View source for Particle physics

{{Short description|Study of subatomic particles and forces}}
{{Use Oxford spelling|date=September 2016}}
{{Use dmy dates|date=July 2020}}
{{Standard model of particle physics}}

'''Particle physics''' or '''high-energy physics''' is the study of [[Elementary particle|fundamental particles]] and [[fundamental interaction|forces]] that constitute [[matter]] and [[radiation]]. The field also studies combinations of elementary particles up to the scale of [[protons]] and [[neutrons]], while the study of combinations of protons and neutrons is called [[nuclear physics]].

The fundamental particles in the [[universe]] are classified in the [[Standard Model]] as [[fermion]]s (matter particles) and [[boson]]s (force-carrying particles). There are three [[Generation (particle physics)|generations]] of fermions, although ordinary matter is made only from the first fermion generation. The first generation consists of [[Up quark|up]] and [[down quark]]s which form [[proton]]s and [[neutron]]s, and [[electron]]s and [[electron neutrino]]s. The three fundamental interactions known to be mediated by bosons are [[electromagnetism]], the [[weak interaction]], and the [[strong interaction]].

[[Quark|Quarks]] form [[hadron]]s, but cannot exist on their own. Hadrons that contain an odd number of quarks are called [[baryon]]s and those that contain an even number are called [[meson]]s. Two baryons, the [[proton]] and the [[neutron]], make up most of the mass of ordinary matter. Mesons are unstable and the longest-lived last for only a few hundredths of a [[microsecond]]. They occur after collisions between particles made of quarks, such as fast-moving protons and neutrons in [[cosmic ray]]s. Mesons are also produced in [[cyclotron]]s or other [[particle accelerator]]s.

Particles have corresponding [[antiparticle]]s with the same [[mass]] but with opposite [[electric charge]]s. For example, the antiparticle of the [[electron]] is the [[positron]]. The electron has a negative electric charge, the positron has a positive charge. These antiparticles can theoretically form a corresponding form of matter called [[antimatter]]. Some particles, such as the [[photon]], are their own antiparticle.

These [[elementary particle]]s are excitations of the [[Field (physics)#Quantum fields|quantum fields]] that also govern their interactions. The dominant theory explaining these fundamental particles and fields, along with their dynamics, is called the [[Standard Model]]. The [[Quantum gravity|reconciliation of gravity]] to the current particle physics theory is not solved; many theories have addressed this problem, such as [[loop quantum gravity]], [[string theory]] and [[Supersymmetry|supersymmetry theory]].

Experimental particle physics is the study of these particles in [[Radioactive decay|radioactive]] processes and in particle accelerators such as the [[Large Hadron Collider]]. Theoretical particle physics is the study of these particles in the context of [[cosmology]] and [[quantum mechanics|quantum theory]]. The two are closely interrelated: the [[Higgs boson]] was postulated theoretically before being confirmed by experiments.

== History ==
{{main|History of subatomic physics}}
[[File:Rutherford_Scattering.svg|alt=see caption|thumb|The [[Geiger–Marsden experiments]] observed that a small fraction of the alpha particles experienced strong deflection when being struck by the gold foil.]]
The idea that all [[matter]] is fundamentally composed of [[elementary particle]]s dates from at least the 6th century BC.<ref>{{cite web |title=Fundamentals of Physics and Nuclear Physics |url=http://novelresearchinstitute.org/library/PhysNuclphys196p.pdf |url-status=usurped |archive-url=https://web.archive.org/web/20121002214053/http://novelresearchinstitute.org/library/PhysNuclphys196p.pdf |archive-date=2 October 2012 |access-date=21 July 2012}}</ref> In the 19th century, [[John Dalton]], through his work on [[stoichiometry]], concluded that each element of nature was composed of a single, unique type of particle.<ref name="MARK I. GROSSMAN">{{cite journal |title=John Dalton and the London Atomists |year=2014 |pmc=4213434 |last1=Grossman |first1=M. I. |journal=Notes and Records of the Royal Society of London |volume=68 |issue=4 |pages=339–356 |doi=10.1098/rsnr.2014.0025 }}</ref> The word ''[[atom]]'', after the Greek word ''[[wikt:ἄτομος|atomos]]'' meaning "indivisible", has since then denoted the smallest particle of a [[chemical element]], but physicists later discovered that atoms are not, in fact, the fundamental particles of nature, but are conglomerates of even smaller particles, such as the [[electron]]. The early 20th century explorations of [[nuclear physics]] and [[quantum physics]] led to proofs of [[nuclear fission]] in 1939 by [[Lise Meitner]] (based on experiments by [[Otto Hahn]]), and [[nuclear fusion]] by [[Hans Bethe]] in that same year; both discoveries also led to the development of [[nuclear weapon]]s. Bethe's 1947 calculation of the [[Lamb shift]] is credited with having "opened the way to the modern era of particle physics".<ref>{{Cite book |last1=Brown |first1=Gerald Edward |url=https://archive.org/details/hansbethehisphys0000unse/page/161 |title=Hans Bethe and His Physics |last2=Lee |first2=Chang-Hwan |date=2006 |publisher=World Scientific Publishing |isbn=978-981-256-609-6 |location=Singapore |page=161}}</ref>

