In particle physics, the quarks are one of the two families of subatomic 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 are generally believed to never exist alone but only in groups of two or three (and, more recently, five); all searches for free quarks since 1977 have yielded negative results. 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.
Six different quarks are known; search for 4th-generation quarks is underway. The known quarks are:
| Name | Charge | Estimated mass (MeV) |
|---|---|---|
| Up (u) | +2/3 | 1.5 to 4.5 1 |
| Down (d) | -1/3 | 5 to 8.5 1 |
| Charm / Centre (c) | +2/3 | 1,000 to 1,400 |
| Strange / Sideways (s) | -1/3 | 80 to 155 |
| Top / Truth (t) | +2/3 | 174,300 ± 5,100 |
| Bottom / Beauty (b) | -1/3 | 4,000 to 4,500 |
1. The estimates of u and d masses are not uncontroversial and still actively being investigated; in fact, there are even suggestions in literature that the u quark could be essentially massless.
Ordinary matter such as protons and neutrons are composed of quarks of the UP or DOWN variety only. A proton contains two UP quarks and one DOWN quark, giving a total charge of +1. A neutron is made of two DOWN quarks and one UP quark, giving a total charge of zero. The other varieties of quarks can only be produced in particle accelerators, and degenerate quickly to the UP and DOWN quarks. (Electrons do not contain quarks, but are of a different type of particle called a Lepton).
According to the theory of quantum chromodynamics (QCD), 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 atomic 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 two new quarks (a quark and an anti-quark) pop 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.
See also: Rubik's Cube for an interesting parallel.