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Quark
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A quark (, or ) is a generic type of physical particle that interacts via the strong force and that forms one of the two basic constituents of matter, the other being the lepton. Various species of quarks combine in specific ways to form protons and neutrons, in each case taking exactly three quarks to make the composite particle in question.
There are six different types of quark, usually known as flavors: up, down, charm, strange, top, and bottom. (Their names do not indicate anything about their properties, but were chosen arbitrarily based on the need to name them something that could be easily remembered and used.) The charm, strange, top, and bottom varieties are highly unstable, and are believed to have decayed within a fraction of a second after the Big Bang – though they can be briefly recreated and studied by scientists. However, the "up" and "down" varieties are abundant and are distinguished by (amongst other things) their electric charge. It is this which makes the difference when quarks clump together to form protons or neutrons: a proton is made up of two "up quarks" and one "down quark", yielding a net charge of +1, while a neutron contains one "up quark" and two "down quarks", yielding a net charge of 0.
In nature, quarks are always found bound together in groups like this, and never in isolation, because of a phenomenon known as confinement. These groups of quarks are called hadrons, with groups of two quarks known specifically as mesons and groups of three quarks as baryons. Various combinations of the six flavors account for all of the 200 or more known mesons and baryons.
Quarks are the only fundamental particles that interact through all four of the fundamental forces.
Antiparticles of quarks are called antiquarks, which have properties similar to those of the quark, but an opposite charge. Notation of antiquarks follows that of antimatter in general: an up quark is denoted by , and an up antiquark is denoted by .
PropertiesThe following table summarizes the key properties of the six known quarks:
Quark flavor properties| Generation | Weak
Isospin | Flavor | Name | Symbol | Charge
e | Mass
MeV/c2 | Antiparticle | Antiparticle
Symbol | | 1 | +˝ | Iz=+˝ | Up | | +? | 1.5 – 4.0 | Antiup | | | 1 | -˝ | Iz=-˝ | Down | | -? | 4 – 8 | Antidown | | | 2 | +˝ | C=1 | Charm | | +? | 1150 – 1350 | Anticharm | | | 2 | -˝ | S=-1 | Strange | | -? | 80 – 130 | Antistrange | | | 3 | +˝ | T=1 | Top | | +? | 170900 ± 1800 | Antitop | | | 3 | -˝ | B'=-1 | Bottom | | -? | 4100 – 4400 | Antibottom | |
- The quantum numbers of the top and bottom quarks are sometimes known as truth and beauty respectively, as an alternative to topness and bottomness.
Flavor
Each flavor of quark is assigned a baryon number (B = 1/3), an internal property which differentiates quarks from leptons, and a vanishing lepton number (L = 0). They have fractional electric charge, Q, either Q = +2/3 or Q = -1/3, depending on their flavor. Each quark is assigned a weak isospin: Tz = +1/2 for an up-type quark and Tz = -1/2 for a down-type quark. Each doublet of weak isospin defines a generation of quarks. There are three generations, and hence six flavors of quarks — the up and down flavors comprise the first generation, the strange and charm the second, and the top and bottom the third.
The number of generations of quarks and leptons are equal in the standard model, with two flavors in each generation. The number of generations of leptons with a light neutrino is strongly constrained by experiments at the LEP in CERN and by observations of the abundance of helium in the universe. Precision measurement of the lifetime of the Z boson at LEP constrains the number of light neutrino generations to be three. Astronomical observations of helium abundance give consistent results. 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 in order to remain stable. Additional limits on extra quark generations come from measurements of quark mixing performed by the experiments Belle and BaBar.
Each flavor defines a quantum number which is conserved under the strong interactions, but not the weak interactions. The magnitude of flavor changing in the weak interaction is encoded into a structure called the CKM matrix. This also encodes the CP violation allowed in the Standard Model.
Spin Quantum numbers corresponding to non-Abelian symmetries like rotations require more care in extraction, since they are not additive. 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. Further, the fact that the lightest baryons have spin-1/2 implies that each quark can have spin S = 1/2. The spins of excited mesons and baryons are completely consistent with this assignment.
Color Since quarks are fermions, the Pauli exclusion principle implies that the three valence quarks must be in an antisymmetric combination in a baryon. However, the charge Q = 2 baryon, (which is one of four isospin Iz = 3/2 baryons) can only be made of three quarks with parallel spins. Since this configuration is symmetric under interchange of the quarks, it implies that there exists another internal quantum number, which would then make the combination antisymmetric. This is given the name "color", although it has nothing to do with the perception of the frequency (or wavelength) of light, which is the usual meaning of color. This quantum number is the charge involved in the gauge theory called quantum chromodynamics (QCD).
The only other colored particle is the gluon, which is the gauge boson of QCD. Like all other non-Abelian gauge theories (and unlike quantum electrodynamics) the gauge bosons interact with one another by the same force that affects the quarks.
Color is a gauged SU(3) symmetry. Quarks are placed in the fundamental representation, 3, and hence come in three colors (red, green, and blue). Gluons are placed in the adjoint representation, 8, and hence come in eight varieties.