Throughout the 1950s and 1960s, a bewildering variety of particles was found in collisions of particles from beams of increasingly high energy. It was referred to informally as the "[[particle zoo]]". Important discoveries such as the [[CP violation]] by [[James Cronin]] and [[Val Fitch]] brought new questions to [[Baryon asymmetry|matter-antimatter imbalance]].<ref>{{Cite web |date=2021-03-01 |title=Antimatter |url=https://home.cern/science/physics/antimatter |url-status=live |archive-url=https://web.archive.org/web/20180911042958/https://home.cern/topics/antimatter |archive-date=11 September 2018 |access-date=12 March 2021}}</ref> After the formulation of the Standard Model during the 1970s, physicists clarified the origin of the particle zoo. The large number of particles was explained as combinations of a (relatively) small number of more fundamental particles and framed in the context of [[Quantum field theory|quantum field theories]]. This reclassification marked the beginning of modern particle physics.<ref>{{cite book |last1=Weinberg |first1=Steven |title=The quantum theory of fields |date=1995–2000 |publisher=Cambridge University Press |isbn=978-0-521-67053-1 |location=Cambridge}}</ref><ref>{{Cite journal |last=Jaeger |first=Gregg |date=2021 |title=The Elementary Particles of Quantum Fields |journal=Entropy |volume=23 |issue=11 |page=1416 |bibcode=2021Entrp..23.1416J |doi=10.3390/e23111416 |pmc=8623095 |pmid=34828114 |doi-access=free}}</ref>

== Standard Model ==
{{Main|Standard Model}}

The current state of the classification of all elementary particles is explained by the [[Standard Model]], which gained widespread acceptance in the mid-1970s after [[experimental confirmation]] of the existence of [[quark]]s. It describes the [[strong interaction|strong]], [[weak interaction|weak]], and [[electromagnetism|electromagnetic]] [[fundamental interaction]]s, using mediating [[gauge boson]]s. The species of gauge bosons are eight [[gluon]]s, [[W and Z bosons|{{SubatomicParticle|W boson-}}, {{SubatomicParticle|W boson+}} and {{SubatomicParticle|Z boson}} bosons]], and the [[photon]].<ref name="Baker p 120">{{cite book |last=Baker |first=Joanne |title=50 quantum physics ideas you really need to know |date=2013 |isbn=978-1-78087-911-6 |publication-place=London |pages=120–123 |oclc=857653602}}</ref> The Standard Model also contains 24 [[fundamental particle|fundamental]] [[fermion]]s (12 particles and their associated anti-particles), which are the constituents of all [[matter]].<ref name="pdg">{{cite journal |last=Nakamura |first=K. |date=1 July 2010 |title=Review of Particle Physics |journal=Journal of Physics G: Nuclear and Particle Physics |volume=37 |issue=7A |pages=1–708 |bibcode=2010JPhG...37g5021N |doi=10.1088/0954-3899/37/7A/075021 |pmid=10020536 |doi-access=free |hdl-access=free |hdl=10481/34593}}</ref> Finally, the Standard Model also predicted the existence of a type of [[boson]] known as the [[Higgs boson]]. On 4 July 2012, physicists with the Large Hadron Collider at CERN announced they had found a new particle that behaves similarly to what is expected from the Higgs boson.<ref>{{cite journal |last=Mann |first=Adam |date=28 March 2013 |title=Newly Discovered Particle Appears to Be Long-Awaited Higgs Boson |url=https://www.wired.com/wiredscience/2012/07/higgs-boson-discovery/ |url-status=live |journal=Wired Science |archive-url=https://web.archive.org/web/20140211212906/http://www.wired.com/wiredscience/2012/07/higgs-boson-discovery/ |archive-date=11 February 2014 |access-date=6 February 2014}}</ref>

The Standard Model, as currently formulated, has 61 elementary particles.<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> Those elementary particles can combine to form composite particles, accounting for the hundreds of other species of particles that have been discovered since the 1960s. The Standard Model has been found to agree with almost all the [[experiment]]al tests conducted to date. However, most particle physicists believe that it is an incomplete description of nature and that a more fundamental theory awaits discovery (See [[Theory of Everything]]). In recent years, measurements of [[neutrino]] [[rest mass|mass]] have provided the first experimental deviations from the Standard Model, since neutrinos do not have mass in the Standard Model.<ref>{{cite web |title=Neutrinos in the Standard Model |url=https://t2k-experiment.org/neutrinos/in-the-standard-model |url-status=live |archive-url=https://web.archive.org/web/20191016010901/https://t2k-experiment.org/neutrinos/in-the-standard-model/ |archive-date=16 October 2019 |access-date=15 October 2019 |publisher=The T2K Collaboration}}</ref>

