View source for Up quark

{{Short description|Type of quark}}
{{Infobox Particle
 |bgcolour        =
 |name            = Up quark
 |image           =
 |caption         =
 |num_types       =
 |composition     = [[elementary particle]]
 |statistics      = [[fermion]]ic
 |group           = [[quark]]
 |generation      = first
 |interaction     = [[Strong interaction|strong]], [[Weak interaction|weak]], [[Electromagnetic interaction|electromagnetic]], [[gravity]]
 |particle        =
 |antiparticle    = up antiquark ({{SubatomicParticle|Up antiquark}})
 |theorized       = [[Murray Gell-Mann]] (1964){{br}}[[George Zweig]] (1964)
 |discovered      = [[SLAC]] (1968)
 |symbol          = {{SubatomicParticle|up quark}}
 |mass            = {{val|2.2|+0.5|-0.4|ul=MeV/c2}}<ref name="PDG2018">
 {{cite journal
  |author=M. Tanabashi et al. (Particle Data Group)
  |title=Review of Particle Physics
  |year= 2018
  |doi=10.1103/PhysRevD.98.030001
  |volume=98
  |issue=3
  |pages=1–708
  |journal=Physical Review D
  |pmid=10020536
  |url=http://pdglive.lbl.gov/DataBlock.action?node=Q123UM
  |doi-access=free
  |bibcode=2018PhRvD..98c0001T
  |hdl=10044/1/68623
  |hdl-access=free
 }}</ref>
 |decay_time      =
 |decay_particle  = stable or [[down quark]] + [[positron]] + [[electron neutrino]]
 |electric_charge = +{{sfrac|2|3}}&nbsp;[[Elementary charge|''e'']]
 |color_charge    = yes
 |spin            = {{sfrac|1|2}}&nbsp;[[reduced Planck constant|''ħ'']]
 |num_spin_states =
 |weak_isospin    = {{nowrap|[[Chirality (physics)|LH]]: +{{sfrac|1|2}}, [[Chirality (physics)|RH]]: 0}}
 |weak_hypercharge= {{nowrap|[[Chirality (physics)|LH]]: +{{sfrac|1|3}}, [[Chirality (physics)|RH]]: +{{sfrac|4|3}}}}
}}

The '''up quark''' or '''u quark''' (symbol: u) is the lightest of all [[quark]]s, a type of [[elementary particle]], and a significant constituent of [[matter]]. It, along with the [[down quark]], forms the [[neutron]]s (one up quark, two down quarks) and [[proton]]s (two up quarks, one down quark) of [[atomic nucleus|atomic nuclei]]. It is part of the [[generation (physics)|first generation]] of matter, has an [[electric charge]] of +{{sfrac|2|3}}&nbsp;[[elementary charge|''e'']] and a [[Quark#Mass|bare mass]] of {{val|2.2|+0.5|-0.4|ul=MeV/c2}}.<ref name="PDG2018"/> Like all [[quark]]s, the up quark is an [[elementary particle|elementary]] [[fermion]] with [[Spin (physics)|spin]] [[spin-1/2|{{sfrac|1|2}}]], and experiences all four [[fundamental interaction]]s: [[gravitation]], [[electromagnetism]], [[weak interaction]]s, and [[strong interaction]]s. The [[antiparticle]] of the up quark is the '''up antiquark''' (sometimes called ''antiup quark'' or simply ''antiup''), which differs from it only in that some of its properties, such as [[electric charge|charge]] have [[additive inverse|equal magnitude but opposite sign]].

Its existence (along with that of the [[down quark|down]] and [[strange quark]]s) was postulated in 1964 by [[Murray Gell-Mann]] and [[George Zweig]] to explain the [[Eightfold Way (physics)|Eightfold Way]] classification scheme of [[hadron]]s. The up quark was first observed by experiments at the [[Stanford Linear Accelerator Center]] in 1968.

== History ==
{{Standard model of particle physics}}
In the beginnings of particle physics (first half of the 20th century), [[hadrons]] such as [[proton]]s, [[neutron]]s and [[pion]]s were thought to be [[elementary particle]]s. However, as new hadrons were discovered, the '[[particle zoo]]' grew from a few particles in the early 1930s and 1940s to several dozens of them in the 1950s. The relationships between each of them were unclear until 1961, when [[Murray Gell-Mann]]<ref>
{{cite book
 |author=M. Gell-Mann
 |year=2000 |orig-date=1964
 |chapter=The Eightfold Way: A theory of strong interaction symmetry
 |editor=M. Gell-Mann, Y. Ne'eman
 |title=The Eightfold Way
 |page=11
 |publisher=[[Westview Press]]
 |isbn=978-0-7382-0299-0
}}{{br}}Original:{{cite journal
 |author=M. Gell-Mann
 |year=1961
 |title=The Eightfold Way: A theory of strong interaction symmetry
 |journal=Synchrotron Laboratory Report CTSL-20
 |publisher=[[California Institute of Technology]]
}}</ref> and [[Yuval Ne'eman]]<ref>
{{cite book
 |author=Y. Ne'eman
 |year=2000 |orig-date=1964
 |chapter=Derivation of strong interactions from gauge invariance
 |editor=M. Gell-Mann, Y. Ne'eman
 |title=The Eightfold Way
 |publisher=[[Westview Press]]
 |isbn=978-0-7382-0299-0
}}{{br}}Original {{cite journal
 |author=Y. Ne'eman
 |year=1961
 |title=Derivation of strong interactions from gauge invariance
 |journal=[[Nuclear Physics (journal)|Nuclear Physics]]
 |volume=26 |pages=222–229
 |doi=10.1016/0029-5582(61)90134-1
 |bibcode = 1961NucPh..26..222N
 |issue=2
}}</ref> (independently of each other) proposed a hadron classification scheme called the [[Eightfold way (physics)|Eightfold Way]], or in more technical terms, [[SU(3)]] [[flavor symmetry]].

