# Up quark

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The **up quark** or **u quark** (symbol: u) is the lightest of all [quarks](/source/Quark), a type of [elementary particle](/source/Elementary_particle), and a significant constituent of [matter](/source/Matter). It, along with the [down quark](/source/Down_quark), forms the [neutrons](/source/Neutron) (one up quark, two down quarks) and [protons](/source/Proton) (two up quarks, one down quark) of [atomic nuclei](/source/Atomic_nucleus). It is part of the [first generation](/source/Generation_(physics)) of matter, has an [electric charge](/source/Electric_charge) of +2⁄3 [*e*](/source/Elementary_charge) and a [bare mass](/source/Quark#Mass) of 2.2 MeV/c2.[1] Like all [quarks](/source/Quark), the up quark is an [elementary](/source/Elementary_particle) [fermion](/source/Fermion) with [spin](/source/Spin_(physics)) [1⁄2](/source/Spin-1/2), and experiences all four [fundamental interactions](/source/Fundamental_interaction): [gravitation](/source/Gravitation), [electromagnetism](/source/Electromagnetism), [weak interactions](/source/Weak_interaction), and [strong interactions](/source/Strong_interaction). The [antiparticle](/source/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 [charge](/source/Electric_charge) have [equal magnitude but opposite sign](/source/Additive_inverse).

Its existence (along with that of the [down](/source/Down_quark) and [strange quarks](/source/Strange_quark)) was postulated in 1964 by [Murray Gell-Mann](/source/Murray_Gell-Mann) and [George Zweig](/source/George_Zweig) to explain the [Eightfold Way](/source/Eightfold_Way_(physics)) classification scheme of [hadrons](/source/Hadron). The up quark was first observed by experiments at the [Stanford Linear Accelerator Center](/source/Stanford_Linear_Accelerator_Center) in 1968.

## History

In the beginnings of particle physics (first half of the 20th century), [hadrons](/source/Hadrons) such as [protons](/source/Proton), [neutrons](/source/Neutron) and [pions](/source/Pion) were thought to be [elementary particles](/source/Elementary_particle). However, as new hadrons were discovered, the '[particle zoo](/source/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](/source/Murray_Gell-Mann)[2] and [Yuval Ne'eman](/source/Yuval_Ne'eman)[3] (independently of each other) proposed a hadron classification scheme called the [Eightfold Way](/source/Eightfold_way_(physics)), or in more technical terms, [SU(3)](/source/SU(3)) [flavor symmetry](/source/Flavor_symmetry).

This classification scheme organized the hadrons into [isospin multiplets](/source/Isospin), but the physical basis behind it was still unclear. In 1964, Gell-Mann[4] and [George Zweig](/source/George_Zweig)[5][6] (independently of each other) proposed the [quark model](/source/Quark_model), then consisting only of up, [down](/source/Down_quark), and [strange quarks](/source/Strange_quark).[7] 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](/source/Stanford_Linear_Accelerator_Center).[8][9] [Deep inelastic scattering](/source/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](/source/Quark_model)).[10]

At first people were reluctant to describe the three bodies as quarks, instead preferring [Richard Feynman](/source/Richard_Feynman)'s [parton](/source/Parton_(particle_physics)) description,[11][12][13] but over time the quark theory became accepted (see *[November Revolution](/source/November_Revolution_(physics))*).[14]

## Mass

Despite being extremely common, the [bare mass](/source/Quark#Mass) of the up quark is not well determined, but probably lies between 1.8 and 3.0 MeV/c2.[15] [Lattice QCD](/source/Lattice_QCD) calculations give a more precise value: 2.01 ± 0.14 MeV/c2.[16]

When found in [mesons](/source/Meson) (particles made of one quark and one [antiquark](/source/Antiparticle)) or [baryons](/source/Baryon) (particles made of three quarks), the 'effective mass' (or 'dressed' mass) of quarks [becomes greater](/source/Quark#Mass) because of the [binding energy](/source/Quantum_chromodynamics_binding_energy) caused by the [gluon field](/source/Gluon) between each quark (see *[Mass–energy equivalence](/source/Mass%E2%80%93energy_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](/source/Down_quark)
- [Isospin](/source/Isospin)
- [Quark model](/source/Quark_model)
- [Quantum Mechanics](/source/Quantum_Mechanics)