== Subatomic particles ==
{| class="wikitable" style="float:right; margin-left:1em; background:#FFF;"
|+ Elementary Particles
|-
!
! scope="col" | [[Generation (particle physics)|Types]]
! scope="col" | [[Generation (particle physics)|Generations]]
! scope="col" | [[Antiparticle]]
! scope="col" | [[Color charge|Colours]]
! scope="col" | Total
|-
! scope="row" style="background:#DAF;" | [[Quark]]s
| rowspan="2" align="center"| 2
| rowspan="2" align="center"| 3
| align="center"|Pair
| align="center"|3
| align="center"|36
|-
! scope="row" style="background:#AF7;" | [[Lepton]]s
| align="center"|Pair
| align="center"|None
| align="center"|12
|-
! scope="row" style="background:#F97;" | [[Gluon]]s
|rowspan="5" align="center"|1
|rowspan="5" align="center"|None
| align="center"|Own
| align="center"|[[Gluon#Eight color states|8]]
| align="center"|8
|-
! scope="row" style="background:#F97;" | [[Photon]]
| align="center"|Own
| align="center" rowspan="4"|None
| align="center"|1
|-
! scope="row" style="background:#F97;" | [[W and Z bosons|Z Boson]]
| align="center"|Own
| align="center"|1
|-
! scope="row" style="background:#F97;" | [[W and Z bosons|W Boson]]
| align="center"|Pair
| align="center"|2
|-
! scope="row" style="background:#FE7;" | [[Higgs boson|Higgs]]
| align="center"|Own
| align="center"|1
|-
! colspan="5" align="right" ! scope="col" | Total number of (known) elementary particles:
| align="center"|'''61'''
|}
Modern particle physics research is focused on [[subatomic particle]]s, including atomic constituents, such as [[electron]]s, [[proton]]s, and [[neutron]]s (protons and neutrons are composite particles called [[baryon]]s, made of [[quark]]s), that are produced by [[Radioactive decay|radioactive]] and [[scattering]] processes; such particles are [[photon]]s, [[neutrino]]s, and [[muon]]s, as well as a wide range of [[exotic particle]]s.<ref>{{cite book |last1=Terranova |first1=Francesco |title=A Modern Primer in Particle and Nuclear Physics. |date=2021 |publisher=Oxford Univ. Press |isbn=978-0-19-284524-5}}</ref> All particles and their interactions observed to date can be described almost entirely by the Standard Model.<ref name="Baker p 120" />

Dynamics of particles are also governed by [[quantum mechanics]]; they exhibit [[wave–particle duality]], displaying particle-like behaviour under certain experimental conditions and [[wave]]-like behaviour in others. In more technical terms, they are described by [[quantum state]] vectors in a [[Hilbert space]], which is also treated in [[quantum field theory]]. Following the convention of particle physicists, the term ''[[elementary particle]]s'' is applied to those particles that are, according to current understanding, presumed to be indivisible and not composed of other particles.<ref name="braibant"/>

=== Quarks and leptons ===
{{Main|Quark|Lepton}}
[[File:Beta_Negative_Decay.svg|thumb|A [[Feynman diagram]] of the [[Beta decay|{{SubatomicParticle|Beta-}}&nbsp;decay]], showing a neutron (n, udd) converted into a proton (p, udu). "u" and "d" are the [[Up quark|up]] and [[down quark]]s, "{{Subatomic particle|electron}}" is the [[electron]], and "{{Subatomic particle|Electron antineutrino}}" is the [[Electron Antineutrino|electron antineutrino]].]]
Ordinary [[matter]] is made from first-[[Generation (particle physics)|generation]] quarks ([[Up quark|up]], [[Down quark|down]]) and leptons ([[electron]], [[electron neutrino]]).<ref name="Povh02">{{cite book |author=Povh |first1=B. |title=Particles and Nuclei: An Introduction to the Physical Concepts |last2=Rith |first2=K. |last3=Scholz |first3=C. |last4=Zetsche |first4=F. |last5=Lavelle |first5=M. |date=2004 |publisher=Springer |isbn=978-3-540-20168-7 |edition=4th |chapter=Part I: Analysis: The building blocks of matter |quote=Ordinary matter is composed entirely of first-generation particles, namely the u and d quarks, plus the electron and its neutrino. |access-date=28 July 2022 |chapter-url=https://books.google.com/books?id=rJe4k8tkq7sC&q=povh+%22building+blocks+of+matter%22&pg=PA9 |archive-url=https://web.archive.org/web/20220422024501/https://books.google.com/books?id=rJe4k8tkq7sC&q=povh+%22building+blocks+of+matter%22&pg=PA9 |archive-date=22 April 2022 |url-status=live}}</ref> Collectively, quarks and leptons are called [[fermion]]s, because they have a [[quantum spin]] of [[half-integer]]s (−1/2, 1/2, 3/2, etc.). This causes the fermions to obey the [[Pauli exclusion principle]], where no two particles may occupy the same [[quantum state]].<ref>{{cite book |author=Peacock |first=K. A. |url=https://archive.org/details/quantumrevolutio00peac |title=The Quantum Revolution |publisher=[[Greenwood Publishing Group]] |year=2008 |isbn=978-0-313-33448-1 |page=[https://archive.org/details/quantumrevolutio00peac/page/n143 125] |url-access=limited}}</ref> Quarks have fractional [[Elementary charge|elementary electric charge]] (−1/3 or 2/3)<ref>{{cite book |author=Quigg |first=C. |title=The New Physics for the Twenty-First Century |publisher=[[Cambridge University Press]] |year=2006 |isbn=978-0-521-81600-7 |editor=G. Fraser |page=91 |chapter=Particles and the Standard Model}}</ref> and leptons have whole-numbered electric charge (0 or -1).<ref>{{Cite book |last1=Serway |first1=Raymond A. |url=https://books.google.com/books?id=ecYWAAAAQBAJ |title=Physics for Scientists and Engineers, Volume 2 |last2=Jewett |first2=John W. |date=2013-01-01 |publisher=Cengage Learning |isbn=978-1-285-62958-2 |language=en}}</ref> Quarks also have [[color charge]], which is labeled arbitrarily with no correlation to actual light [[color]] as red, green and blue.<ref name="R. Nave">{{cite web |author=Nave |first=R. |title=The Color Force |url=http://hyperphysics.phy-astr.gsu.edu/hbase/forces/color.html#c2 |url-status=live |archive-url=https://web.archive.org/web/20181007142048/http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/color.html#c2 |archive-date=7 October 2018 |access-date=2009-04-26 |work=[[HyperPhysics]] |publisher=[[Georgia State University]], Department of Physics and Astronomy}}</ref> Because the interactions between the quarks store energy which can convert to other particles when the quarks are far apart enough, quarks cannot be observed independently. This is called [[color confinement]].<ref name="R. Nave"/>

