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c rays years before the quark model was proposed these particles were deemed strange because they had unusually long lifetimes 61 glashow who co proposed the charm quark with bjorken is quoted as saying we called our construct the charmed quark for we were fascinated and pleased by the symmetry it brought to the subnuclear world 62 the names top and bottom coined by harari were chosen because they are logical partners for up and down quarks 41 42 61 alternative names for top and bottom quarks are truth and beauty nb 4 but these names have somewhat fallen out of use 66 while truth never did catch on accelerator complexes devoted to massive production of bottom quarks are sometimes called beauty factories 67 properties edit electric charge edit see also electric charge quarks have fractional electric charge values either 1 3 or 2 3 times the elementary charge e depending on flavor up charm and top quarks collectively referred to as up type quarks have a charge of 2 3 e down strange and bottom quarks down type quarks have a charge of 1 3 e antiquarks have the opposite charge to their corresponding quarks up type antiquarks have charges of 2 3 e and down type antiquarks have charges of 1 3 e since the electric charge of a hadron is the sum of the charges of the constituent quarks all hadrons have integer charges the combination of three quarks baryons three antiquarks antibaryons or a quark and an antiquark mesons always results in integer charges 68 for example the hadron constituents of atomic nuclei neutrons and protons have charges of 0 e and 1 e respectively the neutron is composed of two down quarks and one up quark and the proton of two up quarks and one down quark 12 spin edit see also spin physics 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 69 spin can be represented by a vector whose length is measured in units of the reduced planck constant ħ pronounced h bar for quarks a measurement of the spin vector component along any axis can only yield the values ħ 2 or ħ 2 for this reason quarks are classified as spin 1 2 particles 70 the component of spin along a given axis by convention the z axis is often denoted by an up arrow for the value 1 2 and a down arrow for the value 1 2 placed after the symbol for flavor for example an up quark with a spin of 1 2 along the z axis is denoted by u 71 weak interaction edit main article weak interaction feynman diagram of beta decay of a neutron into proton electron and electron antineutrino via a virtual w boson time is flowing upwards the ckm matrix discussed below encodes the probability of this and other quark decays 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 this flavor transformation mechanism causes the radioactive process of beta decay in which a neutron n splits into a proton p an electron e and an electron antineutrino ν e see picture this occurs when one of the down quarks in the neutron u d d decays into an up quark by emitting a virtual w boson transforming the neutron into a proton u u d the w boson then decays into an electron and an electron antineutrino 72 n p e ν e beta decay hadron notation u d d u u d e ν e beta decay quark notation 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 the strengths of the weak interactions between the six quarks the intensities of the lines are determined by the elements of the ckm matrix 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 73 v u d v u s v u b v c d v c s v c b v t d v t s v t b 0 974 0 225 0 003 0 225 0 973 0 041 0 009 0 040 0 999 displaystyle begin bmatrix v_ mathrm ud v_ mathrm us v_ mathrm ub v_ mathrm cd v_ mathrm cs v_ mathrm cb v_ mathrm td v_ mathrm ts v_ mathrm tb end bmatrix approx begin bmatrix 0 974 0 225 0 003 0 225 0 973 0 041 0 009 0 040 0 999 end bmatrix where v ij represents the tendency of a quark of flavor i to change into a quark of flavor j or vice versa nb 5 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 74 together the ckm and pmns matrices describe all flavor transformations but the links between the two are not yet clear 75 strong interaction and color charge edit see also color charge and strong interaction all types of hadrons have zero total color charge the pattern of strong charges for the three colors of quark three antiquarks and eight gluons with two of zero charge overlapping 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 nb 6 each of them is complemented by an anticolor antiblue antigreen and antired every quark carries a color while every antiquark carries an anticolor 76 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 a quark with color charge ξ plus an antiquark with color charge ξ will result in a color charge of 0 or white color and the formation of a meson 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 different anticolor charges will result in the same white color charge and the formation of a baryon or antibaryon 77 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 78 just as the laws of physics are independent of which directions in space are designated x y and z and remain unchanged if the coordinate axes are rotated to a new orientation the physics of quantum chromodynamics is independent of which directions in three dimensional color space are identified as blue red and green 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 79 forms a triplet a three component quantum field that transforms under the fundamental representation of su 3 c 80 the requirement that