, the [[Higgs boson]] was announced to have been observed at CERN's Large Hadron Collider. [[Peter Higgs]] who first posited the existence of the Higgs boson was present at the announcement.<ref>
{{cite news
 |first=Lizzy |last=Davies
 |date=4 July 2014
 |title=Higgs boson announcement live: CERN scientists discover subatomic particle
 |url=https://www.theguardian.com/science/blog/2012/jul/04/higgs-boson-discovered-live-coverage-cern
 |newspaper=[[The Guardian]]
 |access-date=2012-07-06 |df=dmy-all
}}</ref> The Higgs boson is believed to have a mass of approximately {{val|125|u=GeV/c2}}.<ref>
{{cite web
 |first=Lucas |last=Taylor
 |date=4 Jul 2014
 |title=Observation of a new particle with a mass of 125 GeV
 |url=http://cms.web.cern.ch/news/observation-new-particle-mass-125-gev
 |publisher=[[Compact Muon Solenoid|CMS]]
 |access-date=2012-07-06 |df=dmy-all
}}</ref> The [[statistical significance]] of this discovery was reported as 5&nbsp;sigma, which implies a certainty of roughly 99.99994%. In particle physics, this is the level of significance required to officially label experimental observations as a [[Discovery (observation)|discovery]]. Research into the properties of the newly discovered particle continues.

==== Graviton ====

{{Main|Graviton}}

The [[graviton]] is a hypothetical elementary spin-2 particle proposed to mediate gravitation. While it remains undiscovered due to [[Graviton#Experimental observation|the difficulty inherent in its detection]], it is sometimes included in tables of elementary particles.<ref name=PFI /> The conventional graviton is massless, although some models containing massive [[Kaluza–Klein theory|Kaluza–Klein]] gravitons exist.<ref>{{cite journal |arxiv=0910.1535 |bibcode=2010PhLB..682..446C |title=Massless versus Kaluza-Klein gravitons at the LHC |journal=Physics Letters B |volume=682 |issue=4–5 |pages=446–449 |last1=Calmet |first1=Xavier |last2=de Aquino |first2=Priscila |last3=Rizzo |first3=Thomas G. |year=2010 |doi=10.1016/j.physletb.2009.11.045 |hdl=2078/31706|s2cid=16310404 }}</ref>

== Beyond the Standard Model ==
Although experimental evidence overwhelmingly confirms the predictions derived from the [[Standard Model]], some of its parameters were added arbitrarily, not determined by a particular explanation, which remain mysterious, for instance the [[hierarchy problem]]. Theories [[beyond the Standard Model]] attempt to resolve these shortcomings.

=== Grand unification ===
{{Main|Grand Unified Theory}}

One extension of the Standard Model attempts to combine the [[electroweak interaction]] with the [[strong interaction]] into a single 'grand unified theory' (GUT). Such a force would be [[spontaneous symmetry breaking|spontaneously broken]] into the three forces by a [[Higgs mechanism|Higgs-like mechanism]]. This breakdown is theorized to occur at high energies, making it difficult to observe unification in a laboratory. The most dramatic prediction of grand unification is the existence of [[X and Y bosons]], which cause [[proton decay]]. The non-observation of proton decay at the [[Super-Kamiokande]] neutrino observatory rules out the simplest GUTs, however, including SU(5) and SO(10).

=== Supersymmetry ===
{{Main|Supersymmetry}}

Supersymmetry extends the Standard Model by adding another class of symmetries to the [[Lagrangian (field theory)|Lagrangian]]. These symmetries exchange [[fermion]]ic particles with [[boson]]ic ones. Such a symmetry predicts the existence of [[supersymmetric particle]]s, abbreviated as ''[[sparticle]]s'', which include the [[slepton]]s, [[squark]]s, [[neutralino]]s, and [[chargino]]s. Each particle in the Standard Model would have a superpartner whose [[Spin (physics)|spin]] differs by {{1/2}} from the ordinary particle. Due to the [[supersymmetry breaking|breaking of supersymmetry]], the sparticles are much heavier than their ordinary counterparts; they are so heavy that existing [[particle collider]]s would not be powerful enough to produce them. Some physicists believe that sparticles will be detected by the [[Large Hadron Collider]] at [[CERN]].

