What is a quark?

Question

What is a quark?

Answer 1

you won't ask this question again will you????

Answer 2

1) In physics a quark is a tiny invisible fundamental particle even smaller than an electron . They were present at the big bang and are found all over the Universe. There are 6 different types up quarks, down quarks,strange, charmed, bottom and top quarks.

2) Quark is a type of very low fat fromage frais type cheese used in cooking.

3)A quark is a term made up by the author Mark Twain

Answer 3

Quark - any of a set of elementary particls that bind together in various combinations to form hadrons



This might be an amazing thought to you but this word is in the dictionary.

Answer 4

In particle physics, quarks are one of the two basic constituents of matter (the other Standard Model fermions are the leptons).

Answer 5

Quark Express is a layout program for graphic design.

Answer 6

quark stranges and charm



From an album in the seventies by an esoteric band called Hawkwind



heheh and the usual microphysics stuff

Answer 7

lol - I don't think it's 'a quark' its just 'quark'. Its a type of very low fat curd. A bit like cream cheese but much less fat. Unless I am being stupid of course... sounds like a marshland bird.



Have just read all above answers. As I suspected, I am indeed stupid.

Answer 8

it's a basic subatomic particle........ unless there's some sort of skin care product with the same name (trying to work out why this question is in 'health and beauty'?)

Answer 9

Quarks are sub-atomic particles.



THE SUBATOMIC WORLD



Particles Prior to Accelerators:

By the mid 1930s, the understanding of the fundamental structure of matter seemed almost complete. Decades before, Rutherford had shown that atoms have relatively tiny but massive nuclei. The quantum theory had made sense of atomic spectra and electron orbitals. The discovery of the neutron had explained nuclear isotopes. So protons, neutrons, and electrons provided the building blocks of all matter. Some puzzles remained, however:

What holds the protons and neutrons together to form the nucleus? What are the forces involved in the radioactive decays of nuclei that make alpha, beta, and gamma rays?



Enter the Accelerator:

To study the nucleus and the interactions of neutrons and protons that form it, physicists needed a tool that could probe within the tiny nucleus, as earlier scattering experiments had probed within the atom. The accelerator is a tool that allows physicists to resolve very small structures by producing particles with very high momentum and thus short wavelength. The wavelength () of the associated wave is inversely proportional to the momentum (p) of the particle (= h/p), where h = Planck's constant.



Particle experiments study collisions of high energy particles produced at accelerators. In modern experiments, large multi-layered detectors surround the collision point. Each layer of the detector serves a separate function in tracking and identifying each of the many particles that may be produced in a single collision.



The Particle Explosion:

To the surprise of the physicists, accelerator experiments revealed that the world of particles was very rich; many more particle types similar to protons and neutrons (called baryons) - and a whole new family of particles called mesons - were discovered. By the early 1960s a hundred or so types of particles had been identified, and physicists still had no complete understanding of the fundamental forces.



The Quark Proposal:

In 1964, two physicists - Murray Gell-Mann and George Zweig - independently hit upon the idea that neutrons and protons and all those new particles could be explained by a few types of yet smaller objects; Gell-Mann called them quarks. They could explain all the observed baryons and mesons with just three types of quarks (now called up, down, and strange) and their antiquarks. The revolutionary part of their idea was that they had to assign the quarks electric charges of 2/3 and -1/3 in units of the proton charge; such charges had never been observed!



Antiquarks are the antimatter partners of quarks; they have the same masses as, but the opposite charge from, the corresponding quarks. When a quark meets an antiquark, they may annihilate, disappearing to give some other form of energy.



The Standard Model:

Nearly thirty years and many experiments later, the quark idea has been confirmed. It is now part of the Standard Model of Fundamental Particles and Interactions. New discoveries have shown that there are six types of quarks (given the odd names of up, down, strange, charm, bottom, and top, in order of increasing mass). Also, there are six types of particles including the electron, called leptons. The Standard Model accounts for the strong, weak, and electromagnetic interactions of the quarks and leptons, and thus explains the patterns of nuclear binding and decays.



The Particles Made from Quarks:

The reason that fractional electric charges like those of quarks have not been seen is that the quarks are never found separately, but only inside composite particles called hadrons. There are two classes of hadrons: baryons, which contain three quarks, and mesons, which contain one quark and one antiquark. The sample hadron tables on the Standard Model chart give a few examples of the many known particles. Particles made from the first five quark types have been produced and studied at accelerators. The top quark is so massive it took many years and very high-energy accelerators to produce it. The top quark was finally discovered in April 1995 at Fermilab.



The Leptons:

In contrast to the quarks, any of the six leptons may by found by itself. The electron is the best known lepton. Two other charged leptons, the muon, (discovered in 1936) and the tau (discovered in 1975) differ from the electron only in that they are more massive than it.



The other three leptons are very elusive particles called neutrinos, which have no electric charge and very little, if any, mass. There is one type of neutrino corresponding to each type of electrically charged lepton. For each of the six leptons there is an antilepton with equal mass and opposite charge.



Forces and Interactions:

Now we know the building blocks of matter, but we must also ask: What holds it together? All forces are due to the underlying interactions of the particles. Interactions come in four types: gravitational, electromagnetic, strong, and weak. Gravity is perhaps the most familiar force to us, but it is not included in the Standard Model because its effects are tiny in particle processes and, furthermore, physicists have not yet figured out how to include it.



Electromagnetic forces are also familiar; they are responsible for binding the electrons to the nucleus to form electrically-neutral atoms. Atoms combine to form molecules or crystals because of electromagnetic effects due to their charged substructure. Most everyday forces, such as the support of the floor or friction, are due to the electromagnetic forces in matter that resist displacement of atoms or electrons from their equilibrium positions in the material.