There are three known generations of quarks (up and down, [[Strange quark|strange]] and [[Charm quark|charm]], [[Top quark|top]] and [[Bottom quark|bottom]]) and leptons (electron and its neutrino, [[muon]] and [[Muon neutrino|its neutrino]], [[Tau (particle)|tau]] and [[Tau neutrino|its neutrino]]), with strong indirect evidence that a fourth generation of fermions does not exist.<ref>{{cite journal |author=Decamp |first=D. |year=1989 |title=Determination of the number of light neutrino species |url=https://cds.cern.ch/record/201511 |journal=[[Physics Letters B]] |volume=231 |issue=4 |pages=519–529 |bibcode=1989PhLB..231..519D |doi=10.1016/0370-2693(89)90704-1 |hdl-access=free |hdl=11384/1735}}</ref>

=== Bosons ===
{{Main|Boson}}

Bosons are the [[Force carrier|mediators or carriers]] of fundamental interactions, such as [[electromagnetism]], the [[weak interaction]], and the [[strong interaction]].<ref name="DarkMatter">{{cite book |author=Carroll, Sean |author-link = Sean M. Carroll | title=Guidebook |publisher=The Teaching Company |year=2007 |isbn=978-1-59803-350-2 |series=Dark Matter, Dark Energy: The dark side of the universe |at=Part&nbsp;2, p.&nbsp;43 |quote=...&nbsp;boson: A force-carrying particle, as opposed to a matter particle (fermion). Bosons can be piled on top of each other without limit. Examples are photons, gluons, gravitons, weak bosons, and the Higgs boson. The spin of a boson is always an integer: 0, 1, 2, and so on&nbsp;...}}</ref> Electromagnetism is mediated by the [[photon]], the [[Quantum|quanta]] of [[light]].<ref>"Role as gauge boson and polarization" §5.1 in {{cite book |last1=Aitchison |first1=I. J. R. |url={{google books |plainurl=y |id=ZJ-ZY8NW9TIC}} |title=Gauge Theories in Particle Physics |last2=Hey |first2=A. J. G. |publisher=[[IOP Publishing]] |year=1993 |isbn=978-0-85274-328-7}}</ref>{{rp|29–30}} The weak interaction is mediated by the [[W and Z bosons]].<ref>{{cite book |first=Peter |last=Watkins |url=https://books.google.com/books?id=J808AAAAIAAJ&pg=PA70 |title=Story of the W and Z |publisher=[[Cambridge University Press]] |year=1986 |isbn=978-0-521-31875-4 |location=Cambridge |page=70 |access-date=28 July 2022 |archive-url=https://web.archive.org/web/20121114055111/http://books.google.co.uk/books?id=J808AAAAIAAJ&pg=PA70 |archive-date=14 November 2012 |url-status=live}}</ref> The strong interaction is mediated by the [[gluon]], which can link quarks together to form composite particles.<ref name="HyperPhysics">{{cite web |author=Nave |first=C. R. |title=The Color Force |url=http://hyperphysics.phy-astr.gsu.edu/hbase/forces/color.html |url-status=live |archive-url=https://web.archive.org/web/20181007142048/http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/color.html |archive-date=7 October 2018 |access-date=2012-04-02 |work=[[HyperPhysics]] |publisher=[[Georgia State University]], Department of Physics}}</ref> Due to the aforementioned color confinement, gluons are never observed independently.<ref name=":0">{{cite journal |last=Debrescu |first=B. A. |year=2005 |title=Massless Gauge Bosons Other Than The Photon |journal=[[Physical Review Letters]] |volume=94 |issue=15 |article-number=151802 |arxiv=hep-ph/0411004 |bibcode=2005PhRvL..94o1802D |doi=10.1103/PhysRevLett.94.151802 |pmid=15904133 |s2cid=7123874}}</ref> The [[Higgs boson]] gives mass to the W and Z bosons via the [[Higgs mechanism]]<ref name="PDG">{{cite web |author1=Bernardi, G. |author2=Carena, M. |author3=Junk, T. |year=2007 |title=Higgs bosons: Theory and searches |url=http://pdg.lbl.gov/2008/reviews/higgs_s055.pdf |url-status=live |archive-url=https://web.archive.org/web/20181003190309/http://pdg.lbl.gov/2008/reviews/higgs_s055.pdf |archive-date=3 October 2018 |access-date=28 July 2022 |series=Review: Hypothetical particles and Concepts |publisher=Particle Data Group}}</ref> – the gluon and photon are expected to be [[Massless particle|massless]].<ref name=":0" /> All bosons have an integer quantum spin (0 and 1) and can have the same [[quantum state]].<ref name="DarkMatter" />

