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 andneutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation; they can only be found within baryons or mesons. Forthis reason, much of what is known about quarks has been drawn from observations of the hadrons themselves.
There are six types of quarks, known as flavors: up, down, strange, charm, bottom,and top. 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 massstate 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, top, and bottom quarks can only be produced in highenergy collisions (such as those involving cosmic rays and in particle accelerators).
Quarks have various intrinsic properties, including electric charge, color charge, mass, and spin. Quarks are theonly 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. For every quark flavor there is a corresponding type ofantiparticle, known as antiquark, that differs from the quark only in that some of its properties have equal magnitude but opposite sign.
The quark model was independently proposed byphysicists 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...