- For other uses of this term, see: Quark (disambiguation)
1974 discovery photograph of a possible charmed baryon, now identified as the Σc++
In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. They are one of the two kinds of spin-½ fermions (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 color-neutral groups of two or three (and possibly five or more); all searches for free quarks since 1977 have yielded negative results. Quarks are differentiated from leptons, the other family of fermions, by color charge. In addition, leptons (such as the electron, the muon, and the neutrino) have integral electric charge (−1 or 0 in units of the proton charge) while quarks have fractional electric charge (+⅔ or −⅓; antiquarks have charge −⅔ or +⅓ and antileptons have charge +1 or 0).
Table of quarks
Estimated mass (MeV)
||1.5 to 4 1
||4 to 8 1
||80 to 130
||1,150 to 1,350
||4,100 to 4,400
||178,000 ± 4,300
1. Estimates of quark masses, being subject to considerable theoretical uncertainty, are controversial and still actively being investigated. There have been suggestions in literature that the u quark could be massless, but this is nearly ruled out by recent results. Since we never see individual quarks their masses must be deduced indirectly. Different ways of doing this can give somewhat different values for the masses. The values in this table are found using the minimal subtraction scheme.
2. The names beauty and truth were originally suggested for the bottom and top quarks respectively. These names are no longer used among physicists but are still mentioned occasionally.
Ordinary matter such as protons and neutrons are composed of quarks of the up and 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 decay quickly into the up and down quarks. (Electrons do not contain quarks, but are of a different type of particle called leptons).
The six varieties of quark are sometimes called flavors.
Families of quarks
All the quarks that appear in ordinary matter are either up or down quarks. However, in very high-energy situations, other quarks appear. The first "extra" quark discovered was called a strange quark; as higher-energy collisions became possible, the charm, bottom, and top quarks were discovered. These extra quarks seem to be merely higher-mass copies of ordinary quarks, just as the muon and the tauon are higher-mass copies of the electron.
One might wonder whether there are yet more families of quarks with even higher masses. Research at CERN has provided strong evidence that no such families exist. This experiment relied on accurate determination of the width in masses of the Z boson; by a subtle series of calculations, the numbers obtained could be shown to contradict the possibility that more families of quarks exist. See  for more information.
The number of families of quarks also affects the only other really high-energy situation we know of — the early Universe. The initial distribution of elements can be predicted using the Standard Model; any model with more heavy quarks would lead to a fraction of initial Helium-4 that is different from what is observed. Thus the number of quarks is confirmed by astronomical observations as well. See  for more information.
According to the theory of quantum chromodynamics (QCD), quarks possess a property metaphorically called "color charge". Instead of just one charge type (with two signs, + and − in electromagnetism), color charge comes in 3 types. Quarks' colors are called "red", "green", or "blue" to suggest the primary colors, while anti-quarks are anti-red or "cyan", anti-green or "magenta", and anti-blue or "yellow". Due to confinement (described below), only color-neutral or "white" particles can exist separately: particles possessing color must be part of a "white" composite. 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 color force, which is transferred by gluons, particles that themselves carry color charge (one color and one anti-color). Therefore, colors of quarks are not static, but are constantly changed by gluons, though the composite hadron always remains neutral. In addition to holding quarks together in mesons and baryons, a residual effect of the color force, the strong nuclear force, holds the protons and neutrons together in the atomic nucleus.
Because the carriers of the strong force, the gluons, are themselves colored, the force between two quarks increases as the quarks are separated. Due to this mechanism, called confinement, quarks are almost never found free; they are always bound into color-neutral baryons or mesons. When we try to separate quarks, as happens in particle accelerator collisions, at some point it is more energetically favorable for a new quark/anti-quark pair to pop out of the vacuum than to allow the quarks to separate further. As a result of this, 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. This process is called hadronization or fragmentation, and is one of the least understood processes in particle physics. But if the pressure and temperature of the nucleonic reaction are high enough, a quark-gluon plasma forms, offering the first evidence of a free quark state.