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 b and parity r. Usually these properties are directly determined by experiments. However, confinement makes it impossible to measure these properties of quarks. Instead, they must be inferred from measurable properties of the composite particles which are made up of quarks. Such inferences are usually most easily made for certain additive quantum numbers called flavors.
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. The quarks (and antiquarks) which impart quantum numbers to hadrons are called "valence" quarks. Apart from these, any hadron may contain an indefinite number of virtual quarks, antiquarks and gluons which together contribute nothing to their quantum numbers. Such virtual quarks are called "sea" quarks.
It is now believed that so-called "neutron stars", collapsed remnants of a massive star in which the protons and electrons degenerate and combine to form neutrons, might actually exist instead in the form of up, down and strange quarks as a single "atom" in what is called a quark star.
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. This was expounded upon by Frank Wilczek, H. David Politzer and David Gross who concluded that the more quarks separated, the greater the attraction due to the strong force, making it impossible to separate the quarks into free particles. This has been called asymptotic freedom, for which Gross, Politzer, and Wilczek was awarded the Nobel Prize in Physics in 2004.
Confinement began as an experimental observation, and is expected to follow from the modern theory of strong interactions, called quantum chromodynamics (QCD). Although there is no mathematical derivation of confinement in QCD, it is easy to show using lattice gauge theory.
However, it may be possible to change the confinement by creating dense or hot quark matter. These new phases of QCD matter have been predicted theoretically, and experimental searches for them have now started at the RHIC. Under some theories, sufficient energy input [by high-speed relativistic collisions such as at the RHIC and planned at the LHC might also generate strange quarks arising from the vacuum, which could recombine with the up and down quarks to form a new type of nucleon called a strangelet or strange quark matter. Wilczek cautioned that there might be concern for an "ice-9" type reaction, in which a strangelet engaged in runaway fusion with normal nuclei, in a Letter to the Editor of Scientific American in 1999. However, he concluded that there likely should be no cause for concern, as most theories show such strangelets to be positively charged, and would repulse normal nuclei due to the charge repulsion of Coulomb's law.
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 quantum chromodynamics, 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 the up quark has mass is important, since there would be no strong CP problem if it were massless. The absolute values of the masses are currently determined from QCD sum rules (also called spectral function sum rules) and lattice QCD. 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. The constituent masses do not have any further dynamical meaning.
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. Thus, its mass can be directly measured from the resulting decay products. This can only be done at the Tevatron which is the only particle accelerator energetic enough to produce top quarks in abundance.
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 scheme grouped together particles with isospin and strangeness using a unitary symmetry derived from current algebra, which we today recognize as part of the approximate chiral symmetry of QCD. This is a global flavor SU(3) symmetry, which should not be confused with the gauge symmetry of QCD.
In this scheme the lightest mesons (spin-0) and baryons (spin-˝) are grouped together into octets, 8, of flavor symmetry. A classification of the spin-3/2 baryons into the representation 10 yielded a prediction of a new particle, , the discovery of which in 1964 led to wide acceptance of the model. The missing representation 3 was identified with quarks.
This scheme was called the eightfold way by Gell-Mann, a clever conflation of the octets of the model with the eightfold way of Buddhism. He also chose the name quark and attributed it to the sentence “Three quarks for Muster Mark” in James Joyce's Finnegans Wake. In reply to the common claim that he did not actually believe that quarks were real physical entities, Gell-Mann has been quoted as saying - "That is baloney. I have explained so many times that I believed from the beginning that quarks were confined inside objects like neutrons and protons, and in my early papers on quarks I described how they could be confined either by an infinite mass and infinite binding energy, or by a potential rising to infinity, which is what we believe today to be correct. Unfortunately, I referred to confined quarks as 'fictitious', meaning that they could not emerge to be utilized for applications such as catalysing nuclear fusion."
Analysis of certain properties of high energy reactions of hadrons led Richard Feynman to postulate substructures of hadrons, which he called partons (since they form part of hadrons). A scaling of deep inelastic scattering cross sections derived from current algebra by James Bjorken received an explanation in terms of partons. When Bjorken scaling was verified in an experiment in 1969, it was immediately realized that partons and quarks could be the same thing. With the proof of asymptotic freedom in QCD in 1973 by David Gross, Frank Wilczek and David Politzer the connection was firmly established.
The charm quark was postulated by Sheldon Glashow, John Iliopoulos and Luciano Maiani in 1970 to prevent unphysical flavor changes in weak decays which would otherwise occur in the standard model. The discovery in 1974 of the meson which came to be called the J/? led to the recognition that it was made of a charm quark and its antiquark.
The existence of a third generation of quarks was predicted by Makoto Kobayashi and Toshihide Maskawa in 1973 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.
Origin of the wordThe word was originally coined by Murray Gell-Mann as a nonsense word rhyming with "pork", but without a spelling. Later, he found the word "quark" in James Joyce's book Finnegans Wake, and used the spelling but not the pronunciation:
- Three quarks for Muster Mark!
- Sure he has not got much of a bark
- And sure any he has it's all beside the mark.
Primary and secondary sources
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