Above a certain energy threshold, pairs of quarks and antiquarks are created. The Quark and the Jaguar: Two terms are used in referring to a quark's mass:
All searches for a fourth generation of quarks and other elementary fermions have failed,   and there is strong indirect evidence that no more than three generations exist. Only first-generation up and down quarks occur commonly in nature. Heavier quarks can only be created in high-energy collisions such as in those involving cosmic rays , and decay quickly; however, they are thought to have been present during the first fractions of a second after the Big Bang , when the universe was in an extremely hot and dense phase the quark epoch.
Studies of heavier quarks are conducted in artificially created conditions, such as in particle accelerators. Having electric charge, mass, color charge, and flavor, quarks are the only known elementary particles that engage in all four fundamental interactions of contemporary physics: However, since no successful quantum theory of gravity exists, gravitation is not described by the Standard Model.
See the table of properties below for a more complete overview of the six quark flavors' properties. The quark model was independently proposed by physicists Murray Gell-Mann  and George Zweig   in At the time of the quark theory's inception, the " particle zoo " included, amongst other particles, a multitude of hadrons.
Gell-Mann and Zweig posited that they were not elementary particles, but were instead composed of combinations of quarks and antiquarks. Their model involved three flavors of quarks, up , down , and strange , to which they ascribed properties such as spin and electric charge. There was particular contention about whether the quark was a physical entity or a mere abstraction used to explain concepts that were not fully understood at the time.
In less than a year, extensions to the Gell-Mann—Zweig model were proposed. Sheldon Lee Glashow and James Bjorken predicted the existence of a fourth flavor of quark, which they called charm.
The addition was proposed because it allowed for a better description of the weak interaction the mechanism that allows quarks to decay , equalized the number of known quarks with the number of known leptons , and implied a mass formula that correctly reproduced the masses of the known mesons. In , deep inelastic scattering experiments at the Stanford Linear Accelerator Center SLAC showed that the proton contained much smaller, point-like objects and was therefore not an elementary particle.
The strange quark's existence was indirectly validated by SLAC's scattering experiments: In a paper, Glashow, John Iliopoulos and Luciano Maiani presented the so-called GIM mechanism to explain the experimental non-observation of flavor-changing neutral currents.
This theoretical model required the existence of the as-yet undiscovered charm quark. The charm quarks were observed bound with charm antiquarks in mesons. The discovery finally convinced the physics community of the quark model's validity. In the following years a number of suggestions appeared for extending the quark model to six quarks. Of these, the paper by Haim Harari  was the first to coin the terms top and bottom for the additional quarks. In , the bottom quark was observed by a team at Fermilab led by Leon Lederman.
For some time, Gell-Mann was undecided on an actual spelling for the term he intended to coin, until he found the word quark in James Joyce 's book Finnegans Wake: Sure he hasn't got much of a bark And sure any he has it's all beside the mark.
In , 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.
Zweig preferred the name ace for the particle he had theorized, but Gell-Mann's terminology came to prominence once the quark model had been commonly accepted. The quark flavors were given their names for several reasons. The up and down quarks are named after the up and down components of isospin , which they carry. Since the electric charge of a hadron is the sum of the charges of the constituent quarks, all hadrons have integer charges: Spin is an intrinsic property of elementary particles, and its direction is an important degree of freedom.
It is sometimes visualized as the rotation of an object around its own axis hence the name " spin " , though this notion is somewhat misguided at subatomic scales because elementary particles are believed to be point-like. A quark of one flavor can transform into a quark of another flavor only through the weak interaction, one of the four fundamental interactions in particle physics. By absorbing or emitting a W boson , any up-type quark up, charm, and top quarks can change into any down-type quark down, strange, and bottom quarks and vice versa.
Both beta decay and the inverse process of inverse beta decay are routinely used in medical applications such as positron emission tomography PET and in experiments involving neutrino detection. While the process of flavor transformation is the same for all quarks, each quark has a preference to transform into the quark of its own generation. The relative tendencies of all flavor transformations are described by a mathematical table , called the Cabibbo—Kobayashi—Maskawa matrix CKM matrix.
Enforcing unitarity , the approximate magnitudes of the entries of the CKM matrix are: There exists an equivalent weak interaction matrix for leptons right side of the W boson on the above beta decay diagram , called the Pontecorvo—Maki—Nakagawa—Sakata matrix PMNS matrix. According to quantum chromodynamics QCD , quarks possess a property called color charge.