## References

1. M. Tanabashi et al. (Particle Data Group) (2018). ["Review of Particle Physics"](http://pdglive.lbl.gov/DataBlock.action?node=Q123UM). *Physical Review D*. **98** (3): 1–708. [Bibcode:2018PhRvD..98c0001T](https://ui.adsabs.harvard.edu/abs/2018PhRvD..98c0001T). [doi:10.1103/PhysRevD.98.030001](https://doi.org/10.1103/PhysRevD.98.030001). [hdl:10044/1/68623](https://hdl.handle.net/10044/1/68623). [PMID 10020536](https://pubmed.ncbi.nlm.nih.gov/10020536)

1. M. Gell-Mann (2000 [1964]). "The Eightfold Way: A theory of strong interaction symmetry". *The Eightfold Way*. M. Gell-Mann, Y. Ne'eman (ed.). [Westview Press](/source/Westview_Press). p. 11. ISBN 978-0-7382-0299-0. Original:M. Gell-Mann (1961). "The Eightfold Way: A theory of strong interaction symmetry". *Synchrotron Laboratory Report CTSL-20*. [California Institute of Technology](/source/California_Institute_of_Technology)

1. Y. Ne'eman (2000 [1964]). "Derivation of strong interactions from gauge invariance". *The Eightfold Way*. M. Gell-Mann, Y. Ne'eman (ed.). [Westview Press](/source/Westview_Press). ISBN 978-0-7382-0299-0. Original Y. Ne'eman (1961). "Derivation of strong interactions from gauge invariance". *[Nuclear Physics](/source/Nuclear_Physics_(journal))*. **26** (2): 222–229. [Bibcode:1961NucPh..26..222N](https://ui.adsabs.harvard.edu/abs/1961NucPh..26..222N). [doi:10.1016/0029-5582(61)90134-1](https://doi.org/10.1016/0029-5582(61)90134-1)

1. M. Gell-Mann (1964). "A Schematic Model of Baryons and Mesons". *[Physics Letters](/source/Physics_Letters)*. **8** (3): 214–215. [Bibcode:1964PhL.....8..214G](https://ui.adsabs.harvard.edu/abs/1964PhL.....8..214G). [doi:10.1016/S0031-9163(64)92001-3](https://doi.org/10.1016/S0031-9163(64)92001-3)

1. G. Zweig (1964). ["An SU(3) Model for Strong Interaction Symmetry and its Breaking"](https://cds.cern.ch/record/352337). *Cern-Th-401*. [doi:10.17181/CERN-TH-401](https://doi.org/10.17181/CERN-TH-401)

1. G. Zweig (1964). ["An SU(3) Model for Strong Interaction Symmetry and its Breaking: II"](https://cds.cern.ch/record/570209). *Cern-Th-412*. [doi:10.17181/CERN-TH-412](https://doi.org/10.17181/CERN-TH-412)

1. B. Carithers, P. Grannis (1995). ["Discovery of the Top Quark"](http://www.slac.stanford.edu/pubs/beamline/25/3/25-3-carithers.pdf). *[Beam Line](/source/Beam_Line_(journal))*. **25** (3): 4–16. Retrieved 2008-09-23.

1. Bloom, E. D.; Coward, D.; Destaebler, H.; Drees, J.; Miller, G.; Mo, L.; Taylor, R.; Breidenbach, M. et al. (1969). "High-Energy Inelastic *e*–*p* Scattering at 6° and 10°". *[Physical Review Letters](/source/Physical_Review_Letters)*. **23** (16): 930–934. [Bibcode:1969PhRvL..23..930B](https://ui.adsabs.harvard.edu/abs/1969PhRvL..23..930B). [doi:10.1103/PhysRevLett.23.930](https://doi.org/10.1103/PhysRevLett.23.930)

1. M. Breidenbach; Friedman, J.; Kendall, H.; Bloom, E.; Coward, D.; Destaebler, H.; Drees, J.; Mo, L.; Taylor, R. et al. (1969). "Observed Behavior of Highly Inelastic Electron–Proton Scattering". *[Physical Review Letters](/source/Physical_Review_Letters)*. **23** (16): 935–939. [Bibcode:1969PhRvL..23..935B](https://ui.adsabs.harvard.edu/abs/1969PhRvL..23..935B). [doi:10.1103/PhysRevLett.23.935](https://doi.org/10.1103/PhysRevLett.23.935). [S2CID 2575595](https://api.semanticscholar.org/CorpusID:2575595)