In particle processes the forces are described as due to the exchange of particles; for each type of force there is an associated carrier particle. The carrier particle of the electromagnetic force is the photon; gamma ray is the name given a photon from a nuclear transition.



For distances much larger than the size of an atomic nucleus, the remaining two forces have only tiny effects -- so we never notice them in everyday life. But we depend on them for the existence of all the stuff from which the world is made, and for the decay processes that make some types of matter unstable.



The strong force holds quarks together to form hadrons; its carrier particles are whimsically called gluons because they so successfully "glue" the quarks together. The binding of protons and neutrons to form nuclei is a residual strong interaction effect due to their strongly-interacting quark and gluon constituents. Leptons have no strong interactions.



Weak interactions are the only processes in which a quark can change to another type of quark, or a lepton to another lepton. They are responsible for the fact that all the more massive quarks and leptons decay to produce lighter quarks and leptons. That is why stable matter around us contains only electrons and the lightest two quark types (up and down). The carrier particles of weak interactions are the W and Z bosons. Beta decay of nuclei was the first observed weak process: in a nucleus where there is sufficient energy a neutron becomes a proton and gives off an electron and an antielectron neutrino. This decay changes the atomic number of the nucleus. Beta ray is the name given to the emerging electron.



So now we have explained beta and gamma rays; what about the alpha? The alpha particle is a helium nucleus - one of the products of a nuclear fission. Fission is the breakup of a massive nucleus into smaller nuclei; this occurs when the sum of the masses of the smaller nuclei is less than the mass of the parent nucleus. This is a residual strong interaction effect.



What Questions Remain?

The Standard Model answers many of the questions of the structure and stability of matter with its six types of quarks, six of leptons, and the four force types.



But the Standard Model leaves many other questions unanswered: Why are there three types of quarks and leptons of each charge? Is there some pattern to their masses? Are there more types of particles and forces to be discovered at yet higher-energy accelerators? Are the quarks and leptons really fundamental, or do they, too, have substructure? How can the gravitational interactions be included? What particles form the dark matter in the universe?



Questions such as these drive particle physicists to build and operate new accelerators, so that higher-energy collisions can provide clues to their answers.



I think that just about covers it and a bit more.

Answer 10

well in Germany we eat it, its like yogurt just thicker and it has no taste, i use to mix it with fruit and a little milk, very nice shake. And we use it for cake , mix it in the sponge, the kids love it.

Answer 11

this is a sub atomic particle. Three in protons and three in neutrons.

Answer 12

It's a subatomic particle.

Answer 13

a sub atomic particle.

Answer 14

Captain Kirk's best friend!

Answer 15

he he

Answer 16

Quark

From Wikipedia, the free encyclopedia

Jump to: navigation, search

For other uses, see Quark (disambiguation).



These are the 6 quarks and their most likely decay modes. Mass decreases moving from right to left.In particle physics, quarks are one of the two basic constituents of matter (the other Standard Model fermions are the leptons). Antiparticles of quarks are called antiquarks. Quarks are the only fundamental particles that interact through all four of the fundamental forces. The derivation of this word comes from the book Finnegans Wake by James Joyce.



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); an exception is the top quark, which decays so quickly that it does not hadronize, and can therefore be observed more directly via its decay products. 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.



Contents [hide]

1 Free quarks

2 Confinement and quark properties

3 Flavour

4 Spin

5 Colour

6 Quark masses

6.1 Current quark mass

6.2 Valence quark mass

6.3 Heavy quark masses

7 Properties of quarks

8 Antiquarks

9 Substructure

10 History

11 See also

12 References and external links

12.1 Primary and secondary sources

12.2 Other references







[edit]

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. However, it may be possible to change the volume of 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.



[edit]

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. 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.



[edit]

Flavour

Flavour in particle physics

Flavour quantum numbers

Lepton number: L

Baryon number: B

Electric charge: Q

Weak hypercharge: YW

Weak isospin: Tz

Isospin: I, Iz

Hypercharge: Y

Strangeness: S

Charm: C

Bottomness: B'

Topness: T



--------------------------------------------------------------------------------



Y=B+S+C+B'+T

Q=Iz+Y/2

Q=Tz+YW/2

B−L



--------------------------------------------------------------------------------



Related topics:



CPT symmetry

CKM matrix

CP symmetry

Chirality



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: 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 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.



[edit]

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 J = 1/2. The spins of excited mesons and baryons are completely consistent with this assignment.



[edit]

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 Iz = 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 colour. This quantum number is the charge involved in the gauge theory called quantum chromodynamics (QCD).



The only other coloured 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.



Colour is a gauged SU(3) symmetry. Quarks are placed in the fundamental representation, 3, and hence come in three colours. Gluons are placed in the adjoint representation, 8, and hence come in eight varieties. For more on this, see the article on colour charge.



[edit]

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.



[edit]

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 ≠ 0 is important, since there would be no strong CP problem if mu were to vanish. 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.



[edit]

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.



[edit]

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.



[edit]

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



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.

[edit]

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 .



[edit]

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.



[edit]

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 flavour 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 flavour 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 [1]. 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 1973 to prevent unphysical flavour 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 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.



[edit]

See also

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.

[edit]

References and external links

[edit]

Primary and secondary sources

Griffiths, David J. (1987). Introduction to Elementary Particles. Wiley, John & Sons, Inc. ISBN 0-471-60386-4.

Povh, Bogdan (1995). Particles and Nuclei: An Introduction to the Physical Concepts. Springer-Verlag. 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

[edit]

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