The down quark is a first-generation quark with a charge of ? 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.

Mass

The bar 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]

Hadrons containing down quarks

Some of the hadrons containing down quarks include:

• Charged pions () are mesons containing an up quark and an down antiquark, or vice versa.
• Neutral pions () is a linear combination of up quark-up antiquark and down quark-down antiquark, as are the and ? mesons.
• The and flavorless mesons are linear combinations of several quark-antiquark pairs, including down quark-down antiquark.
• A large number of detected baryons contain one or more down quarks. Like the nucleons, the Delta baryons are made of only up and down quarks: the contains no down quark, but the contains one, the contains two, and the contains three.

See also

• Isospin
• Quark model

References

1. ↑
2. ↑
3. ↑
4. ↑
5. ↑

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