Down quark

Down quark

Composition	Elementary particle
Statistics	Fermionic
Generation	First
Interactions	Strong, Weak, Electromagnetic force, Gravity
Symbol	d
Antiparticle	Down antiquark (d)
Theorized	Murray Gell-Mann (1964) George Zweig (1964)
Discovered	SLAC (1968)
Mass	4.1-5.7 MeV/c2[1]
Decays into	Up quark
Electric charge	−1⁄3 e
Color charge	Yes
Spin	1⁄2
Weak isospin	LH: −1⁄2, RH: 0
Weak hypercharge	LH: 1⁄3, RH: −2⁄3

The down quark or d quark (from its symbol, d) is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the up quark, forms the neutrons (one up quark, two down quarks) and protons (two up quarks, one down quark) of atomic nuclei. It is part of the first generation of matter, has an electric charge of −¹⁄₃ e and a bare mass of 4.1–5.7 MeV/c².^[1] Like all quarks, the down quark is an elementary fermion with spin-¹⁄₂, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. The antiparticle of the down quark is the down antiquark (sometimes called antidown quark or simply antidown), which differs from it only in that some of its properties have equal magnitude but opposite sign.

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

History

In the beginnings of particle physics (first half of the 20th century), hadrons such as protons, neutron and pions were thought to be elementary particles. 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 was unclear until 1961, when Murray Gell-Mann^[2] and Yuval Ne'eman^[3] (independently of each other) proposed a hadron classification scheme called the Eightfold Way, or in more technical terms, SU(3) flavor symmetry.

This classification scheme organized the hadrons into isospin multiplets, but the physical basis behind it was still unclear. In 1964, Gell-Mann^[4] and George Zweig^[5][6] (independently of each other) proposed the quark model, then consisting only of up, down, and strange quarks.^[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.^[8][9] 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).^[10]

At first people were reluctant to identify the three-bodies as quarks, instead preferring Richard Feynman's parton description,^[11][12][13] but over time the quark theory became accepted (see November Revolution).^[14]

Mass

Despite being extremely common, the bare mass of the down quark is not well determined, but probably lies between 4.1 and 5.7 MeV/c².^[1] When found in mesons (particles made of one quark and one antiquark) or baryons (particles made of three quarks), the 'effective mass' (or 'dressed' mass) of quarks becomes greater because of the binding energy caused by the gluon field between each quarks (see mass–energy equivalence). For example, the effective mass of down quarks in a proton is around 330 MeV/c². Because the bare mass of down quarks is so light, it cannot be straightforwardly calculated because relativistic effects have to be taken into account.

See also

• Isospin
• Quark model

References

1. ^ ^{a b c} K. Nakamura et al. (Particle Data Group) (2011). "PDGLive Particle Summary 'Quarks (u, d, s, c, b, t, b', t', Free)'". Particle Data Group. http://pdg.lbl.gov/2011/tables/rpp2011-sum-quarks.pdf. Retrieved 2011-08-08. 
2. ^ M. Gell-Mann (2000) [1964]. "The Eightfold Way: A theory of strong interaction symmetry". In M. Gell-Manm, Y. Ne'emann. The Eightfold Way. Westview Press. p. 11. ISBN 0-7382-0299-1. 
   Original: M. Gell-Mann (1961). "The Eightfold Way: A theory of strong interaction symmetry". Synchroton Laboratory Report CTSL-20 (California Institute of Technology) 
3. ^ Y. Ne'emann (2000) [1964]. "Derivation of strong interactions from gauge invariance". In M. Gell-Manm, Y. Ne'emann. The Eightfold Way. Westview Press. ISBN 0-7382-0299-1. 
   Original Y. Ne'emann (1961). "Derivation of strong interactions from gauge invariance". Nuclear Physics 26: 222. Bibcode 1961NucPh..26..222N. doi:10.1016/0029-5582(61)90134-1. 
4. ^ M. Gell-Mann (1964). "A Schematic Model of Baryons and Mesons". Physics Letters 8 (3): 214–215. Bibcode 1964PhL.....8..214G. doi:10.1016/S0031-9163(64)92001-3. 
5. ^ G. Zweig (1964). "An SU(3) Model for Strong Interaction Symmetry and its Breaking". CERN Report No.8181/Th 8419. 
6. ^ G. Zweig (1964). "An SU(3) Model for Strong Interaction Symmetry and its Breaking: II". CERN Report No.8419/Th 8412. 
7. ^ B. Carithers, P. Grannis (1995). "Discovery of the Top Quark" (PDF). Beam Line (SLAC) 25 (3): 4–16. http://www.slac.stanford.edu/pubs/beamline/25/3/25-3-carithers.pdf. Retrieved 2008-09-23. 
8. ^ E. D. Bloom et al. (1969). "High-Energy Inelastic e–p Scattering at 6° and 10°". Physical Review Letters 23 (16): 930–934. Bibcode 1969PhRvL..23..930B. doi:10.1103/PhysRevLett.23.930. 
9. ^ M. Breidenbach et al. (1969). "Observed Behavior of Highly Inelastic Electron–Proton Scattering". Physical Review Letters 23 (16): 935–939. Bibcode 1969PhRvL..23..935B. doi:10.1103/PhysRevLett.23.935. 
10. ^ J. I. Friedman. "The Road to the Nobel Prize". Hue University. http://www.hueuni.edu.vn/hueuni/en/news_detail.php?NewsID=1606&PHPSESSID=909807ffc5b9c0288cc8d137ff063c72. Retrieved 2008-09-29. 
11. ^ R. P. Feynman (1969). "Very High-Energy Collisions of Hadrons". Physical Review Letters 23 (24): 1415–1417. Bibcode 1969PhRvL..23.1415F. doi:10.1103/PhysRevLett.23.1415. 
12. ^ S. Kretzer et al. (2004). "CTEQ6 Parton Distributions with Heavy Quark Mass Effects". Physical Review D 69 (11): 114005. arXiv:hep-ph/0307022. Bibcode 2004PhRvD..69k4005K. doi:10.1103/PhysRevD.69.114005. 
13. ^ D. J. Griffiths (1987). Introduction to Elementary Particles. John Wiley & Sons. p. 42. ISBN 0-471-60386-4. 
14. ^ M. E. Peskin, D. V. Schroeder (1995). An introduction to quantum field theory. Addison–Wesley. p. 556. ISBN 0-201-50397-2. 

Further reading

• A. Ali, G. 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. doi:10.1140/epjh/e2011-10047-1. 
• R. Nave. "Quarks". HyperPhysics. Georgia State University, Department of Physics and Astronomy. http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/quark.html. Retrieved 2008-06-29. 
• A. Pickering (1984). Constructing Quarks. University of Chicago Press. pp. 114–125. ISBN 0226667995.