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The Standard Model.
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A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never found in isolation; they can only be found within hadrons.

Quarks have various intrinsic properties, including electric charge, color charge, spin, and mass. Quarks are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions, also known as fundamental forces (electromagnetism, gravitation, strong interaction, and weak interaction), as well as the only known particles whose electric charges are not integer multiples of the elementary charge.

The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. All six flavors of quark have since been observed in accelerator experiments; the top quark, first observed at Fermilab in 1995, was the last to be discovered. The Standard Model is the theoretical framework describing all the currently known elementary particles, as well as the unobserved Higgs boson. This model contains six flavors of quarks (q), named up (u), down (d), charm (c), strange (s), top (t), and bottom (b). Up and down quarks are generally stable and the most common in the universe, whereas charm, strange, top, and bottom quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators).

Quarks are spin-1/2 particles, implying that they are fermions according to the spin-statistics theorem. They are subject to the Pauli exclusion principle, which states that no two identical fermions can simultaneously occupy the same quantum state. This is in contrast to bosons (particles with integer spin), of which any number can be in the same state. Unlike leptons, quarks possess color charge, which causes them to engage in the strong interaction. The resulting attraction between different quarks causes the formation of composite particles known as hadrons.

A hadron is a composite particle made of quarks held together by the strong force (atoms and molecules are held together by the electromagnetic force). Hadrons are baryons (made of three quarks--includes protons and neutrons), and mesons (made of one quark and one antiquark).

Baryons participate in the strong interaction. Leptons, on the other hand, are not composed of quarks and as such do not participate in the strong interaction. Electrons (the other major component of atoms) are leptons.