Everything about Quark totally explained
A
quark ( or ) is a generic type of
physical particle 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 don't 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 (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.
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.
Quarks are the only
fundamental particles that interact through all four of the
fundamental forces.
Antiparticles of quarks are called
antiquarks.
Properties
The following table summarizes the key properties of the six known quarks:
» =20.1.
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 don't 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.
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 can't 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 up antiquark 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's 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 recognize as part of the approximate chiral symmetry of QCD. This is a global flavor
SU(3) symmetry, which shouldn't 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,, 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 didn't actually believe that quarks were real physical entities, Gell-Mann has been quoted as saying - "
That is baloney. I've 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 couldn't 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/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 couldn't 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 word
The 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 hasn't got much of a bark
» And sure any he's it's all beside the mark.
In this context, the word rhymes with "mark", and "bark", but the physics term is pronounced "kwork". Gell-Mann's own explanation:
» In 1963, when I assigned the name "quark" to the fundamental constituents of the nucleon, I'd the sound first, without the spelling, which could have been "kwork". Then, in one of my occasional perusals of Finnegans Wake, by James Joyce, I came across the word "quark" in the phrase "Three quarks for Muster Mark". Since "quark" (meaning, for one thing, the cry of the gull) was clearly intended to rhyme with "Mark," as well as "bark" and other such words, I'd to find an excuse to pronounce it as "kwork". But the book represents the dream of a publican named Humphrey Chimpden Earwicker. Words in the text are typically drawn from several sources at once, like the "portmanteau" words in "Through the Looking Glass". From time to time, phrases occur in the book that are partially determined by calls for drinks at the bar. I argued, therefore, that perhaps one of the multiple sources of the cry "Three quarks for Muster Mark" might be "Three quarts for Mister Mark," in which case the pronunciation "kwork" wouldn't be totally unjustified. In any case, the number three fitted perfectly the way quarks occur in nature.
The phrase "three quarks" is a particularly good fit (as mentioned in the above quote), as at the time, there were only three known quarks, and since quarks appear in groups of three in baryons.
In Joyce's use, it's seabirds giving "three quarks", akin to three cheers, "quark" having a meaning of the cry of a gull (probably
onomatopoeia, like "quack" for ducks). The word is also a pun on the relationship between
Munster and its provincial capital,
Cork.
Further Information
Get more info on 'Quark'.
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