down quark
The down quark is a first-generation quark with a charge of -(1/3)e. It is the second-lightest of all the six flavoured quarks. Its bare mass is not well determined, but probably lies 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, however, 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.
Hadrons containing down quarks
Some of the hadrons containing down quarks include:
 Charged Pions () are mesons containing an up quark and an anti-down quark, or vice versa.
The neutral pion () is a linear combination of up-antiup and down-antidown, as are the and ω mesons.
The and 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 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
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