=== Antiparticles and color charge ===
{{Main|Antiparticle|Color charge}}

Most aforementioned particles have corresponding [[antiparticle]]s, which compose [[antimatter]]. Normal particles have positive [[Lepton number|lepton]] or [[baryon number]], and antiparticles have these numbers negative.<ref>{{cite journal |last=Tsan |first=Ung Chan |date=2013 |title=Mass, Matter, Materialization, Mattergenesis and Conservation of Charge |journal=International Journal of Modern Physics E |volume=22 |issue=5 |page=1350027 |bibcode=2013IJMPE..2250027T |doi=10.1142/S0218301313500274 |quote=Matter conservation means conservation of baryonic number ''A'' and leptonic number ''L'', ''A'' and ''L'' being algebraic numbers. Positive ''A'' and ''L'' are associated to matter particles, negative ''A'' and ''L'' are associated to antimatter particles. All known interactions do conserve matter.}}</ref> Most properties of corresponding antiparticles and particles are the same, with a few gets reversed; the electron's antiparticle, positron, has an opposite charge. To differentiate between antiparticles and particles, a plus or negative sign is added in [[superscript]]. For example, the electron and the positron are denoted {{Subatomic particle|Electron}} and {{Subatomic particle|positron}}.<ref name="raith">{{cite book |last1=Raith |first1=W. |title=Constituents of Matter: Atoms, Molecules, Nuclei and Particles |last2=Mulvey |first2=T. |publisher=[[CRC Press]] |year=2001 |isbn=978-0-8493-1202-1 |pages=777–781}}</ref> However, in the case that the particle has a charge of 0 (equal to that of the antiparticle), the antiparticle is denoted with a line above the symbol. As such, an electron neutrino is {{Math|{{Subatomic Particle|Electron Neutrino}}}}, whereas its antineutrino is {{Math|{{Subatomic Particle|Electron Antineutrino}}}}. When a particle and an antiparticle interact with each other, they are [[Annihilation|annihilated]] and convert to other particles.<ref>{{cite web |title=Antimatter |url=http://www.lbl.gov/abc/Antimatter.html |url-status=live |archive-url=https://web.archive.org/web/20080823180515/http://www.lbl.gov/abc/Antimatter.html |archive-date=23 August 2008 |access-date=3 September 2008 |publisher=[[Lawrence Berkeley National Laboratory]]}}</ref> Some particles, such as the photon or gluon, have no antiparticles.{{Citation needed|date=July 2022}}

Quarks and gluons additionally have color charges, which influences the strong interaction. Quark's color charges are called red, green and blue (though the particle itself have no physical color), and in antiquarks are called antired, antigreen and antiblue.<ref name="R. Nave"/> The gluon can have [[Gluon|eight color charges]], which are the result of quarks' interactions to form composite particles (gauge symmetry [[SU(3)]]).<ref name="PeskinSchroeder">Part III of {{cite book |last1=Peskin |first1=M. E. |url=https://archive.org/details/introductiontoqu0000pesk |title=An Introduction to Quantum Field Theory |last2=Schroeder |first2=D. V. |publisher=[[Addison–Wesley]] |year=1995 |isbn=978-0-201-50397-5 |url-access=registration}}</ref>

=== Composite ===
{{Main|Composite particle}}
[[File:Quark_structure_proton.svg|thumb|A [[proton]] consists of two up quarks and one down quark, linked together by [[gluon]]s. The quarks' color charge are also visible.]]
The [[neutron]]s and [[proton]]s in the [[Atomic nucleus|atomic nuclei]] are [[baryon]]s – the neutron is composed of two down quarks and one up quark, and the proton is composed of two up quarks and one down quark.<ref name="Knowing2">{{cite book |author=Munowitz |first=M. |title=Knowing |publisher=[[Oxford University Press]] |year=2005 |isbn=0-19-516737-6 |page=35}}</ref> A baryon is composed of three quarks, and a [[meson]] is composed of two quarks (one normal, one anti). Baryons and mesons are collectively called [[hadron]]s. Quarks inside hadrons are governed by the strong interaction, thus are subjected to [[quantum chromodynamics]] (color charges). The [[Bound state|bounded]] quarks must have their color charge to be neutral, or "white" for analogy with [[Additive color|mixing the primary colors]].<ref>{{cite book |author=Schumm |first=B. A. |url=https://archive.org/details/deepdownthingsbr00schu/page/131 |title=Deep Down Things |publisher=[[Johns Hopkins University Press]] |year=2004 |isbn=978-0-8018-7971-5 |pages=[https://archive.org/details/deepdownthingsbr00schu/page/131 131–132]}}</ref> More [[exotic hadron]]s can have other types, arrangement or number of quarks ([[tetraquark]], [[pentaquark]]).<ref>{{cite journal |last=Close |first=F. E. |year=1988 |title=Gluonic Hadrons |journal=Reports on Progress in Physics |volume=51 |pages=833–882 |bibcode=1988RPPh...51..833C |doi=10.1088/0034-4885/51/6/002 |number=6|s2cid=250819208 }}</ref>

An atom is made from protons, neutrons and electrons.<ref>{{Cite book |last1=Kofoed |first1=Melissa |last2=Miller |first2=Shawn |date=July 2024 |title=Introductory Chemistry |url=https://uen.pressbooks.pub/introductorychemistry/}}</ref> By modifying the particles inside a normal atom, [[exotic atom]]s can be formed.<ref>§1.8, ''Constituents of Matter: Atoms, Molecules, Nuclei and Particles'', Ludwig Bergmann, Clemens Schaefer, and Wilhelm Raith, Berlin, Germany: Walter de Gruyter, 1997, {{ISBN|3-11-013990-1}}.</ref> A simple example would be the [[hydrogen-4.1]], which has one of its electrons replaced with a muon.<ref>{{Cite journal |last1=Fleming |first1=D. G. |last2=Arseneau |first2=D. J. |last3=Sukhorukov |first3=O. |last4=Brewer |first4=J. H. |last5=Mielke |first5=S. L. |last6=Schatz |first6=G. C. |last7=Garrett |first7=B. C. |last8=Peterson |first8=K. A. |last9=Truhlar |first9=D. G. |date=28 Jan 2011 |title=Kinetic Isotope Effects for the Reactions of Muonic Helium and Muonium with H<sub>2</sub> |url=https://www.science.org/doi/abs/10.1126/science.1199421 |journal=Science |volume=331 |issue=6016 |pages=448–450 |doi=10.1126/science.1199421 |pmid=21273484 |bibcode=2011Sci...331..448F |s2cid=206530683|url-access=subscription }}</ref>

