A fundamental component of matter; symbol q. Though there is experimental support for quarks, none has been observed directly. Six quark types are known, identified by their flavours: up, down, strange, charm, top, and bottom. The most recently discovered quark is the top quark, seen in protonantiproton collisions at Fermilab in 1994; it has a mass of about 177 GeV, about as much as an entire gold atom, and vastly more than any other quark species, especially the light u, d, and s quarks. Each flavour quark carries one of three possible colours. Quarks have spin ½, and charges of ±? or ±?. For example, the up quark (u) has charge +? and the down quark (d) charge ??. Protons comprise uud, neutrons udd. According to current theory, subatomic particles composed of quarks are bound together by gluons. Baryons are made of three quarks; mesons of quark/antiquark pairs.
The names of quark types (Up, Down, Strange, Charm, Bottom, and Top) were also chosen arbitrarily based on the need to name them something that could be easily remembered and used.
An important property of quarks is called confinement, which states that individual quarks are not seen because they are always confined inside subatomic particles called hadrons (e.g., protons and neutrons);
Free quarks
No search for free quarks or fractional electric charges has returned convincing evidence. The absence of free quarks has therefore been incorporated into the notion of confinement, which, it is believed, the theory of quarks must possess.
Confinement and quark properties
Every subatomic particle is completely described by a small set of observables such as mass m and quantum numbers, such as spin J and parity P.
The composite particles made of quarks and antiquarks are the hadrons. These include the mesons which get their quantum numbers from a quark and an antiquark, and the baryons, which get theirs from three quarks.
Flavor
Each quark is assigned a baryon number, B = the down-type quark flavours are down, strange, and bottom (each list is in the order of increasing mass).
The number of generations of quarks and leptons are equal in the standard model. Results of direct searches for a fourth generation give limits on the mass of the lightest possible fourth generation quark. The most stringent limit comes from analysis of results from the Tevatron collider at Fermilab, and shows that the mass of a fourth-generation quark must be greater than 190 GeV. Additional limits on extra quark generations come from measurements of quark mixing performed by the experiments Belle and BaBar. In the quark model one builds mesons out of a quark and an antiquark, whereas baryons are built from three quarks. Since mesons are bosons (having integer spins) and baryons are fermions (having half-integer spins), the quark model implies that quarks are fermions.
Colour
Since quarks are fermions, the Pauli exclusion principle implies that the three valence quarks must be in an antisymmetric combination in a baryon.
Quark masses
Although one speaks of quark mass in the same way as the mass of any other particle, the notion of mass for quarks is complicated by the fact that quarks cannot be found free in nature. As a result, the notion of a quark mass is a theoretical construct, which makes sense only when one specifies exactly the procedure used to define it.
Current quark mass
The approximate chiral symmetry of QCD, for example, allows one to define the ratio between various (up, down and strange) quark masses through combinations of the masses of the pseudo-scalar meson octet in the quark model through chiral perturbation theory, giving
The fact that mu ≠ Masses determined in this manner are called current quark masses. The connection between different definitions of the current quark masses needs the full machinery of renormalization for its specification.
Valence quark mass
Another, older, method of specifying the quark masses was to use the Gell-Mann-Nishijima mass formula in the quark model, which connect hadron masses to quark masses. The masses so determined are called constituent quark masses, and are significantly different from the current quark masses defined above.
Heavy quark masses
The masses of the heavy charm and bottom quarks are obtained from the masses of hadrons containing a single heavy quark (and one light antiquark or two light quarks) and from the analysis of quarkonia. Lattice QCD computations using the heavy quark effective theory (HQET) or non-relativistic quantum chromodynamics (NRQCD) are currently used to determine these quark masses.
The top quark is sufficiently heavy that perturbative QCD can be used to determine its mass. Before its discovery in 1995, the best theoretical estimates of the top quark mass are obtained from global analysis of precision tests of the Standard Model. The top quark, however, is unique amongst quarks in that it decays before having a chance to hadronize.
Properties of quarks
The following table summarizes the key properties of the six known quarks:
| Generation |
Weak Isospin |
Flavour | Name | Symbol | Charge / e | Mass / MeV.c-2 |
|---|---|---|---|---|---|---|
| 1 | + 1/2 | Iz=+1/2 | Up | u | + 2/3 | 1.5 to 4.0 |
| 1 | − 1/2 | Iz=−1/2 | Down | d | − 1/3 | 4 to 8 |
| 2 | − 1/2 | S=−1 | Strange | s | − 1/3 | 80 to 130 |
| 2 | + 1/2 | C=1 | Charm | c | + 2/3 | 1150 to 1350 |
| 3 | − 1/2 | B′=−1 | Bottom | b | − 1/3 | 4100 to 4400 |
| 3 | + 1/2 | T=1 | Top | t | + 2/3 | 171400 ± 2100 |
Antiquarks
The additive quantum numbers of antiquarks are equal in magnitude and opposite in sign to those of the quarks. CPT symmetry forces them to have the same spin and mass as the corresponding quark. Notation of antiquarks follows that of antimatter in general: an up quark is denoted by , and an anti-up quark is denoted by .
Substructure
Some extensions of the Standard Model begin with the assumption that quarks and leptons have substructure.
History
The notion of quarks evolved out of a classification of hadrons developed independently in 1961 by Murray Gell-Mann and Kazuhiko Nishijima, which nowadays goes by the name of the quark model. The negative results of quark search experiments caused Gell-Mann to hold that quarks were mathematical fiction.
The charm quark was postulated by Sheldon Glashow, Iliopoulos and Maiani in 1973 to prevent unphysical flavour changes in weak decays which would otherwise occur in the standard model.
The existence of a third generation of quarks was predicted by Kobayashi and Maskawa who realized that the observed violation of CP symmetry by neutral kaons could not be accommodated into the Standard Model with two generations of quarks. The bottom quark was discovered in 1977 and the top quark in 1996 at the Tevatron collider in Fermilab. Particle Data Group on quarks A schematic model of baryons and mesons, by Murray Gell-Mann (1964) Observation of the top quark at Fermilab
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