Quark

Three colored balls (symbolizing quarks) connected pairwise by springs (symbolizing gluons), all inside a gray circle (symbolizing a proton). The colors of the balls are red, green, and blue, to parallel each quark's color charge. The red and blue balls are labeled "u" (for "up" quark) and the green one is labeled "d" (for "down" quark).
A four-by-four table of particles. Columns are three generations of matter (fermions) and one of forces (bosons). In the first three columns, two rows contain quarks and two leptons. The top two rows' columns contain up (u) and down (d) quarks, charm (c) and strange (s) quarks, top (t) and bottom (b) quarks, and photon (γ) and gluon (g), respectively. The bottom two rows' columns contain electron neutrino (ν sub e) and electron (e), muon neutrino (ν sub μ) and muon (μ), and tau neutrino (ν sub τ) and tau (τ), and Z sup 0 and W sup ± weak force. Mass, charge, and spin are listed for each particle.
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. 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.

The Standard Model is the theoretical framework describing all the currently known elementary particles. This model contains six flavors of quarks (
q
), named up (
u
), down (
d
), charm (
c
), strange (
s
), top (
t
), and bottom (
b
). Antiparticles of quarks are called antiquarks, and are denoted by a bar over the symbol for the corresponding quark, such as
u
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-​12 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).

This page was last edited on 25 May 2018, at 23:29.
Reference: https://en.wikipedia.org/wiki/Quark under CC BY-SA license.

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