=== Hypothetical ===
The [[graviton]] is a hypothetical particle that can mediate the gravitational interaction, but it has not been detected or completely reconciled with current theories.<ref>{{cite news |last=Sokal |first=A. |author-link=Alan Sokal |date=July 22, 1996 |title=Don't Pull the String Yet on Superstring Theory |work=[[The New York Times]] |url=https://query.nytimes.com/gst/fullpage.html?res=9D0DE7DB1639F931A15754C0A960958260 |url-status=live |access-date=March 26, 2010 |archive-url=https://web.archive.org/web/20081207212917/https://query.nytimes.com/gst/fullpage.html?res=9D0DE7DB1639F931A15754C0A960958260 |archive-date=7 December 2008}}</ref> Many other hypothetical particles have been proposed to address the limitations of the Standard Model. Notably, [[Supersymmetry|supersymmetric]] particles aim to solve the [[hierarchy problem]], [[axion]]s address the [[strong CP problem]], and various other particles are proposed to explain the origins of [[dark matter]] and [[dark energy]].

== Experimental laboratories ==
[[File:02 Fermilab - Fermi National Accelerator Laboratory - American particle accelerator Fermilab near Chicago Illinois.jpg|thumb|Fermi National Accelerator Laboratory, USA]]
The world's major particle physics laboratories are:
* [[Brookhaven National Laboratory]] ([[Long Island]], New York, [[United States]]). Its main facility is the [[Relativistic Heavy Ion Collider]] (RHIC), which collides [[Relativistic nuclear collisions|heavy ions]] such as gold ions and polarized protons. It is the world's first heavy ion collider, and the world's only polarized proton collider.<ref>{{Cite journal|last1=Harrison|first1=M.|last2=Ludlam|first2=T.|last3=Ozaki|first3=S.|date=March 2003|title=RHIC project overview|url=https://zenodo.org/record/1259915|journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment|volume=499|issue=2–3|pages=235–244|doi=10.1016/S0168-9002(02)01937-X|bibcode=2003NIMPA.499..235H|access-date=16 September 2019|archive-date=15 April 2021|archive-url=https://web.archive.org/web/20210415022754/https://zenodo.org/record/1259915|url-status=live}}</ref><ref>{{Cite journal|last=Courant|first=Ernest D.|title=Accelerators, Colliders, and Snakes|date=December 2003|journal=[[Annual Review of Nuclear and Particle Science]]|volume=53|issue=1|pages=1–37|doi=10.1146/annurev.nucl.53.041002.110450|bibcode=2003ARNPS..53....1C|issn=0163-8998|doi-access=}}</ref>
* [[Budker Institute of Nuclear Physics]] ([[Novosibirsk]], [[Russia]]). Its main projects are now the electron-positron [[collider]]s [[VEPP-2000]],<ref>{{cite web |url=http://vepp2k.inp.nsk.su/ |title=index |publisher=Vepp2k.inp.nsk.su |access-date=21 July 2012 |archive-url=https://web.archive.org/web/20121029223656/http://vepp2k.inp.nsk.su/ |archive-date=29 October 2012 }}</ref> operated since 2006, and VEPP-4,<ref>{{cite web |url=http://v4.inp.nsk.su/index.en.html |title=The VEPP-4 accelerating-storage complex |publisher=V4.inp.nsk.su |access-date=21 July 2012 |archive-url=https://web.archive.org/web/20110716074832/http://v4.inp.nsk.su/index.en.html |archive-date=16 July 2011 }}</ref> started experiments in 1994. Earlier facilities include the first electron–electron beam–beam [[collider]] VEP-1, which conducted experiments from 1964 to 1968; the electron-positron [[collider]]s VEPP-2, operated from 1965 to 1974; and, its successor VEPP-2M,<ref>{{cite web |url=http://www.inp.nsk.su/activity/old/vepp2m/index.ru.shtml |title=VEPP-2M collider complex |language=ru |publisher=Inp.nsk.su |access-date=21 July 2012 |archive-date=3 December 2013 |archive-url=https://web.archive.org/web/20131203005149/http://www.inp.nsk.su/activity/old/vepp2m/index.ru.shtml |url-status=live }}</ref> performed experiments from 1974 to 2000.<ref>{{cite web |url=http://englishrussia.com/2012/01/21/the-budker-institute-of-nuclear-physics/ |title=The Budker Institute of Nuclear Physics |publisher=English Russia |date=21 January 2012 |access-date=23 June 2012 |archive-date=28 June 2012 |archive-url=https://web.archive.org/web/20120628191134/http://englishrussia.com/2012/01/21/the-budker-institute-of-nuclear-physics |url-status=live }}</ref>
