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| − | In [[particle physics]], an '''elementary particle''' or ''fundamental particle'' is a particle not known to have substructure; that is, it is not made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic particles of the [[universe]] from which all larger particles are made. In the modern theory of particle physics, the [[Standard Model]], the [[quark]]s, [[lepton]]s, and [[gauge boson]]s are elementary particles.<ref>{{cite book | author=Gribbon, John | title=Q is for Quantum - An Encyclopedia of Particle Physics | publisher=Simon & Schuster | year=2000 | id=ISBN 068485578X}}</ref><ref>{{cite book | author=Clark, John, E.O. | title=The Essential Dictionary of Science | publisher=Barnes & Noble | year=2004 | id=ISBN 0760746168}}</ref> Historically, the [[hadron]]s ([[meson]]s and [[baryon]]s such as the [[proton]] and [[neutron]]) and even whole [[atom]]s were once regarded as elementary particles.
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| − | Elementary particles are grouped according to [[spin (physics)|spin]]: particles normally associated with [[matter]] are [[fermion]]s, having [[half-integer]] spin; they are divided into twelve [[flavour (particle physics)|flavour]]s. Particles associated with [[fundamental force]]s are [[boson]]s, having [[integer]] spin.<ref>{{cite book | author=Veltman, Martinus | title=Facts and Mysteries in Elementary Particle Physics | publisher=World Scientific | year=2003 | id=ISBN 981238149X}}</ref>
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| − | *'''[[Fermion]]s:'''
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| − | ::[[Quark]]s — [[up quark|up]], [[down quark|down]], [[strange quark|strange]], [[charm quark|charm]], [[bottom quark|bottom]], [[top quark|top]]
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| − | ::[[Lepton]]s — [[electron]], [[muon]], [[tau lepton|tau]], [[electron neutrino]], [[muon neutrino]], [[tau neutrino]]
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| − | *'''[[Boson]]s:'''
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| − | ::[[Gauge boson]]s – [[gluon]], [[W and Z bosons]], [[photon]]
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| − | ::Other bosons — [[Higgs boson]], [[graviton]]
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| − | == Standard Model ==
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| − | {{main|Standard Model}}
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| − | The Standard Model of particle physics contains 12 flavours of elementary [[fermion]]s, plus their corresponding [[antiparticle]]s, as well as elementary [[boson]]s that mediate the forces and the still undiscovered [[Higgs boson]]. However, the Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, since it is fundamentally incompatible with [[Albert Einstein|Einstein]]'s [[general relativity]]. There are likely to be hypothetical elementary particles not described by the Standard Model, such as the [[graviton]], the particle that would carry the [[gravity|gravitational force]] or the [[sparticle]]s, [[supersymmetry|supersymmetric]] partners of the ordinary particles.
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| − | === Fundamental fermions ===
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| − | {{main|fermion}}
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| − | The 12 fundamental fermionic flavours are divided into three [[generation (particle physics)|generations]] of four particles each. Six of the particles are [[quarks]]. The remaining six are [[leptons]], three of which are [[neutrino]]s, and the remaining three of which have an electric charge of −1: the electron and its two cousins, the [[muon]] and the [[tau lepton]].