=== String theory ===
{{Main|String theory}}

String theory is a model of physics whereby all "particles" that make up [[matter]] are composed of strings (measuring at the Planck length) that exist in an 11-dimensional (according to [[M-theory]], the leading version) or 12-dimensional (according to [[F-theory]]<ref>{{cite journal |doi=10.1016/0550-3213(96)00172-1 |arxiv=hep-th/9602022 |bibcode=1996NuPhB.469..403V |title=Evidence for F-theory |year=1996 |last1=Vafa |first1=Cumrun |journal=Nuclear Physics B |volume=469 |issue=3 |pages=403–415|s2cid=6511691 }}</ref>) universe. These strings vibrate at different frequencies that determine mass, electric charge, color charge, and spin. A "string" can be open (a line) or closed in a loop (a one-dimensional sphere, that is, a circle). As a string moves through space it sweeps out something called a ''[[world line#World lines as a tool to describe events|world sheet]]''. String theory predicts 1- to 10-branes (a 1-[[Membrane (M-theory)|brane]] being a string and a 10-brane being a 10-dimensional object) that prevent tears in the "fabric" of space using the [[uncertainty principle]] (e.g., the electron orbiting a hydrogen atom has the probability, albeit small, that it could be anywhere else in the universe at any given moment).

String theory proposes that our universe is merely a 4-brane, inside which exist the three space dimensions and the one time dimension that we observe. The remaining 7&nbsp;theoretical dimensions either are very tiny and curled up (and too small to be macroscopically accessible) or simply do not/cannot exist in our universe (because they exist in a grander scheme called the "[[multiverse]]" outside our known universe).

Some predictions of the string theory include existence of extremely massive counterparts of ordinary particles due to vibrational excitations of the fundamental string and existence of a massless spin-2 particle behaving like the [[graviton]].

=== Technicolor ===
{{Main|Technicolor (physics)}}

Technicolor theories try to modify the Standard Model in a minimal way by introducing a new QCD-like interaction. This means one adds a new theory of so-called Techniquarks, interacting via so called Technigluons. The main idea is that the Higgs boson is not an elementary particle but a bound state of these objects.

=== Preon theory ===
{{Main|Preon}}

According to preon theory there are one or more orders of particles more fundamental than those (or most of those) found in the Standard Model. The most fundamental of these are normally called preons, which is derived from "pre-quarks". In essence, preon theory tries to do for the Standard Model what the Standard Model did for the [[particle zoo]] that came before it. Most models assume that almost everything in the Standard Model can be explained in terms of three to six more fundamental particles and the rules that govern their interactions. Interest in preons has waned since the simplest models were experimentally ruled out in the 1980s.

=== Acceleron theory ===
[[Acceleron]]s are the hypothetical [[subatomic particle]]s that integrally link the newfound mass of the [[neutrino]] to the [[dark energy]] conjectured to be accelerating the [[metric expansion of space|expansion of the universe]].<ref name=acceleron />

In this theory, neutrinos are influenced by a new force resulting from their interactions with accelerons, leading to dark energy. Dark energy results as the universe tries to pull neutrinos apart.<ref name=acceleron>
{{cite web
 |date=28 Jul 2004
 |url=https://www.sciencedaily.com/releases/2004/07/040728090338.htm
 |title=New theory links neutrino's slight mass to accelerating Universe expansion
 |website=[[ScienceDaily]]
 |access-date=2008-06-05 |df=dmy-all
}}</ref> Accelerons are thought to interact with matter more infrequently than they do with neutrinos.<ref>
{{cite news
 |url=https://astronomy.com/news/2004/07/acceleron-anyone
 |title=Acceleron, anyone?
 |first=Francis |last=Reddy
 |date=2004-07-27
 |magazine=Astronomy
 |access-date=2020-04-20 |df=dmy-all
}}</ref>

== See also ==
{{Portal|Physics}}
{{col div|colwidth=30em}}
* [[Asymptotic freedom]]
* [[List of particles]]
* [[Physical ontology]]
* [[Quantum field theory]]
* [[Quantum gravity]]
* [[Quantum triviality]]
* [[UV fixed point]]
* [[Periodic Table]]
{{colend}}

