Down quark

 ( Quarks ) 

The down quark (symbol: d) is a type of elementary particle, and a major constituent of matter. The down quark is the second-lightest of all , and combines with other quarks to form composite particles called hadrons. Down quarks are most commonly found in atomic nucleus, where it combines with to form and . The proton is made of one down quark with two up quarks, and the neutron is made up of two down quarks with one up quark. Because they are found in every single known atom, down quarks are present in all everyday matter that we interact with.

The down quark is part of the first generation of matter, has an electric charge of −  e and a bare mass of . Like all quarks, the down quark is an elementary fermion with spin , and experiences all four fundamental interactions: gravitation, electromagnetism, , 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 additive inverse.

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

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History In the beginnings of particle physics (first half of the 20th century), such as , , and 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

M. Gell-Mann (2025). 9780738202990, Westview Press. ISBN 9780738202990

Original: and Yuval Ne'eman

Y. Ne'eman (2025). 9780738202990, Westview Press. ISBN 9780738202990

Original (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, but the physical basis behind it was still unclear. In 1964, Gell-Mann and George Zweig (independently of each other) proposed the quark model, then consisting only of up quark, down, and strange quark quarks. 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. 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).

At first people were reluctant to identify the three-bodies as quarks, instead preferring Richard Feynman's parton description,

but over time the quark theory became accepted (see November Revolution).

M. E. Peskin, D. V. Schroeder (1995). 9780201503975, Addison–Wesley. . ISBN 9780201503975

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Mass Despite being extremely common, the bare mass of the down quark is not well determined, but probably lies between . Lattice QCD calculations give a more precise value: .

When found in (particles made of one quark and one antiparticle) or (particles made of three quarks), the 'effective mass' (or 'dressed' mass) of quarks becomes greater because of the binding energy caused by the gluon between quarks (see mass–energy equivalence). For example, the effective mass of down quarks in a proton is around . Because the bare mass of down quarks is so small, it cannot be straightforwardly calculated because relativistic effects have to be taken into account.

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Further reading

• A. Pickering (1984). 9780226667997, University of Chicago Press. ISBN 9780226667997

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