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.
The Standard Model is the theoretical framework describing all the currently known elementary particles. This model contains six flavors of quarks (
), named up (
), down (
), charm (
), strange (
), top (
), and bottom (
). Antiparticles of quarks are called antiquarks, and are denoted by a bar over the symbol for the corresponding quark, such as
for an up antiquark. As with antimatter in general, antiquarks have the same mass, mean lifetime, and spin as their respective quarks, but the electric charge and other charges have the opposite sign.
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), any number of which 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 (see "Strong interaction and color charge" below).