A quark is a physical particle that interacts via all four fundamental forces and that is placed under the classification of fermion. and that forms one of the two basic constituents of matter, the other being the lepton.
There are six different types of quarks, usually known as flavors: up, down, charm, strange, top, and bottom.The charm, strange, top, are highly unstable, and can only be recreated and observed under special conditions. However, the "up" and "down" varieties are in abudant existence throughout the universe. In addition to flavor, quarks are assigned various other properties, such as electrical and color charge, mass, and spin.
In nature, quarks are always found bound together in groups, and never in isolation, due to the phenomenon known as confinement, a process that is controlled by another type of particle known as a gluon, particles that can be thought of as subatomic gluing agents. Groups of quarks are called hadrons; with groups of two quarks forming a meson, and with groups of three quarks, generally, forming baryons. Various combinations of the six flavors account for all of the 200 or more known mesons and baryons.
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 u, and an up antiquark is denoted by u.
The six flavors of quarks and their most likely decay modes. Mass increases moving from left to right.
The notion of quarks evolved out of a classification of hadrons developed independently in 1964 by Murray Gell-Mann and George Zweig, which today is called 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 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. 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/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.
The word was originally coined by Murray Gell-Mann as the sound atomic ducks make, 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.
In 1963, when I assigned the name "quark" to the fundamental constituents of the nucleon, I had 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 had 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" would not be totally unjustified. In any case, the number three fitted perfectly the way quarks occur in nature.
—Murray Gell-Mann, The Quark and the Jaguar: Adventures in the Simple and the Complex
Every subatomic particle is completely described by a small set of observables such as electrical charge, mass, spin and so on. Usually these properties are directly determined by experiments and related scientific observations. However, it is in the very nature of the quark that they cannot be found in singularity. 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. Such information can then be used to determine further details about the quark.
Quarks come in six types, or "flavors". This term, "flavor", in fact has nothing at all to do with the typical human experience of flavor, but is an arbitrarily named property that takes its name from a simple everyday word that is easy to comprehend and work with. These quark flavors are given the names up, down, top, bottom, charm and strange.Only the former two flavors are actually in frequent occurrence in theuniverse, while the rest are very rare and very quickly decaying andcan only be reproduced and observed in specialized conditions. The sixflavors are differentiated by properties such as mass, spin andelectrical charge. They are grouped in pairs according to how theyinteract with the weak nuclear force, and grouped into three"generations", ordered by mass: up and down, with the least mass, formthe first generation, charm and strange the second, and top and bottomthe third. See the table at the bottom of the section for a more complete analysis.
Only the up and down varieties are in regular existence throughoutthe universe. As such, it is only these two quarks that actually bindtogether and form the hadrons, which are the particles that constitutethe atom. It is at this point in the hadronization process thatelectrical charge becomes important. An up quark carries an electricalcharge of positive value, specifically +2/3, while a down quark holdscharge of negative value, specifically –1/3. Speaking regarding theconstituents of the atom, one of the central particles in physics,inverse combinations of three quarks give the proton and neutron and their respective charges. Two up quarks and one down give a netcharge of +1, resulting in the proton, while the neutron gains itscharge of neutral 0 from a combination of two downs and an up. Variousother quark combinations result in the production of numerous hadronforms, each differing based on the meld of the properties andelectrical charges of the collective quark group.
It is important to note that for every quark flavor, there is anantiquark, resulting in a possible twelve different flavors. Much likeantimatter in general, antiquarks possess similar, but completelyinverse, properties to quarks, with every quality reversed. Antiquarksand their anti-properties become important particles when it comes tothe hadronization of two quark entities, which is explained further onin the article.
In summation, flavors represent the most basic dividers between thevarious quarks, each flavor differentiated by a set of properties thatdistinguishes it from the next. Flavor, of itself, is not a true"property" of quarks, but rather flavor exists as a term assigning toquarks a set of various other real properties. In addition, flavor ofitself does not instigate quark attraction or repulsion, flavor becomesquite insignificant outside of the charge that comes with flavor whenattraction is considered, and it is instead the property of "color"that dictates the quark attraction matrix.
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. Quarks have two spin states, spin up and spin down. The combination of color, spin and charge gives a total of 36 possible quark states.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.
