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Quark
 A proton is composed of two up quarks, one down quark, and the gluons that mediate the forces "binding" them together. The color assignment of individual quarks is arbitrary, but all three colors must be present; red, blue and green are used as an analogy to the primary colors that together produce a white color. | |
| Composition | elementary particle |
|---|---|
| Statistics | fermionic |
| Generation | 1st, 2nd, 3rd |
| Interactions | strong, weak, electromagnetic, gravitation |
| Symbol | q |
| Antiparticle | antiquark ( q ) |
| Theorized |
|
| Discovered | SLAC (c. 1968) |
| Types | 6 (up, down, strange, charm, bottom, and top) |
| Electric charge | +2/3 e, −1/3 e |
| Color charge | yes |
| Spin | 1/2 ħ |
| Baryon number | 1/3 |
A quark (/kwɔːrk, kwɑːrk/) 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.[1] All commonly observable matter is composed of up quarks, down quarks and electrons. Owing to a phenomenon known as color confinement, quarks are never found in isolation; they can be found only within hadrons, which include baryons (such as protons and neutrons) and mesons, or in quark–gluon plasmas.[2][3][nb 1] For this reason, much of what is known about quarks has been drawn from observations of hadrons.
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, 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.
There are six types, known as flavors, of quarks: up, down, charm, strange, top, and bottom.[4] Up and down quarks have the lowest masses of all quarks. The heavier quarks rapidly change into up and down quarks through a process of particle decay: the transformation from a higher mass state to a lower mass state. Because of this, up and down quarks are generally stable and the most common in the universe, whereas strange, charm, bottom, and top quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators). For every quark flavor there is a corresponding type of antiparticle, known as an antiquark, that differs from the quark only in that some of its properties (such as the electric charge) have equal magnitude but opposite sign.
The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964.[5] 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.[6][7] Accelerator program experiments have provided evidence for all six flavors. The top quark, first observed at Fermilab in 1995, was the last to be discovered.[5]
- ^ "Quark (subatomic particle)". Encyclopædia Britannica. Retrieved 29 June 2008.
- ^ R. Nave. "Confinement of Quarks". HyperPhysics. Georgia State University, Department of Physics and Astronomy. Retrieved 29 June 2008.
- ^ R. Nave. "Bag Model of Quark Confinement". HyperPhysics. Georgia State University, Department of Physics and Astronomy. Retrieved 29 June 2008.
- ^ R. Nave. "Quarks". HyperPhysics. Georgia State University, Department of Physics and Astronomy. Retrieved 29 June 2008.
- ^ a b B. Carithers; P. Grannis (1995). "Discovery of the Top Quark" (PDF). Beam Line. 25 (3): 4–16. Retrieved 23 September 2008.
- ^ E. D. Bloom; et al. (1969). "High-Energy Inelastic e–p Scattering at 6° and 10°". Physical Review Letters. 23 (16): 930–934. Bibcode:1969PhRvL..23..930B. doi:10.1103/PhysRevLett.23.930.
- ^ M. Breidenbach; et al. (1969). "Observed Behavior of Highly Inelastic Electron–Proton Scattering". Physical Review Letters. 23 (16): 935–939. Bibcode:1969PhRvL..23..935B. doi:10.1103/PhysRevLett.23.935. OSTI 1444731. S2CID 2575595.
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