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Down quark
From Wikipedia, the free encyclopedia
| Down Quark | |
| Composition: | Elementary particle |
|---|---|
| Family: | Fermion |
| Group: | Quark |
| Generation: | First |
| Mass: | 4 - 8 MeV/c2 |
| Electric charge: | -1/3 e |
| Spin: | ½ |
The down quark is a first-generation quark with a charge of -(1/3)e. It is the second-lightest of all quarks, with a bare mass of between 4 and 8 MeV. According to the Standard Model of particle physics, it and the up quark are the fundamental constituents of the nucleons; the proton contains one down quark and two up quarks, while the neutron contains two down quarks and one up quark. (Note that the majority of the mass in nucleons comes from the energy in the gluon field holding the quarks together, and not the quark masses themselves.)
Down quarks were named when Gell-Mann and Zweig developed the quark model in 1964, and the first evidence for them was found in deep inelastic scattering experiments at SLAC in 1967.
Some of the hadrons containing down quarks include:
- Charged Pions (π±) are mesons containing an up quark and an anti-down quark, or vice versa.
- The neutral pion (π0) is a linear combination of up-antiup and down-antidown, as are the ρ and ω mesons.
- The η and η' flavorless mesons are linear combinations of several quark-antiquark pairs, including down-antidown.
- A large number of detected baryons contain one or more down quarks. Like the nucleons, the Δ baryons are made of only up and down quarks: the Δ+ contains one down quark, the Δ0 contains two, and the Δ− contains three.
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| Fermions: Quarks: (Up · Down · Strange · Charm · Bottom · Top) | Leptons: (Electron · Muon · Tau · Neutrinos) | |
| Gauge bosons: Photon | W and Z bosons | Gluons | |
| Not yet observed: Higgs boson | Graviton | Other hypothetical particles | |
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