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A quark () is a type of 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 directly observed or found in isolation; they can be found only within hadrons, which exist as either baryons (which include protons and neutrons) or as mesons. For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves.
Quarks have various intrinsic properties, including electric charge, mass, color charge, and spin. They are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions (electromagnetism, gravitation, strong interaction, and weak interaction)—also known as fundamental forces. And quarks are the only known particles whose electric charges are not integer multiples of their elementary charges.
There are six types (or flavors) of quarks: up, down, charm, strange, top, and bottom (see infobox above, "Generations of matter"). The up/down generation (also called "family") of quarks has the lowest masses of all the generations (families). The heavier quarks rapidly change into up/down quarks through a process known as particle decay, which is the persistent transformation from a higher mass state to a lower mass state. Thus, up/down quarks are generally stable and the most common in the universe, whereas the charm/strange and top/bottom families of quarks can only be produced in high energy collisions (such as those involving cosmic rays and particle accelerators). For every quark flavor there is a corresponding type of antiparticle known as an antiquark, which differs from the quark only in that some of its properties have equal magnitude but opposite sign.
The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until deep inelastic scattering experiments at the Stanford Linear Accelerator Center in 1968. Accelerator experiments have provided evidence for all six flavors. The top quark was the last to be discovered at Fermilab in 1995.
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