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| − | {| width=100%
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| − | |+ '''''Particle Generations'''''
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| − | |- valign=top
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| − | | ''First generation''
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| − | * [[electron]]: ''e<sup>−</sup>''
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| − | * [[electron-neutrino]]: ''ν<sub>e</sub>''
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| − | * [[up quark]]: ''u''
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| − | * [[down quark]]: ''d''
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| − | | ''Second generation''
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| − | * [[muon]]: ''μ<sup>−</sup>''
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| − | * [[muon-neutrino]]: ''ν<sub>μ</sub>''
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| − | * [[charm quark]]: ''c''
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| − | * [[strange quark]]: ''s''
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| − | | ''Third generation''
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| − | * [[tau lepton]]: ''τ<sup>−</sup>''
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| − | * [[tau-neutrino]]: ''ν<sub>τ</sub>''
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| − | * [[top quark]]: ''t''
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| − | * [[bottom quark]]: ''b''
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| − | |}
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| − | ==== Antiparticles ====
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| − | {{main|antimatter}}
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| − | There are also 12 fundamental fermionic antiparticles which correspond to these 12 particles. The [[positron]] ''e<sup>+</sup>'' corresponds to the electron and has an electric charge of +1 and so on:
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| − | {| width=100%
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| − | |+ '''''Antiparticles'''''
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| − | |- valign=top
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| − | | ''First generation''
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| − | * [[positron]]: ''e<sup>+</sup>''
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| − | * electron-antineutrino: <math> \bar{\nu}_e </math>
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| − | * up antiquark: <math> \bar{u} </math>
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| − | * down antiquark: <math> \bar{d} </math>
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| − | | ''Second generation''
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| − | * positive muon: ''μ<sup>+</sup>''
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| − | * muon-antineutrino: <math> \bar{\nu}_\mu </math>
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| − | * charm antiquark: <math> \bar{c} </math>
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| − | * strange antiquark: <math> \bar{s} </math>
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| − | | ''Third generation''
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| − | * positive tau lepton: ''τ<sup>+</sup>''
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| − | * tau-antineutrino: <math> \bar{\nu}_\tau </math>
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| − | * top antiquark: <math> \bar{t} </math>
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| − | * bottom antiquark: <math> \bar{b} </math>
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| − | |}
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| − | ==== Quarks ====
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| − | {{main|quark}}
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| − | Quarks and antiquarks have never been detected to be isolated, 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. However, gluons are themselves color charged, 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; effectively, they can never separate from one another.
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| − | However, color charged particles may combine to form color neutral [[composite particle]]s called [[hadron]]s. A quark may pair up to 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]]. Or three quarks can exist together: one quark is "red", another "blue", another "green". These three colored quarks together form a color neutral [[baryon]]. Or three antiquarks can exist together: one antiquark is "antired", another "antiblue", another "antigreen". These three anticolored antiquarks form a color neutral [[antibaryon]].
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| − | 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 +2/3 or −1/3, whereas antiquarks have corresponding electric charges of either −2/3 or +1/3.
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| − | 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.
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| − | === Fundamental bosons ===
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| − | {{main|boson}}
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| − | In the Standard Model, vector ([[spin (physics)|spin]]-1) bosons ([[gluon]]s, [[photon]]s, and the [[W and Z bosons]]) mediate forces, while the [[Higgs boson]] (spin-0) is responsible for particles having intrinsic [[mass]].
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| − | ==== Gluons ====
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| − | {{main|gluon}}
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| − | Gluons are the mediators of the [[strong interaction]] and carry both [[color charge|colour]] and anticolour. Although gluons are massless, they are never observed in [[particle detector|detectors]] due to [[colour confinement]]; rather, they produce [[particle jet|jets]] of [[hadron]]s, similar to single [[quark]]s. The first evidence for gluons came from annihilations of electrons and positrons at high energies which sometimes produced [[three jet event|three jets]] — a quark, an antiquark, and a gluon.
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| − | ==== Electroweak bosons ====
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| − | {{main|W and Z bosons}}
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| − | There are three [[weak gauge boson]]s: ''W<sup>+</sup>'', ''W<sup>−</sup>'', and ''Z<sup>0</sup>''; these mediate the [[weak interaction]]. The massless [[photon]] mediates the [[electromagnetism|electromagnetic interaction]].
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| − | ==== Higgs boson ====
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| − | {{main|higgs boson}}
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| − | 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 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 massless (the photon). Although the Higgs mechanism has become an accepted part of the Standard Model, the [[Higgs boson]] itself has not yet been observed in detectors. Indirect evidence for the Higgs boson suggests its mass lies below about 200 GeV. In this case, the [[Large Hadron Collider|LHC]] experiments will be able to discover this last missing piece of the Standard Model.
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| − | == Beyond the Standard Model ==
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| − | Although all experimental evidence confirms the predictions of the [[Standard Model]], many physicists find this model to be unsatisfactory due to its many undetermined parameters, many [[fundamental particle]]s, the non-observation of the [[Higgs boson]] and other more theoretical considerations such as the [[hierarchy problem]]. There are many speculative theories beyond the Standard Model which attempt to rectify these deficiencies.
