In
particle physics, the 'quark' (pronounced ) is one of the two basic constituents of
matter (the other is the
lepton). It is quarks that make up
protons and
neutrons, with there being exactly three quarks within each kind of particle.
There are six different types of quark, usually known as
flavors:
up,
down,
charm,
strange,
top, and
bottom. (Their names were chosen arbitrarily based on the need to name them ''something'' that could be easily remembered and used.) The strange, charm, bottom and top varieties are highly unstable and died out within a fraction of a second after the
Big Bang; they can be recreated and studied by particle physicists. The up and down varieties survive in profusion, and are distinguished by (among 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.
Quarks are the only
fundamental particles that interact through all four of the
fundamental forces.
Antiparticles of quarks are called 'antiquarks'.
Isolated quarks are never found naturally; they are almost always found in groups of two (
mesons) or groups of three (
baryons) called
hadrons. This is a direct consequence of
confinement.
Origin of the word
The word was originally coined by
Murray Gell-Mann as a nonsense word rhyming with "pork".
[1] Later, he found the same word in
James Joyce's book ''
Finnegans Wake'', where seabirds give "three quarks", akin to three cheers (probably
onomatopoeically imitating a seabird call, like "quack" for ducks, as well as making a pun on the relationship between
Munster and its provincial capital,
Cork) in the passage ''"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."'' Further explanation for the use of the word "quark" may be derived from the fact that, at the time, there were only three known quarks in existence.
Free quarks
1974 discovery photograph of a possible charmed baryon, now identified as the Σc++
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 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.
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 'S' and
parity 'P'. 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
flavours.
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'.
Flavour
Each quark is assigned a
baryon number, 'B = 1/3', and a vanishing
lepton number 'L = 0'. They have fractional
electric charge, 'Q', either 'Q = +2/3' or 'Q = −1/3'. The former are called ''up-type quarks'', the latter, ''down-type quarks''. Each ''quark'' is assigned a
weak isospin: 'T
z = +1/2' for an ''up-type quark'' and 'T
z = −1/2' for a ''down-type quark''. Each doublet of weak isospin defines a '
generation' of quarks. There are three generations, and hence six
flavours of quarks — the ''up-type'' quark flavours are up, charm and top; 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. 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. Additional limits on extra quark generations come from measurements of quark mixing performed by the experiments
Belle and
BaBar.
Each flavour defines a quantum number which is conserved under the
strong interactions, but not the
weak interactions. The magnitude of flavour 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. The flavour quantum numbers are described in detail in the article on
flavour.
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.
Colour
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, 'Δ
++' (which is one of four isospin 'I
z = 3/2 'baryons) can only be made of three 'u' 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 "
colour", 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. For more on this, see the article on
color charge.
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.
Properties of quarks
The following table summarizes the key properties of the six known quarks:
:{| class="wikitable"
! Generation
! Weak
Isospin
! Flavor
! Name
! Symbol
! Charge /
e
! Mass /
MeV·
c-2
! Antiparticle
! Symbol
|-
| 1
| +½
| I
z=+½
|
Up
| u
| +⅔
| 1.5 – 4.0
| Antiup
|
|-
| 1
| -½
| I
z=-½
|
Down
| d
| -⅓
| 4 – 8
| Antidown
|
|-
| 2
| -½
| S=-1
|
Strange
| s
| -⅓
| 80 – 130
| Antistrange
|
|-
| 2
| +½
| C=1
|
Charm
| c
| +⅔
| 1150 – 1350
| Anticharm
|
|-
| 3
| -½
| B'=-1
|
Bottom
| b
| -⅓
| 4100 – 4400
| Antibottom
|
|-
| 3
| +½
| T=1
|
Top
| t
| +⅔
| 170900 ± 1800
[2]
| Antitop
|
|}
★ Top quark mass from the
Tevatron Electroweak Working Group
★ Other quark masses from
Particle Data Group; these masses are given in the
MS-bar scheme.
★ The quantum numbers of the top and bottom quarks are sometimes known as truth and beauty respectively, as an alternative to topness and bottomness.
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. Tests of CPT symmetry cannot be performed directly on quarks and antiquarks, due to confinement, but can be performed on hadrons. 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'. In other words, these models assume that the elementary particles of the Standard Model are in fact composite particles, made of some other elementary constituents. Such an assumption is open to experimental tests, and these theories are severely constrained by data. At present there is no evidence for such substructure. For more details see the article on
preons.
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 recognise 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-½) 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, Ω
−, 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''
[3]. The negative results of quark search experiments caused Gell-Mann to hold that quarks were mathematical fiction.
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,
Iliopoulos and
Maiani in 1970 to prevent unphysical flavor changes in weak decays which would otherwise occur in the
standard model. The discovery in 1975 of the
meson which came to be called the
J/ψ 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.
See also
★
Quark model
★
Fundamental forces and
strong interactions
★
Gluons
★
Quantum chromodynamics, the
quark model and
partons.
★
Confinement,
deconfinement,
quark matter and
asymptotic freedom
★
Standard model overview and
details, the
CKM matrix and
CP symmetry.
References and external links
1. Gribbin, John. "Richard Feynman: A Life in Science" Dutton 1997, pg 194.
2. Summary of Top Mass Results - March 2007
3. http://www.bartleby.com/61/67/Q0016700.html
Primary and secondary sources
★
Introduction to Elementary Particles, Griffiths, David J., , , Wiley, John & Sons, Inc, 1987, ISBN 0-471-60386-4
★
Particles and Nuclei: An Introduction to the Physical Concepts, Povh, Bogdan, , , Springer-Verlag, 1995, ISBN 0-387-59439-6
★
Particle Data Group on quarks
★
A schematic model of baryons and mesons, by Murray Gell-Mann (1964)
★
Observation of the top quark at Fermilab
★
NanoReisen-A very educational site on Quarks and many other things beyond the nanoscale.
Other references
★
Quark dance
★
A Positron Named Priscilla — A description of CERN’s experiment to count the families of quarks
★
The original English word ''quark'' and its adaptation to particle physics
★
An elementary popular introduction
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