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The up quark or u quark (symbol: u) is the lightest of all , a type of elementary particle, and a significant constituent of matter. It, along with the down quark, forms the (one up quark, two down quarks) and (two up quarks, one down quark) of atomic nucleus. It is part of the first generation of matter, has an electric charge of + e and a bare mass of . Like all , the up quark is an elementary fermion with spin , and experiences all four fundamental interactions: gravitation, electromagnetism, , and strong interactions. The antiparticle of the up quark is the up antiquark (sometimes called antiup quark or simply antiup), which differs from it only in that some of its properties, such as electric charge have additive inverse.
Its existence (along with that of the down quark and ) was postulated in 1964 by Murray Gell-Mann and George Zweig to explain the Eightfold Way classification scheme of . The up quark was first observed by experiments at the Stanford Linear Accelerator Center in 1968.
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, down quark, and . 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 describe the three bodies as quarks, instead preferring Richard Feynman's parton description, but over time the quark theory became accepted (see November Revolution).
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 each quark (see Mass–energy equivalence). The bare mass of up quarks is so light, it cannot be straightforwardly calculated because relativistic effects have to be taken into account.
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