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The up quark is an entity described by the Standard Model theory of physics. It is a first-generation quark with a charge of +(2/3)e. It is the lightest of all quarks, with a bare mass ranging between 1.5 and 4 MeV. According to the Standard Model of particle physics, there are six quark types. Ordinary matter, such as atoms, contains electrons and a nucleus. The protons and neutrons inside the nucleus are called nucleons, and the up quark- along with the down quark- are the fundamental constituents of these nucleons. The proton contains two up quarks and a down quark, while the neutron contains one up quark and two down quarks.

The equivalence of mass and energy described in the theory of Special Relativity means that quickly moving particles have greater energy and so appear to have a greater "mass" at high speeds than while at rest.[1] The strength of the strong forces holding the quarks in the nucleus suggests they are moving with relativistic speeds. Therefore, 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. The existence of up quarks was first postulated 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.

Some of the hadrons containing up quarks include:

• Charged Pions (π^±) are mesons containing an up quark and an anti-down quark, or vice versa.
• The neutral pion (π⁰) is a linear combination of up-antiup and down-antidown, as are the ρ and ω mesons.
• The η and η' flavorless mesons are linear combinations of several quark-antiquark pairs, including up-antiup.
• A large number of detected baryons contain one or more up quarks. Like the nucleons, the Δ baryons are made of only up and down quarks: the Δ⁺⁺ contains three up quarks, the Δ⁺ contains two, and the Δ⁰ contains only one.

• Isospin

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