This classification scheme organized the hadrons into [[isospin|isospin multiplet]]s, but the physical basis behind it was still unclear. In 1964, Gell-Mann<ref name="Gell-Man1964">
{{cite journal
 |author=M. Gell-Mann
 |title=A Schematic Model of Baryons and Mesons
 |journal=[[Physics Letters]]
 |volume=8 |issue=3 |pages=214–215
 |year=1964
 |doi=10.1016/S0031-9163(64)92001-3
 |bibcode=1964PhL.....8..214G
}}</ref> and [[George Zweig]]<ref name="Zweig1964a">
{{cite journal
 |author=G. Zweig
 |title=An SU(3) Model for Strong Interaction Symmetry and its Breaking
 |journal=Cern-Th-401
 |url=https://cds.cern.ch/record/352337
 |year=1964
 |doi=10.17181/CERN-TH-401
}}</ref><ref name="Zweig1964b">
{{cite journal
 |author=G. Zweig
 |title=An SU(3) Model for Strong Interaction Symmetry and its Breaking: II
 |journal=Cern-Th-412
 |url=https://cds.cern.ch/record/570209
 |year=1964
 |doi=10.17181/CERN-TH-412
}}</ref> (independently of each other) proposed the [[quark model]], then consisting only of up, [[down quark|down]], and [[strange quark]]s.<ref name="Carithers">
{{cite journal
 |author=B. Carithers, P. Grannis
 |title=Discovery of the Top Quark
 |url=http://www.slac.stanford.edu/pubs/beamline/25/3/25-3-carithers.pdf |journal=[[Beam Line (journal)|Beam Line]]
 |volume=25 |issue=3 |pages=4–16
 |year=1995
 |access-date=2008-09-23
}}</ref> However, while the quark model explained the Eightfold Way, no direct evidence of the existence of quarks was found until 1968 at the [[Stanford Linear Accelerator Center]].<ref name="Bloom">
{{cite journal
 |last1=Bloom
 |first1=E. D.
 |title=High-Energy Inelastic ''e''–''p'' Scattering at 6° and 10°
 |journal=[[Physical Review Letters]]
 |volume=23 |issue=16 |pages=930–934
 |year=1969
 |doi=10.1103/PhysRevLett.23.930
 |bibcode=1969PhRvL..23..930B
 |last2=Coward |first2=D.
 |last3=Destaebler |first3=H.
 |last4=Drees |first4=J.
 |last5=Miller |first5=G.
 |last6=Mo |first6=L.
 |last7=Taylor|first7=R.
 |last8=Breidenbach |first8=M.
 |last9=Friedman |first9=J.
 |last10=Hartmann |first10=G.
 |last11=Kendall |first11=H.
 |display-authors=8
 |doi-access=free
}}</ref><ref name="Breidenbach">
{{cite journal
 |author=M. Breidenbach
 |title=Observed Behavior of Highly Inelastic Electron–Proton Scattering
 |journal=[[Physical Review Letters]]
 |volume=23 |issue=16 |pages=935–939
 |year=1969
 |doi=10.1103/PhysRevLett.23.935
 |bibcode=1969PhRvL..23..935B
 |last2=Friedman |first2=J.
 |last3=Kendall |first3=H.
 |last4=Bloom |first4=E.
 |last5=Coward |first5=D.
 |last6=Destaebler |first6=H.
 |last7=Drees |first7=J.
 |last8=Mo |first8=L.
 |last9=Taylor |first9=R.
 |osti=1444731
 |s2cid=2575595
 |display-authors=etal
}}</ref> [[Deep inelastic scattering]] experiments indicated that protons had substructure, and that protons made of three more-fundamental particles explained the data (thus confirming the [[quark model]]).<ref>
{{cite web
 |author       = J. I. Friedman
 |title        = The Road to the Nobel Prize
 |url          = http://www.hueuni.edu.vn/hueuni/en/news_detail.php?NewsID=1606&PHPSESSID=909807ffc5b9c0288cc8d137ff063c72
 |publisher    = [[Hue University]]
 |access-date   = 2008-09-29
 |archive-url  = https://web.archive.org/web/20081225093044/http://www.hueuni.edu.vn/hueuni/en/news_detail.php?NewsID=1606&PHPSESSID=909807ffc5b9c0288cc8d137ff063c72
 |archive-date = 2008-12-25
 }}</ref>