1. J. I. Friedman. ["The Road to the Nobel Prize"](http://www.hueuni.edu.vn/hueuni/en/news_detail.php?NewsID=1606&PHPSESSID=909807ffc5b9c0288cc8d137ff063c72). [Hue University](/source/Hue_University). [Archived](https://web.archive.org/web/20081225093044/http://www.hueuni.edu.vn/hueuni/en/news_detail.php?NewsID=1606&PHPSESSID=909807ffc5b9c0288cc8d137ff063c72) 2008-12-25 at the Wayback Machine. Retrieved 2008-09-29.

1. R. P. Feynman (1969). ["Very High-Energy Collisions of Hadrons"](http://authors.library.caltech.edu/3871/1/FEYprl69.pdf). *[Physical Review Letters](/source/Physical_Review_Letters)*. **23** (24): 1415–1417. [Bibcode:1969PhRvL..23.1415F](https://ui.adsabs.harvard.edu/abs/1969PhRvL..23.1415F). [doi:10.1103/PhysRevLett.23.1415](https://doi.org/10.1103/PhysRevLett.23.1415)

1. S. Kretzer; Lai, H.; Olness, Fredrick; Tung, W. et al. (2004). "CTEQ6 Parton Distributions with Heavy Quark Mass Effects". *[Physical Review D](/source/Physical_Review_D)*. **69** (11). [arXiv:hep-ph/0307022](https://arxiv.org/abs/hep-ph/0307022). [Bibcode:2004PhRvD..69k4005K](https://ui.adsabs.harvard.edu/abs/2004PhRvD..69k4005K). [doi:10.1103/PhysRevD.69.114005](https://doi.org/10.1103/PhysRevD.69.114005). [S2CID 119379329](https://api.semanticscholar.org/CorpusID:119379329)

1. D. J. Griffiths (1987). *Introduction to Elementary Particles*. [John Wiley & Sons](/source/John_Wiley_%26_Sons). p. 42. ISBN 978-0-471-60386-3.

1. M. E. Peskin, D. V. Schroeder (1995). [*An introduction to quantum field theory*](https://archive.org/details/introductiontoqu0000pesk). [Addison–Wesley](/source/Addison%E2%80%93Wesley). p. [556](https://archive.org/details/introductiontoqu0000pesk/page/556). ISBN 978-0-201-50397-5.

1. J. Beringer ([Particle Data Group](/source/Particle_Data_Group)) et al. (2012). ["PDGLive Particle Summary 'Quarks (u, d, s, c, b, t, b′, t′, Free)'"](http://pdg.lbl.gov/2012/tables/rpp2012-sum-quarks.pdf). [Particle Data Group](/source/Particle_Data_Group). Retrieved 2013-02-21.

1. Cho, Adrian (April 2010). ["Mass of the Common Quark Finally Nailed Down"](https://www.science.org/content/article/mass-common-quark-finally-nailed-down). Science Magazine.

## Further reading

- A. Ali, G. Kramer & Kramer (2011). "JETS and QCD: A historical review of the discovery of the quark and gluon jets and its impact on QCD". *[European Physical Journal H](/source/European_Physical_Journal_H)*. **36** (2): 245. [arXiv:1012.2288](https://arxiv.org/abs/1012.2288). [Bibcode:2011EPJH...36..245A](https://ui.adsabs.harvard.edu/abs/2011EPJH...36..245A). [doi:10.1140/epjh/e2011-10047-1](https://doi.org/10.1140/epjh/e2011-10047-1). [S2CID 54062126](https://api.semanticscholar.org/CorpusID:54062126)
- R. Nave. ["Quarks"](http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/quark.html). *[HyperPhysics](/source/HyperPhysics)*. [Georgia State University](/source/Georgia_State_University), Department of Physics and Astronomy. Retrieved 2008-06-29.
- A. Pickering (1984). *Constructing Quarks*. [University of Chicago Press](/source/University_of_Chicago_Press). pp. 114–125. ISBN 978-0-226-66799-7.

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Adapted from the Wikipedia article [Up quark](https://en.wikipedia.org/wiki/Up_quark) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Up_quark?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.