== Notes ==
{{reflist|25em}}

== Further reading ==

=== General readers ===
* {{cite book |last1=Feynman |first1=Richard Phillips |author-link1=Richard Feynman |url=https://archive.org/details/B-001-000-178/ |title=Elementary particles and the laws of physics: the 1986 Dirac memorial lectures |last2=Weinberg |first2=Steven |author-link2=Steven Weinberg |last3=MacKenzie |first3=Richard |last4=Doust |first4=Paul |date=1991 |publisher=Cambridge university press |isbn=978-0-521-34000-7 |location=Cambridge New York Port Chester [etc.]}}
* {{cite book |last=Ford |first=Kenneth W. |author-link=Kenneth W. Ford |title=The quantum world: Quantum physics for erveryone |date=2004 |publisher=Harvard Univ. Pr |isbn=978-0-674-01342-1 |location=London}}
* {{cite book |last=Greene |first=Brian |author-link=Brian Greene |title=The elegant universe: superstrings, hidden dimensions, and the quest for the ultimate theory |title-link=The Elegant Universe |publisher=Norton |year=1999 |isbn=978-0-393-05858-1 |location=New York London}}
* {{cite book |last1=Gribbin |first1=John |author-link1=John Gribbin |title=Q is for quantum: an encyclopedia of particle physics |last2=Gribbin |first2=Mary |last3=Gribbin |first3=Jonathan |date=1998 |publisher=Free Press |isbn=978-0-684-85578-3 |location=New York, NY}}
* {{cite book |last=Oerter |first=Robert |title=The theory of almost everything: the Standard Model, the unsung triumph of modern physics |date=2006 |publisher=Pi Press |isbn=978-0-452-28786-0 |location=New York, NY}}
* {{cite book |last=Schumm |first=Bruce A. |title=Deep down things: the breathtaking beauty of particle physics |date=2004 |publisher=Johns Hopkins University Press |isbn=978-0-8018-7971-5 |location=Baltimore}}
* {{cite book |first=Martinus |last=Veltman |title=Facts and Mysteries in Elementary Particle Physics |url=https://archive.org/details/factsmysteriesin0000velt |url-access=registration |publisher=[[World Scientific]] |year=2003 |isbn=978-981-238-149-1 |author-link=Martinus Veltman}}
* {{cite book |first=Frank |last=Close |title=Particle Physics: A very short introduction |publisher=[[Oxford University Press]] |location=Oxford |year=2004 |isbn=978-0-19-280434-1 |author-link=Frank Close}}
* {{cite book |last=Seiden |first=Abraham |title=Particle Physics: A comprehensive introduction |publisher=[[Addison Wesley]] |year=2005 |isbn=978-0-8053-8736-0}}

=== Textbooks ===
* {{cite book |last=Bettini |first=Alessandro |title=Introduction to elementary particle physics |date=2008 |publisher=Cambridge Univ. Press |isbn=978-0-521-88021-3 |location=Cambridge}}
* {{cite book |last1=Coughlan |first1=Guy D. |title=The ideas of particle physics: an introduction for scientists |last2=Dodd |first2=James Edmund |date=1994 |publisher=Cambridge Univ. Press |isbn=978-0-521-38677-7 |edition=2., reprint |location=Cambridge}} An undergraduate text for those not majoring in physics.
* {{cite book |last=Griffiths |first=David Jeffrey |title=Introduction to elementary particles |date=1987 |publisher=J. Wiley and sons |isbn=978-0-471-60386-3 |location=New York Chichester Brisbane [etc.]}}
* {{cite book |last=Kane |first=Gordon L. |title=Modern elementary particle physics |publisher=[[Addison-Wesley]] |year=1987 |isbn=978-0-201-11749-3 |edition=2. print |location=Redwood City, Calif.}}
* {{cite book |last=Perkins |first=Donald H. |title=Introduction to high energy physics |date=2000 |publisher=Cambridge University Press |isbn=978-0-521-62196-0 |edition=4th |location=Cambridge; New York}}

== External links ==
The most important address about the current experimental and theoretical knowledge about elementary particle physics is the [[Particle Data Group]], where different international institutions collect all experimental data and give short reviews over the contemporary theoretical understanding.
* {{cite web |url=http://pdg.lbl.gov/ |title=Particle Data Group (home page)}}

other pages are:
* [https://web.archive.org/web/20190719141632/http://particleadventure.org/ particleadventure.org], a well-made introduction also for non physicists
* [http://www.cerncourier.com/main/article/41/2/17 CERNCourier: Season of Higgs and melodrama] {{Webarchive|url=https://web.archive.org/web/20080723144736/http://www.cerncourier.com/main/article/41/2/17 |date=2008-07-23 }}
* [http://www.interactions.org/ Interactions.org], particle physics news
* [http://www.symmetrymagazine.org/ Symmetry Magazine], a joint [[Fermilab]]/[[SLAC]] publication
* [http://www.thingsmadethinkable.com/item/elementary_particles.php Elementary Particles made thinkable], an interactive visualisation allowing physical properties to be compared

{{particles}}

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[[Category:Elementary particles| ]]
[[Category:Quantum mechanics]]
[[Category:Quantum field theory]]
[[Category:Subatomic particles]]