There are three types of color charge, arbitrarily labeled blue , green , and red. Every quark carries a color, while every antiquark carries an anticolor. The system of attraction and repulsion between quarks charged with different combinations of the three colors is called strong interaction , which is mediated by force carrying particles known as gluons ; this is discussed at length below.
The theory that describes strong interactions is called quantum chromodynamics QCD. A quark, which will have a single color value, can form a bound system with an antiquark carrying the corresponding anticolor. The result of two attracting quarks will be color neutrality: This is analogous to the additive color model in basic optics.
Similarly, the combination of three quarks, each with different color charges, or three antiquarks, each with anticolor charges, will result in the same "white" color charge and the formation of a baryon or antibaryon.
In modern particle physics, gauge symmetries — a kind of symmetry group — relate interactions between particles see gauge theories. Color SU 3 commonly abbreviated to SU 3 c is the gauge symmetry that relates the color charge in quarks and is the defining symmetry for quantum chromodynamics. SU 3 c color transformations correspond to "rotations" in color space which, mathematically speaking, is a complex space.
Every quark flavor f , each with subtypes f B , f G , f R corresponding to the quark colors,  forms a triplet: In particular, it implies the existence of eight gluon types to act as its force carriers. Two terms are used in referring to a quark's mass: Most of a hadron's mass comes from the gluons that bind the constituent quarks together, rather than from the quarks themselves. While gluons are inherently massless, they possess energy — more specifically, quantum chromodynamics binding energy QCBE — and it is this that contributes so greatly to the overall mass of the hadron see mass in special relativity.
The Standard Model posits that elementary particles derive their masses from the Higgs mechanism , which is associated to the Higgs boson. The following table summarizes the key properties of the six quarks. Mass and total angular momentum J ; equal to spin for point particles do not change sign for the antiquarks. As described by quantum chromodynamics , the strong interaction between quarks is mediated by gluons, massless vector gauge bosons.
Each gluon carries one color charge and one anticolor charge. In the standard framework of particle interactions part of a more general formulation known as perturbation theory , gluons are constantly exchanged between quarks through a virtual emission and absorption process. When a gluon is transferred between quarks, a color change occurs in both; for example, if a red quark emits a red—antigreen gluon, it becomes green, and if a green quark absorbs a red—antigreen gluon, it becomes red.
Therefore, while each quark's color constantly changes, their strong interaction is preserved. Since gluons carry color charge, they themselves are able to emit and absorb other gluons.
This causes asymptotic freedom: The color field becomes stressed, much as an elastic band is stressed when stretched, and more gluons of appropriate color are spontaneously created to strengthen the field. Above a certain energy threshold, pairs of quarks and antiquarks are created. These pairs bind with the quarks being separated, causing new hadrons to form. This phenomenon is known as color confinement: The only exception is the top quark, which may decay before it hadronizes.
Hadrons contain, along with the valence quarks q v that contribute to their quantum numbers , virtual quark—antiquark q q pairs known as sea quarks q s. Sea quarks form when a gluon of the hadron's color field splits; this process also works in reverse in that the annihilation of two sea quarks produces a gluon.
The result is a constant flux of gluon splits and creations colloquially known as "the sea". Despite this, sea quarks can hadronize into baryonic or mesonic particles under certain circumstances. Under sufficiently extreme conditions, quarks may become deconfined and exist as free particles. In the course of asymptotic freedom , the strong interaction becomes weaker at higher temperatures. Eventually, color confinement would be lost and an extremely hot plasma of freely moving quarks and gluons would be formed.
This theoretical phase of matter is called quark—gluon plasma. The quark—gluon plasma would be characterized by a great increase in the number of heavier quark pairs in relation to the number of up and down quark pairs.
Given sufficiently high baryon densities and relatively low temperatures — possibly comparable to those found in neutron stars — quark matter is expected to degenerate into a Fermi liquid of weakly interacting quarks. This liquid would be characterized by a condensation of colored quark Cooper pairs , thereby breaking the local SU 3 c symmetry.
Because quark Cooper pairs harbor color charge, such a phase of quark matter would be color superconductive ; that is, color charge would be able to pass through it with no resistance.
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