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| − | === Grand unification ===
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| − | {{main|grand unification theory}}
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| − | 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]]. The most dramatic prediction of grand unification is the existence of [[X boson]]s, which cause [[proton decay]]. However, the non-observation of proton decay at [[Super-Kamiokande]] rules out the simplest GUTs, including SU(5) and SO(10).
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| − | === Supersymmetry ===
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| − | {{main|supersymmetry}}
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| − | Supersymmetry extends the Standard Model by adding an additional class of symmetries to the [[Lagrangian]]. These symmetries exchange [[fermion]]ic particles with [[boson]]ic ones. Such a symmetry predicts the existence of '''supersymmetric particles''', 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. However, some physicists believe that sparticles will be detected when the [[Large Hadron Collider]] at [[CERN]] begins running.
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| − | === String theory ===
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| − | {{main|string theory}}
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| − | According to [[string theory|string theorists]], each kind of fundamental particle corresponds to a different pattern of fundamental string. All strings are essentially the same, although they may be open (lines) or closed (loops). Different particles differ in the coordination of their strings. Modern string theories include [[supersymmetry]], making them [[superstring theory|superstring theories]].
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| − | One particular prediction of string theory is the existence of extremely massive counterparts of ordinary particles due to vibrational excitations of the fundamental string. Another important prediction of string theory is the existence of a massless spin-2 particle behaving like the [[graviton]]. By predicting [[gravity]], string theory unifies [[quantum mechanics]] with [[general relativity]], making it the first consistent theory of [[quantum gravity]].
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| − | One problem with string theory is that it predicts that the number of [[dimension]]s for [[spacetime]] much greater than 4 (the number of observed dimensions). These [[extra dimensions]] are supposedly [[compactification (physics)|compactified]] or rolled-up. Other related theories such as [[brane]] theories contain extended extra dimensions, which are hidden from us by our confinement to a brane.
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| − | === Preon theory ===
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| − | {{main|preon}}
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| − | 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 half a dozen 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 1980's.
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| − | == See also ==
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| − | * [[Subatomic particle]]
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| − | * [[Particle physics]]
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| − | * [[List of particles]]
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| − | == References ==
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| − | <div class="references-small">
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| − | <references />
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| − | <!-- Dead note "Seiden": {{cite book | author=Seiden, Abraham | title=Particle Physics - A Comprehensive Introduction | publisher=Addison Wesley | year=2005 | id=ISBN 0805387366}} -->
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| − | <!-- Dead note "schumm": {{cite book | author=Schumm, Bruce, A. | title=Deep Down Things - the Breathtaking Beauty of Particle Physics | publisher=Johns Hopkins University Press | year=2004 | id=ISBN 081087971X}} -->
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| − | #[[Brian Greene]], ''[[The Elegant Universe]]'', W.W.Norton & Company, 1999, ISBN 0-393-05858-1.
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| − | </div>
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| − | == External links ==
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| − | * [[Brian Greene|Greene, Brian,]] "''[http://www.pbs.org/wgbh/nova/elegant/part-flash.html Elementary particles]''". The Elegant Universe, [[Nova (series)|NOVA]] ([[PBS]])
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| − | *[http://particleadventure.org/particleadventure/frameless/standard_model.html particleadventure.org: The Standard Model], *[http://particleadventure.org/particleadventure/frameless/beyond_start.html Unsolved Mysteries. Beyond The Standard Model], *[http://particleadventure.org/particleadventure/frameless/quarknaming.html What is the World Made of? The Naming of Quarks]
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| − | * [http://pdg.lbl.gov/ University of California: Particle Data Group]
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| − | * [http://particleadventure.org/particleadventure/frameless/chart.html particleadventure.org: Particle chart]
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| − | * [http://www.cerncourier.com/main/article/41/2/17 CERNCourier: Season of Higgs and melodrama]
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| − | * [http://plato.phy.ohiou.edu/~hicks/thplus.htm Pentaquark information page]
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| − | {{Wikipedia}}
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| − | [[Category:Subatomic particles]]
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| − | [[Category:Quantum mechanics]]
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