
The down quark or d quark (symbol: d) is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. Together with the up quark, it forms the neutrons (one up quark, two down quarks) and protons (two up quarks, one down quark) of atomic nuclei. It is part of the first generation of matter, has an electric c...
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http://en.wikipedia.org/wiki/Down_quark

(from the article `quark`) ...(). Each carries a fractional value of the electron charge (i.e., a charge less than that of the electron, ). The up quark (charge 23) and down ... ...two types of quark are necessary to build protons and neutrons, the constituents of atomic nuclei. These are the up quark, with a charge of +23, ... The W...
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http://www.britannica.com/eb/a-z/d/71

[
n] - a stable quark with an electric charge of -1/3 and a mass 607 times that of an electron
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http://www.webdictionary.co.uk/definition.php?query=down%20quark

The second flavour of quark (in order of increasing mass), with electric charge -1/3. See also: Bottom, Charm, Quark.
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noun a stable quark with an electric charge of -1/3 and a mass 607 times that of an electron
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the quark having electric charge − times the elementary charge, with strangeness, charm, and other quark quantum numbers equal to 0. Also called
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https://www.infoplease.com/dictionary/down-quark
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