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formation of composite particles known as hadrons see strong interaction and color charge below the quarks that determine the quantum numbers of hadrons are called valence quarks apart from these any hadron may contain an indefinite number of virtual sea quarks antiquarks and gluons which do not influence its quantum numbers 10 there are two families of hadrons baryons with three valence quarks and mesons with a valence quark and an antiquark 11 the most common baryons are the proton and the neutron the building blocks of the atomic nucleus 12 a great number of hadrons are known see list of baryons and list of mesons most of them differentiated by their quark content and the properties these constituent quarks confer the existence of exotic hadrons with more valence quarks such as tetraquarks q q q q and pentaquarks q q q q q was conjectured from the beginnings of the quark model 13 but not discovered until the early 21st century 14 15 16 17 elementary fermions are grouped into three generations each comprising two leptons and two quarks the first generation includes up and down quarks the second strange and charm quarks and the third bottom and top quarks all searches for a fourth generation of quarks and other elementary fermions have failed 18 19 and there is strong indirect evidence that no more than three generations exist nb 2 20 21 22 particles in higher generations generally have greater mass and less stability causing them to decay into lower generation particles by means of weak interactions 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 23 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 electromagnetism gravitation strong interaction and weak interaction 12 gravitation is too weak to be relevant to individual particle interactions except at extremes of energy planck energy and distance scales planck distance 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 history edit murray gell mann 2007 george zweig 2015 the quark model was independently proposed by physicists murray gell mann 24 and george zweig 25 26 in 1964 5 the proposal came shortly after gell mann s 1961 formulation of a particle classification system known as the eightfold way or in more technical terms su 3 flavor symmetry streamlining its structure 27 physicist yuval ne eman had independently developed a scheme similar to the eightfold way in the same year 28 29 an early attempt at constituent organization was available in the sakata model at the time of the quark theory s inception the particle zoo included a multitude of hadrons among other particles 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 24 25 26 the initial reaction of the physics community to the proposal was mixed 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 30 in less than a year extensions to the gell mann zweig model were proposed sheldon 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 31 deep inelastic scattering experiments conducted in 1968 at the stanford linear accelerator center slac and published on october 20 1969 showed that the proton contained much smaller point like objects and was therefore not an elementary particle 6 7 32 physicists were reluctant to firmly identify these objects with quarks at the time instead calling them partons a term coined by richard feynman 33 34 35 the objects that were observed at slac would later be identified as up and down quarks as the other flavors were discovered 36 nevertheless parton remains in use as a collective term for the constituents of hadrons quarks antiquarks and gluons richard taylor henry kendall and jerome friedman received the 1990 nobel prize in physics for their work at slac photograph of the event that led to the discovery of the σ c baryon at the brookhaven national laboratory in 1974 the strange quark s existence was indirectly validated by slac s scattering experiments not only was it a necessary component of gell mann and zweig s three quark model but it provided an explanation for the kaon k and pion π hadrons discovered in cosmic rays in 1947 37 in a 1970 paper glashow john iliopoulos and luciano maiani presented the gim mechanism named from their initials to explain the experimental non observation of flavor changing neutral currents this theoretical model required the existence of the as yet undiscovered charm quark 38 39 the number of supposed quark flavors grew to the current six in 1973 when makoto kobayashi and toshihide maskawa noted that the experimental observation of cp violation nb 3 40 could be explained if there were another pair of quarks charm quarks were produced almost simultaneously by two teams in november 1974 see november revolution one at slac under burton richter and one at brookhaven national laboratory under samuel ting the charm quarks were observed bound with charm antiquarks in mesons the two parties had assigned the discovered meson two different symbols j and ψ thus it became formally known as the j ψ meson the discovery finally convinced the physics community of the quark model s validity 35 in the following years a number of suggestions appeared for extending the quark model to six quarks of these the 1975 paper by haim harari 41 was the first to coin the terms top and bottom for the additional quarks 42 in 1977 the bottom quark was observed by a team at fermilab led by leon lederman 43 44 this was a strong indicator of the top quark s existence without the top quark the bottom quark would have been without a partner it was not until 1995 that the top quark was finally observed also by the cdf 45 and dø 46 teams at fermilab 5 it had a mass much larger than expected 47 almost as large as that of a gold atom 48 etymology edit 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 1939 book finnegans wake 49 three quarks for muster mark sure he hasn t got much of a bark and sure any he has it s all beside the mark the word quark is an old english word meaning to croak 50 and the above quoted lines are about a bird choir mocking king mark of cornwall in the legend of tristan and iseult 51 especially in the german speaking parts of the world there is a widespread legend however that joyce had taken it from the word quark 52 a german word of slavic origin which denotes a curd cheese 53 but is also a colloquial term for trivial nonsense 54 in the legend it is said that he had heard it on a journey to germany at a farmers market in freiburg 55 56 some authors however defend a possible german origin of joyce s word quark 57 gell mann went into further detail regarding the name of the quark in his 1994 book the quark and the jaguar 58 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 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 59 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 60 strange quarks were given their name because they were discovered to be components of the strange particles discovered in cosmic 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 a...
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