A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to formcomposite particles. They are 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, such as baryons (of which protons and neutrons are examples), and mesons.For this 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 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, top, and bottom quarks can only be produced in high energy collisions.
Quarks have various intrinsic properties, including electric charge, mass, color charge and spin. 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. 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 have equal magnitude but opposite sign.
The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. The proposal came shortly after Gell-Mann’s 1961 formulation of a particle classification system known as the Eightfold Way – or, in more technical terms, SU(3)flavor symmetry. Physicist Yuval Ne’eman had independently developed a scheme similar to the Eightfold Way in the same year.
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.