Due to hadronization, quark masses cannot be measured directly and must be inferred from the effects they have on their parent hadron's properties.
There are two different terms used when describing a quark's mass; current quark mass refers to the mass of a quark by itself. On the other hand, a constituent quark refers to a current quark surrounded by a field of gluons, with constituent quark mass being used to ascribe value to these constituent quarks. These two values are typically very different in their relative size, for several reasons, as explained below.
In any one hadron, the majority of the mass comes from the gluons that bind the constituent quarks together, rather than from the actual quarks themselves; the mass of the latter is rendered almost negligible in comparison to the former. While gluons are inherently massless, they still possess ample energy, and it is this energy that contributes so greatly to the overall mass of the bound state.This is demonstrated quite clearly by a common hadron - the proton. Composed of duu, the proton has an overall mass of approximately 938 MeV, of which the three quarks contribute around 15 MeV, the remainder being filled by gluons, or, the mass their energy accounts for.
This makes the calculation of quark mass very difficult, and the complexity of the issue is compounded by the fact that quark separation is not a possibility due to the process known as "color confinement", an issue discussed in the following section. Often, mass values can be ascertained after calculating the mass differences between two related hadrons that have opposing or complimentary quark components; for example, the duu proton to the ddu neutron, where the difference between the two is one down quark to one up quark, the relative masses and the mass differences of which can then be measured by the distance in the overall mass of the two hadrons.
Quarks have an inherent and innate relationship to a type of particle referred to as a gluon, a vector gauge boson. Gluons are responsible for the generation of the color field that ensures that quarks remain bound in hadrons. Gluons are necessarily imbued with both +1 positive color charge and -1 negative anti-color charge. This results in 9 possible gluon color states, divided up into an octet of color-anticolor combinations and an extra neutral color consisting of a mix of all colors and anti-colors.
Gluons are constantly exchanged between quarks through a simple emission and reception process. These gluon exchange events between quarks are extremely frequent, occurring approximately a trillion trillion, or a septillion, times every second. When a gluon is transferred between one quark and another, a color change comes into effect in the receiving and emitting quark due to the gluon's property of possessing a +1 positive charge of one color type and a -1 negative charge of an anticolor type. These constant switches in color within quarks are mediated by the gluons in such a way that a bound hadron, whether a baryon or meson, will continually and constantly retain a dynamic and ever-changing set of color types that will preserve the force of attraction critical to the hadron binding, therefore forever disallowing quarks to exist in isolation. This particular phenomenon is referred to as color confinement, and, judging from scientific observations, is an integral function of the quark's life; all searches for quarks in isolation, or "free" quarks, have been met by failure.It is in this way, through color confinement, that gluons become very necessary components in the stable existence of a bound hadron and in the comprising quarks' properties.
The very nature of the color field the gluon creates is in itself an inherent element in the hadron's indivisible nature, in a way that is extrinsic from the infallible process of color exchange leading to attraction that the gluon mediates. This is demonstrated by the varying strength of the binding force between the constituent quarks of a hadron; as quarks come closer to each other, the binding force actually weakens, but while they drift further apart, the strength of the bind dramatically increases. This is because as the color/attraction field is stressed by the drifting away of a quark, much as an elastic band is stressed when pulled apart, a proportionate and necessary multitude of gluons of appropriate color property are born to strengthen the stretched color field. In this way, an infinite amount of energy would be required to wrench a quark from its hadronized state, as a sufficient number of resisting gluons will be spontaneously born for any other degree of force to counteract the stress.
The matrix of interactions and exchanges that occurs in a hadron model is complicated by the fact that gluons are able to engage in a process of self-exchange; that is, gluons are able to emit gluons and exchange them with another gluon. This property has led to postulations regarding the possible existence of a particle that is purely gluon—a glueball—despite previous observations indicating that gluons cannot exist without attached quarks, and that separation would be impossible anyway due to the nature of the quark-gluon field-quark connection.[t also lends possibility to the rise of hybrid mesons, which are colorless particles consisting of gluon, quark and antiquark bound together.
Some have suggested that particularly massive 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 singular in what is called a quark star. The conversion from neutron star to quark star is referred to as a "quark-nova". This event is predicted to be one of the most powerful in the universe, with calculations revealing that up to 1053 erg of energy might be released from the quark-nova process.
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