* [[File:View inside detector at the CMS cavern LHC CERN.jpg|thumb|[[Compact Muon Solenoid|CMS]] detector for LHC]][[CERN]] (European Organization for Nuclear Research) ([[France|Franco]]-[[Switzerland|Swiss]] border, near [[Geneva]], Switzerland). Its main project is now the [[Large Hadron Collider]] (LHC), which had its first beam circulation on 10 September 2008, and is now the world's most energetic collider of protons. It also became the most energetic collider of heavy ions after it began colliding lead ions. Earlier facilities include the [[Large Electron–Positron Collider]] (LEP), which was stopped on 2 November 2000 and then dismantled to give way for LHC; and the [[Super Proton Synchrotron]], which is being reused as a pre-accelerator for the LHC and for fixed-target experiments.<ref>{{cite web |url=http://info.cern.ch/ |title=Welcome to |publisher=Info.cern.ch |access-date=23 June 2012 |archive-date=5 January 2010 |archive-url=https://web.archive.org/web/20100105103513/http://info.cern.ch/ |url-status=live }}</ref>
* [[DESY]] (Deutsches Elektronen-Synchrotron) ([[Hamburg]], [[Germany]]). Its main facility was the [[Hadron Elektron Ring Anlage]] (HERA), which collided electrons and positrons with protons.<ref>{{cite web |url=http://www.desy.de/index_eng.html |title=Germany's largest accelerator centre |publisher=Deutsches Elektronen-Synchrotron DESY |access-date=23 June 2012 |archive-date=26 June 2012 |archive-url=https://web.archive.org/web/20120626075024/http://www.desy.de/index_eng.html |url-status=live }}</ref> The accelerator complex is now focused on the production of [[synchrotron radiation]] with [[PETRA III]], [[FLASH]] and the [[European XFEL]].
* [[Fermilab|Fermi National Accelerator Laboratory (Fermilab)]] ([[Batavia, Illinois|Batavia]], Illinois, [[United States]]). Its main facility until 2011 was the [[Tevatron]], which collided protons and antiprotons and was the highest-energy particle collider on earth until the Large Hadron Collider surpassed it on 29 November 2009.<ref>{{cite web |url=http://www.fnal.gov/ |title=Fermilab &#124; Home |publisher=Fnal.gov |access-date=23 June 2012 |archive-date=5 November 2009 |archive-url=https://web.archive.org/web/20091105014508/http://www.fnal.gov/pub/publications/index.html |url-status=live }}</ref>
* [[Institute of High Energy Physics]] (IHEP) ([[Beijing]], [[China]]). IHEP manages a number of China's major particle physics facilities, including the [[Beijing Electron–Positron Collider II]](BEPC II), the Beijing Spectrometer (BES), the Beijing Synchrotron Radiation Facility (BSRF), the [[Yangbajain Cosmic Ray National Field Scientific Observatory|International Cosmic-Ray Observatory at Yangbajing]] in Tibet, the [[Daya Bay Reactor Neutrino Experiment]], the [[China Spallation Neutron Source]], the [[Hard X-ray Modulation Telescope]] (HXMT), and the Accelerator-driven Sub-critical System (ADS) as well as the [[Jiangmen Underground Neutrino Observatory]] (JUNO).<ref>{{cite web |url=http://english.ihep.cas.cn/au/ |title=IHEP &#124; Home |publisher=ihep.ac.cn |access-date=29 November 2015 |archive-url=https://web.archive.org/web/20160201061558/http://english.ihep.cas.cn/au/ |archive-date=1 February 2016}}</ref>
* [[KEK]] ([[Tsukuba, Ibaraki|Tsukuba]], [[Japan]]). It is the home of a number of experiments such as the [[K2K experiment]] and its successor [[T2K experiment]], a [[neutrino oscillation]] experiment and [[Belle II experiment|Belle II]], an experiment measuring the [[CP violation]] of [[B meson]]s.<ref>{{cite web|url=http://legacy.kek.jp/intra-e/index.html |title=Kek &#124; High Energy Accelerator Research Organization |publisher=Legacy.kek.jp |access-date=23 June 2012 |archive-url=https://web.archive.org/web/20120621201554/http://legacy.kek.jp/intra-e/index.html |archive-date=21 June 2012 }}</ref>
* [[SLAC National Accelerator Laboratory]] ([[Menlo Park, California|Menlo Park]], California, [[United States]]). Its 2-mile-long linear particle accelerator began operating in 1962 and was the basis for numerous [[electron]] and [[positron]] collision experiments until 2008. Since then the linear accelerator is being used for the [[Linac Coherent Light Source]] [[X-ray laser]] as well as advanced accelerator design research. SLAC staff continue to participate in developing and building many [[particle detector]]s around the world.<ref>{{cite web|title=SLAC National Accelerator Laboratory Home Page|url=http://www6.slac.stanford.edu/|access-date=19 February 2015|archive-date=5 February 2015|archive-url=https://web.archive.org/web/20150205100556/https://www6.slac.stanford.edu/|url-status=live}}</ref>

== Theory ==
{{Quantum field theory|cTopic=Standard model}}
Theoretical particle physics attempts to develop the models, theoretical framework, and mathematical tools to understand current experiments and make predictions for future experiments (see also [[theoretical physics]]). There are several major interrelated efforts being made in theoretical particle physics today.