su 3 c should be local that is that its transformations be allowed to vary with space and time determines the properties of the strong interaction in particular it implies the existence of eight gluon types to act as its force carriers 78 81 mass edit current quark masses for all six flavors in comparison as balls of proportional volumes proton gray and electron red are shown in bottom left corner for scale see also invariant mass two terms are used in referring to a quark s mass current quark mass refers to the mass of a quark by itself while constituent quark mass refers to the current quark mass plus the mass of the gluon particle field surrounding the quark 82 these masses typically have very different values 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 for example a proton has a mass of approximately 938 mev c 2 of which the rest mass of its three valence quarks only contributes about 9 mev c 2 much of the remainder can be attributed to the field energy of the gluons 83 84 see chiral symmetry breaking the standard model posits that elementary particles derive their masses from the higgs mechanism which is associated to the higgs boson it is hoped that further research into the reasons for the top quark s large mass of 173 gev c 2 almost the mass of a gold atom 83 85 might reveal more about the origin of the mass of quarks and other elementary particles 86 size edit in qcd quarks are considered to be point like entities with no structure as of 2014 experimental evidence indicates they have no structure greater than 10 4 times the size of a proton i e less than 10 19 m 87 table of properties edit see also flavour particle physics the following table summarizes the key properties of the six quarks flavor quantum numbers isospin i 3 charm c strangeness s not to be confused with spin topness t and bottomness b are assigned to certain quark flavors and denote qualities of quark based systems and hadrons the baryon number b is 1 3 for all quarks as baryons are made of three quarks for antiquarks the electric charge q and all flavor quantum numbers b i 3 c s t and b are of opposite sign mass and total angular momentum j equal to spin for point particles do not change sign for the antiquarks quark flavor properties 83 particle mass mev c 2 j ħ b q e i 3 c s t b antiparticle name symbol name symbol first generation up u 2 3 0 7 0 5 1 2 1 3 2 3 1 2 0 0 0 0 antiup u down d 4 8 0 5 0 3 1 2 1 3 1 3 1 2 0 0 0 0 antidown d second generation charm c 1275 25 1 2 1 3 2 3 0 1 0 0 0 anticharm c strange s 95 5 1 2 1 3 1 3 0 0 1 0 0 antistrange s third generation top t 173 210 510 710 1 2 1 3 2 3 0 0 0 1 0 antitop t bottom b 4180 30 1 2 1 3 1 3 0 0 0 0 1 antibottom b j total angular momentum b baryon number q electric charge i 3 isospin c charm s strangeness t topness b bottomness notation such as 173 210 510 710 in the case of the top quark denotes two types of measurement uncertainty the first uncertainty is statistical in nature and the second is systematic interacting quarks edit see also color confinement and gluon 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 88 89 90 since gluons carry color charge they themselves are able to emit and absorb other gluons this causes asymptotic freedom as quarks come closer to each other the chromodynamic binding force between them weakens 91 conversely as the distance between quarks increases the binding force strengthens 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 quarks never appear in isolation 92 93 this process of hadronization occurs before quarks formed in a high energy collision are able to interact in any other way the only exception is the top quark which may decay before it hadronizes 94 sea quarks edit 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 95 sea quarks are much less stable than their valence counterparts and they typically annihilate each other within the interior of the hadron despite this sea quarks can hadronize into baryonic or mesonic particles under certain circumstances 96 other phases of quark matter edit main article qcd matter a qualitative rendering of the phase diagram of quark matter the precise details of the diagram are the subject of ongoing research 97 98 under sufficiently extreme conditions quarks may become deconfined out of bound states and propagate as thermalized free excitations in the larger medium in the course of asymptotic freedom the strong interaction becomes weaker at increasing temperatures eventually color confinement would be effectively lost in an extremely hot plasma of freely moving quarks and gluons this theoretical phase of matter is called quark gluon plasma 99 the exact conditions needed to give rise to this state are unknown and have been the subject of a great deal of speculation and experimentation an estimate puts the needed temperature at 1 90 0 02 10 12 kelvin 100 while a state of entirely free quarks and gluons has never been achieved despite numerous attempts by cern in the 1980s and 1990s 101 recent experiments at the relativistic heavy ion collider have yielded evidence for liquid like quark matter exhibiting nearly perfect fluid motion 102 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 it is believed that in the period prior to 10 6 seconds after the big bang the quark epoch the universe was filled with quark gluon plasma as the temperature was too high for hadrons to be stable 103 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...
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