Quark
In particle physics, the quarks are one of the two families of particles thought to be elemental and indivisible (the other being the leptons). Objects made up of quarks are known as hadrons; well known examples are protons and neutrons.
Quarks never exist alone but only in groups. Quarks are differentiated from leptons, the other family of elemental particles, by electric charge. Leptons (such as the electron or the muon) have integral charge (+1, 0 or -1) while quarks have +2/3 or -1/3 charge (antiquarks have -2/3 or +1/3 charge). All quarks have spin 1/2 h.
The six quarks are known as:
Name Charge Mass Up +2/3 4 MeV Down -1/3 8 MeV Charm +2/3 1500 MeV Strange -1/3 150 MeV Top (or truth) +2/3 176,000 MeV Bottom (or beauty) -1/3 4,700 MeV
A proton is made of two up quarks and one down, giving a charge of +1. A neutron is made of two down and one up, giving a charge of zero. Ordinary everyday matter is composed of up and down quarks only; the other quarks can only be produced in particle accelerators and degenerate quickly into those two types.
According to the theory of Quantum Chromodynamics quarks possess another property that is called "color charge" (and that doesn't have anything to do with real color). Instead of just two different charge types (like + and - in electromagnetism), color charge comes in 3 types: "red", "green" and "blue" (6 if we count the "anticharges"). In the theory, only "color neutral" particles can exist. Particles composed of one red, one green and one blue quark are called baryons; the proton and the neutron are the most important examples. Particles composed of a quark and an anti-quark of the corresponding anti-color are called mesons.
Particles of different color charge are attracted and particles of like color charge are repelled by the strong nuclear force, which is transferred by gluons, particles that themselves carry color charge. Therefore, colors of quarks are not static, but are interchanged by gluons, always maintaining the result neutral. This interchange of color charge is thought to result in the strong nuclear force holding quarks together in mesons and baryons; a "secondary" effect of this strong nuclear force is to hold the protons and neutrons together in the atom's nucleus.
Due to the extremely strong nature of the strong force, quarks are never found free. They are always bound into baryons or mesons. When we try to separate quarks in a meson or baryon, as happens in particle accelerators, the strong force actually becomes stronger as they get farther apart. At some point it is more energetically favorable to create two more quarks to cancel out the increasing force, and nature pops two new quarks (a quark and an anti-quark) out of the vacuum. This process is called hadronization or fragmentation, and is one of the least understood processes in particle physics. As a result of fragmentation, when quarks are produced in particle accelerators, instead of seeing the individual quarks in detectors, scientists see "jets" of many color-neutral particles (mesons and baryons), clustered together.
The theory behind quarks was first suggested by physicists Murray Gell-Mann and George Zweig, who found they could explain the properties of many particles by considering them to be composed of these elementary quarks. The name quark comes from "three quarks for Muster Mark"", a nonsense phrase in James Joyce's Finnegans Wake.
In Germany, Quark is a fresh creamery cow milk cheese somewhat similar to American cottage cheese. It is made by letting bacteria ferment milk, with the resulting acid producing curd. The word "Quark" is also used figuratively in conversation, Quark dazugeben means "to add your two cents worth" with a negative connotation. In this example, "Quark" is synonymous to the (more popular) "Senf" (mustard).
In the television series Star Trek Deep Space Nine, Quark is a character portrayed by Armin Shimerman. Quark is an acquisitive Ferengi.