At first people were reluctant to describe the three bodies as quarks, instead preferring [[Richard Feynman]]'s [[parton (particle physics)|parton]] description,<ref>
{{cite journal
 |author=R. P. Feynman
 |title=Very High-Energy Collisions of Hadrons
 |journal=[[Physical Review Letters]]
 |volume=23 |issue=24 |pages=1415–1417
 |year=1969
 |doi=10.1103/PhysRevLett.23.1415
 |bibcode=1969PhRvL..23.1415F
 |url=http://authors.library.caltech.edu/3871/1/FEYprl69.pdf
}}</ref><ref>
{{cite journal
 |author=S. Kretzer
 |title=CTEQ6 Parton Distributions with Heavy Quark Mass Effects
 |journal=[[Physical Review D]]
 |volume=69 |issue=11 |article-number=114005
 |year=2004
 |doi=10.1103/PhysRevD.69.114005
 |arxiv=hep-ph/0307022
 |bibcode = 2004PhRvD..69k4005K
 |last2=Lai |first2=H.
 |last3=Olness |first3=Fredrick
 |last4=Tung |first4=W.
 |s2cid=119379329
 |display-authors=etal
}}</ref><ref name="Griffiths">
{{cite book
 |author=D. J. Griffiths
 |title=Introduction to Elementary Particles
 |page=42
 |publisher=[[John Wiley & Sons]]
 |year=1987
 |isbn=978-0-471-60386-3
}}</ref> but over time the quark theory became accepted (see ''[[November Revolution (physics)|November Revolution]]'').<ref>
{{cite book
 |author=M. E. Peskin, D. V. Schroeder
 |year=1995
 |title=An introduction to quantum field theory
 |url=https://archive.org/details/introductiontoqu0000pesk
 |url-access=registration
 |page=[https://archive.org/details/introductiontoqu0000pesk/page/556 556]
 |publisher=[[Addison–Wesley]]
 |isbn=978-0-201-50397-5
}}</ref>

== Mass ==
Despite being extremely common, the [[Quark#Mass|bare mass]] of the up quark is not well determined, but probably lies between 1.8 and {{val|3.0|ul=MeV/c2}}.<ref name="PDG2012">
{{cite web
 |author=J. Beringer ([[Particle Data Group]])
 |url=http://pdg.lbl.gov/2012/tables/rpp2012-sum-quarks.pdf
 |title=PDGLive Particle Summary 'Quarks (u, d, s, c, b, t, b′, t′, Free)'
 |publisher=[[Particle Data Group]]
 |year=2012
 |access-date=2013-02-21
 |display-authors=etal
}}</ref> [[Lattice QCD]] calculations give a more precise value: {{val|2.01|0.14|ul=MeV/c2}}.<ref name=lqcd>{{cite web|last=Cho|first=Adrian|title=Mass of the Common Quark Finally Nailed Down|url=https://www.science.org/content/article/mass-common-quark-finally-nailed-down|publisher=Science Magazine|date=April 2010}}</ref>

When found in [[meson]]s (particles made of one quark and one [[antiparticle|antiquark]]) or [[baryon]]s (particles made of three quarks), the 'effective mass' (or 'dressed' mass) of quarks [[Quark#Mass|becomes greater]] because of the [[Quantum chromodynamics binding energy|binding energy]] caused by the [[gluon|gluon field]] between each quark (see ''[[Mass–energy equivalence]]''). The bare mass of up quarks is so light, it cannot be straightforwardly calculated because relativistic effects have to be taken into account.

== See also ==
* [[Down quark]]
* [[Isospin]]
* [[Quark model]]
* [[Quantum Mechanics]]

== References ==
{{reflist|30em}}

== Further reading ==
* {{cite journal
 |author=A. Ali, G. Kramer
 |year=2011
 |title=JETS and QCD: A historical review of the discovery of the quark and gluon jets and its impact on QCD
 |journal=[[European Physical Journal H]]
 |volume= 36|issue= 2|page=245
 |doi=10.1140/epjh/e2011-10047-1
 |bibcode = 2011EPJH...36..245A |arxiv = 1012.2288 |last2=Kramer
 |s2cid=54062126
}}
* {{cite web
 |author=R. Nave
 |title=Quarks
 |url=http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/quark.html
 |work=[[HyperPhysics]]
 |publisher=[[Georgia State University]], Department of Physics and Astronomy
 |access-date=2008-06-29
}}
* {{cite book
 |author=A. Pickering
 |title=Constructing Quarks
 |pages=114–125
 |publisher=[[University of Chicago Press]]
 |year=1984
 |isbn=978-0-226-66799-7
}}

{{Particles}}
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{{DEFAULTSORT:Up Quark}}
[[Category:Quarks]]
[[Category:Elementary particles]]