The theory behind quarks was first suggested by physicists Murray Gell-Mann and Yuval Ne'eman , who found they could explain various properties of several mesons by considering them to belong to an 8-dimensional representation of the group SU(3), called 8 for short. This description was called the "Eightfold Way" by Gell-Mann.
Success was found by attaching several baryons to a 10-dimensional representation, culminating in the successful prediction of the Ω−. The physical fact that baryons had distinct antiparticles corresponded to the mathematical fact that the 10-dimensional representation has a distinct dual, of the same dimension. These are called 10 and 10*.
This left one mathematical fact unexplained: the simplest representation of SU(3) is 3-dimensional, and is distinct from its dual (the 3 and the 3*). This would correspond physically to a triplet of particles, with distinct antiparticles. And the mathematics of deriving the 8 and 10 from the 3 would then correspond physically to joining two or three of the new particles.
This step was taken in 1964 independently by Gell-Mann and George Zweig. But the new particles would be slightly unusual. In his 1964 paper (see below) Gell-Mann notes:
A simpler and more elegant scheme can be constructed if we allow non-integral values of the charges. We then refer to the members u, d and s of the triplet as "quarks".
At the end of the paper, he cites James Joyce, Finnegans Wake (1939) p. 383, which contains the less-than-illuminating line "Three quarks for Muster Mark." Gell-Mann states elsewhere that "quark" was originally a nonsense word pronounced <kwôrk> (sounding like "quart"), which he invented a few weeks before coming across the line from Finnegans Wake. Gell-Mann's paper is only 2 pages. He left all the details to the interested reader, of which there were few.
Gell-Mann's approach was based on manipulating the "current algebra" of quantum fields associated with SU(3). He did not claim his quarks were real particles, and would hedge the question if asked.
In contrast, Zweig developed his "aces" as being real particles from the start, and in his two long CERN preprints, provided detailed ace content descriptions of known and unknown particles. He was unable to publish these papers.
At the time, the notion of quarks as real particles was widely considered self-evidently nonsensical. Fractional charges had never been observed, and a spate of new searches came up empty. And because quarks were fermions, the Pauli exclusion principle had to apply, and that meant two or more identical fermions could not share the same quantum states. Two up quarks, for example, could pair together if their spins had opposite orientations. But there was no third orientation available for a third up quark. Worse, the known uuu candidate, the Δ++, had spin 3/2, meaning its constituent quarks had identically oriented spins, so it consisted of three otherwise identical fermions.
This view changed in the early seventies, when deep inelastic scattering experiments showed that protons indeed had subcomponent structure. The details matched well with the quark model, with two surprising twists. The forces between quarks decreased at decreasing separations, while 3 quarks did not account for all the internal energy and momentum.
The 1973 discovery that the mathematics of SU(3) gauge theory was asymptotically free accounted for the first twist. The associated exchange bosons, dubbed gluons, accounted for the second twist. Since the quark force got stronger with distance, it was no longer felt unusual that free quarks were not seen, and since quarks carried 3 color charges, uuu was really uuu, that is, "red up", "blue up", "green up", and there was no contradiction with the exclusion principle.
(The SU(3) of the Eightfold Way is conceptually distinct from the SU(3) of gauge theory. The former is considered an approximate descriptive symmetry involving 3 quark flavors, which number has grown. The latter is considered an exact dynamical symmetry involving 3 quark colors, which has not changed.)
The experimental discovery of weak neutral currents in 1973 led to a new difficulty, however. The electroweak theory, directly applied to 3 quark flavors, led to the prediction that there would be strangeness changing neutral currents at rates that were obviously much too high. Glashow, Iliopoulos, and Maiani had, in 1970, shown that a fourth quark flavor ("charm") would automatically suppress these strangeness changing neutral currents.
It was also realized, in 1971, that SU(3) renormalizability required the total charge of all the fundamental quarks had to cancel the total charge of all the fundamental leptons. The then-identified quarks and leptons did not cancel out. The simplest corrective was again, the "charm" quark.
The unusual J/ψ, discovered in 1974, was immediately explained as a charm-anticharm meson. Subsequent study of it and related particles confirmed this interpretation in great detail, and quarks quickly became the new orthodoxy in particle physics.
Last updated: 06-01-2005 15:42:05