One important branch attempts to better understand the [[Standard Model]] and its tests. Theorists make quantitative predictions of observables at [[collider]] and [[Astroparticle physics|astronomical]] experiments, which along with experimental measurements is used to extract the parameters of the Standard Model with less uncertainty. This work probes the limits of the Standard Model and therefore expands scientific understanding of nature's building blocks. Those efforts are made challenging by the difficulty of calculating high precision quantities in [[quantum chromodynamics]]. Some theorists working in this area use the tools of perturbative [[quantum field theory]] and [[effective field theory]], referring to themselves as [[Phenomenology (physics)|phenomenologists]]. Others make use of [[lattice field theory]] and call themselves ''lattice theorists''.

Another major effort is in model building where model builders develop ideas for what physics may lie [[beyond the Standard Model]] (at higher energies or smaller distances). This work is often motivated by the [[hierarchy problem]] and is constrained by existing experimental data.<ref>{{Cite web |last=Gagnon |first=Pauline |date=March 14, 2014 |title=Standard Model: a beautiful but flawed theory |url=http://www.quantumdiaries.org/2014/03/14/the-standard-model-a-beautiful-but-flawed-theory/ |access-date=September 7, 2023 |website=Quantum Diaries}}</ref><ref>{{Cite web |title=The Standard Model |url=https://home.cern/science/physics/standard-model |access-date=September 7, 2023 |website=CERN}}</ref> It may involve work on [[supersymmetry]], alternatives to the [[Higgs mechanism]], extra spatial dimensions (such as the [[Randall–Sundrum model]]s), [[Preon]] theory, combinations of these, or other ideas. [[Vanishing dimensions theory|Vanishing-dimensions theory]] is a particle physics theory suggesting that systems with higher energy have a smaller number of dimensions.<ref>{{cite web|url=http://atramateria.com/the-vanishing-dimensions-of-the-universe/|title=The vanishing dimensions of the Universe|date=March 22, 2011|publisher=Astra Materia|last1=Corbion|first1=Ashley|access-date=May 21, 2013}}</ref>

A third major effort in theoretical particle physics is [[string theory]]. ''String theorists'' attempt to construct a unified description of [[quantum mechanics]] and [[general relativity]] by building a theory based on small strings, and [[brane]]s rather than particles. If the theory is successful, it may be considered a "[[Theory of Everything]]", or "TOE".<ref>{{Cite web |last=Wolchover |first=Natalie |date=2017-12-22 |title=The Best Explanation for Everything in the Universe |url=https://www.theatlantic.com/science/archive/2017/12/string-theory-everything/548774/ |access-date=2022-03-11 |website=The Atlantic |language=en |archive-date=15 November 2020 |archive-url=https://web.archive.org/web/20201115210213/https://www.theatlantic.com/science/archive/2017/12/string-theory-everything/548774/ |url-status=live }}</ref>

There are other areas of work in theoretical particle physics ranging from [[Particle physics in cosmology|particle cosmology]] to [[loop quantum gravity]].

== Practical applications ==
In principle, all physics (and practical applications developed therefrom) can be derived from the study of fundamental particles. In practice, even if "particle physics" is taken to mean only "high-energy atom smashers", many technologies have been developed during these pioneering investigations that later find wide uses in society. Particle accelerators are used to produce [[Isotopes in medicine|medical isotopes]] for research and treatment (for example, isotopes used in [[PET imaging]]), or used directly in [[external beam radiotherapy]]. The development of [[superconductor]]s has been pushed forward by their use in particle physics. The [[World Wide Web]] and [[touchscreen]] technology were initially developed at [[CERN]]. Additional applications are found in medicine, national security, industry, computing, science, and workforce development, illustrating a long and growing list of beneficial practical applications with contributions from particle physics.<ref>{{cite web |url=http://www.fnal.gov/pub/science/benefits/ |title=Fermilab &#124; Science at Fermilab &#124; Benefits to Society |publisher=Fnal.gov |access-date=23 June 2012 |archive-date=9 June 2012 |archive-url=https://web.archive.org/web/20120609161544/http://www.fnal.gov/pub/science/benefits/ |url-status=live }}</ref>

== Future ==
Major efforts to look for [[physics beyond the Standard Model]] include the [[Future Circular Collider]] proposed for CERN<ref>{{Cite web |title=Muon Colliders Hold a Key to Unraveling New Physics |url=http://www.aps.org/publications/apsnews/202111/muon.cfm |access-date=2023-09-17 |website=www.aps.org |language=en}}</ref> and the [[Particle Physics Project Prioritization Panel]] (P5) in the US that will update the 2014 P5 study that recommended the [[Deep Underground Neutrino Experiment]], among other experiments.

== See also ==
{{columns-list|colwidth=30em|
* [[Atomic physics]]
* [[Astronomy]]
* [[Astroparticle physics]]
* [[Computational particle physics]]
* [[High pressure]]
* [[International Conference on High Energy Physics]]
* [[International Conference on Photonic, Electronic and Atomic Collisions]]
* [[Introduction to quantum mechanics]]
* [[Standard Model]]
* [[List of accelerators in particle physics]]
* [[List of particles]]
* [[Micro black hole]]
* [[Number theory]]
* [[Particle physics and representation theory]]
* [[Resonance (particle physics)]]
* [[Self-consistency principle in high energy physics]]
* [[Stanford Physics Information Retrieval System]]
* [[Timeline of particle physics]]
* [[Track significance]]
* [[Unparticle physics]]
}}

== References ==
{{reflist}}

== External links ==
{{Commons category}}
{{Wikiquote}}

{{Physics-footer}}
{{Particles}}
{{Fundamental interactions}}
{{Standard model of physics}}
{{Authority control}}

[[Category:Particle physics]]