Down quark
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| Composition: | Elementary particle |
| Family: | Fermion |
| Group: | Quark |
| Generation: | First |
| Interaction: | Strong, Weak, Electromagnetic force, Gravity |
| Antiparticle: | Down antiquark (d) |
| Theorized: | Murray Gell-Mann (1964) George Zweig (1964) |
| Discovered: | SLAC (1968) |
| Symbol(s): | d |
| Mass: | 3.5–6.0 MeV/c2 |
| Decays into: | Up quark |
| Electric charge: | −1⁄3 e |
| Color charge: | Yes |
| Spin: | 1⁄2 |
The down quark is a first-generation quark with a charge of −1⁄3 e. It is the second-lightest of all the six flavours of quarks, the lightest being the up quark. Down quarks are most commonly found in nucleons (protons and neutrons); protons contains one down quark and two up quarks, while neutrons contains two down quarks and one up quark.
[edit] Mass
The bare mass of down quarks is not well determined, but probably lies between 3.5 and 6.0 MeV/c2.[1] The down quark is very light compared to nucleons (~940 MeV/c2). The reason is that the majority of the mass of nucleons comes from the energy in the gluon field holding the quarks together, and not the quark masses themselves.
Down quarks were theorized by Murray Gell-Mann and George Zweig when they developed the quark model in 1964 (Zweig called them aces rather than quarks),[2][3] and the first evidence for them was found in deep inelastic scattering experiments at SLAC in 1968.[4][5]
[edit] See also
[edit] References
- ^ C. Amsler et al. (2008). "Review of Particle Physics: Quarks" (PDF). Physics Letters (Particle Data Group) B667 (1): 1–1340. http://pdg.lbl.gov/2008/tables/rpp2008-sum-quarks.pdf.
- ^ M. Gell-Mann (1964). "A Schematic of Baryons and Mesons". Physics Letters 8 (3): 214–215. doi:.
- ^ B. Carithers, P. Grannis. "Discovery of the Top Quark" (PDF). Beam Line (SLAC). http://www.slac.stanford.edu/pubs/beamline/25/3/25-3-carithers.pdf. Retrieved on 2008-09-23.
- ^ E.D. Bloom (1969). "High-Energy Inelastic e-p Scattering at 6° and 10°". Physical Review Letters 23 (16): 930–934. doi:.
- ^ M. Breidenbach (1969). "Observed Behavior of Highly Inelastic Electron-Proton Scattering". Physical Review Letters 23 (16): 935–939. doi:.
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