Physics Investiga tory Project Primary Unit of Elementary pa p articles.
Quar k’s
Quarks: A quark is an Elementary particle and found constituent of Matter . The quark model was independently proposed by physicists Murray Gell-Mann and Geore !wei in "#$%. &uarks combine to form composite particles called 'adrons ( Particles on which strong force acts e.g.:Protons, neutrons, pions, baryons, mesons )* the most stable of which are Protons and +eutrons* the composition of atomic nuclei. ,ue to a phenomenon known as olor connement * quarks are ne/er directly obser/ed or found in isolation0 they can be found only within hadrons* such as baryons and mesons. &uarks ha/e /arious intrinsic properties* includin electric chare* mass* color chare* and spin. &uarks are the only elementary particles in the 1tandard model of particle physics to e2perience all four fundamental interactions* also known as fundamental forces ( electromanetism* ra/itation* stron interaction* and weak interaction )* as well as the only known particles whose electric chares are not inteer multiples of the elementary chare. There are si2 types of quarks* known as 3a/ors4 up* down* strane* charm* top* and bottom. Up and down quarks ha/e the lowest masses of all quarks. The hea/ier quarks rapidly chane into up and down quarks throuh a process of particle decay 4 the transformation from a hiher mass state to a lower mass state. 5ecause of this* up and down quarks
are enerally stable and the most common in the uni/erse* whereas strane* charm* bottom* and top quarks can only be produced in hih enery collisions (such as those in/ol/in cosmic rays and in particle accelerators ). 6or e/ery quark 3a/or there is a correspondin type of antiparticle* known as an antiquark* that di7ers from the quark only in that some of its properties ha/e equal manitude but opposite sin.
&uarks are 8 spin particles* implyin that they are fermions accordin to the spin - statistics theorem. They are sub9ect to the Pauli E2clusion Principle* which states that no two identical fermions can simultaneously occupy the same quantum state. This is in contrast to bosons* any number of which can be in same state. Unlike leptons* quarks possess color chare* which causes them to enae in the stron interaction. The resultin attraction between di7erent quarks causes the formation of composite particles known as hadrons. The quarks which determine the quantum numbers of hadrons are called /alance quarks0 apart from these* any hadron can contain an indenite number of /irtual quarks* antiquarks* and luons which do not in3uence its quantum numbers. There are two families of hadrons4 baryons* with three /alence quarks* and mesons* with a /alence quark and an antiquark. The most common baryons are the proton and the neutron* the buildin blocks of the atomic nucleus. A reat number of hadrons are known* most of them di7erentiated by their quark content and the properties these constituent quarks confer. The e2istence of :e2otic: hadrons with more /alence quarks* such as tetraquarks (qqqq) and pentaquarks (qqqqq)* has been con9ectured but not pro/en. 'owe/er* on ";
"?* the @'b collaboration at E+ reported results consistent with pentaquark states.
Elementary fermions are rouped into three enerations* each comprisin two leptons and two quarks. The rst eneration includes up and down quarks* the second strane and charm quarks* and the third bottom and top quarks. All searches for a fourth eneration of quarks and other elementary fermions ha/e failed* and there is stron indirect e/idence that no more than three enerations e2ist. Particles in hiher enerations enerally ha/e reater mass and less stability* causin them to decay into lowereneration particles by means of weak interactions. Bnly rst-eneration (up and down) quarks occur commonly in nature. 'ea/ier quarks can only be created in hih-enery collisions (such as in those in/ol/in cosmic rays)* and decay quickly0 howe/er* they are thouht to ha/e been present durin the rst fractions of a second after the 5i 5an* when the uni/erse was in an e2tremely hot and dense phase (the quark epoch). 1tudies of hea/ier quarks are conducted in articially created conditions* such as in particle accelerators. 'a/in electric chare* mass* color chare* and 3a/or* quarks are the only known elementary particles that enae in all four fundamental interactions of contemporary physics4 electromanetism* ra/itation* stron interaction* and weak interaction. Gra/itation is too weak to be rele/ant to indi/idual particle interactions e2cept at e2tremes of enery (Planck enery) and distance scales (Planck
distance). 'owe/er* since no successful quantum theory of ra/ity e2ists* ra/itation is not described by the 1tandard Model.
I!"#$% 4The quark model was proposed by physicists &urray 'ell&ann and 'eorge (weig in "#$%. At the time of the &uark theoryCs inception* the )particle *oo+ included* amonst other particles* a multitue of arons . Gell-Mann and !wei posited that they were not elementary particles* but were instead composed of combinations of quarks and antiquarks. Their model in/oked three avors of quarks* up* own* and strange* to which they ascribed properties such as spin and electric chare. There was particular contention about whether the quark was a physical entity or a mere abstraction used to e2plain concepts that were not fully understood at the time. Dn less than a year* e2tensions to the Gell-Mann-!wei model were proposed. 1heldon @ee Glashow and
known leptons* and implied a mass formula that correctly reproduced the masses of the known mesons. Dn "#$F* deep inelastic scatterin e2periments at the !tanfor 0inear 1ccelerator center ( !01 ) showed that the proton contained much smaller* point-like ob9ects and was therefore not an elementary particle. Physicists were reluctant to rmly identify these ob9ects with quarks at the time* instead callin them partons+-a term i/en by ichard 6eynman. The ob9ects were later identied as up and own quarks as the other 3a/ors were disco/ered. +e/ertheless*+parton+ remains in use as a collecti/e term for the constituents of hadrons (quarks* antiquarks* and luons). The strane quarkCs e2istence was indirectly /alidated by 1@ACs scatterin e2periments4 not only necessary component of Gell-Mann and !weiCs three-quark model* but it pro/ided an e2planation for the kaon ( 2 ) and pion ( 3 ) hadrons disco/ered in cosmic rays in "#%H. Dn "#H> paper* Glashow*
harm quarks were produced almost simultaneously by two teams in +o/ember "#H%. Bne at !01 under 4urton
$ichter * and one at 4rookhaven 5ational 0aboratory under !amuel "ing. The charm quarks were obser/ed bound with charm antiquarks in mesons. The two parties had assined the disco/ered meson two di7erent symbols* 6 and 0 thus* it became formally known as the 67 meson. Dn followin years a number of suestions appeared for e2tendin the quark model to si2 quarks. Bf these* in "#H? paper by 'aim 'arari was the rst to coin the terms top and bottom for the additional quarks. Dn "#HH* the bottom quark was obser/ed by a team at 6ermilab led by 0eon 0eerman. This was a stron indicator of the top quarkCs e2istence. Dt was in "##? that top quark was nally obser/ed* also by the 89 and 8 teams at 6ermilab. Dt had a mass much larer than had been pre/iously e2pected* almost as larer as that of a Gold Atom.
tymology 46or some time* Gell-Mann was undecided on an actual spellin for the term he intended to coin* until he found the word Q;1$2 in
"hree Q;1$2! for muster &ark= Gell-Mann went into further detail reardin the name quark in his book "he Quark an the 6aguar.
Dn "#$; he assined the name &uark to the fundamental constituent of 5ucleon. !wei preferred the name ace for the particle he had theoried* but Gell-MannCs terminoloy came to prominence once the quark model had been commonly accepted. The quark 3a/ors were i/en their names for se/eral reasons. The up and own quarks are named after the up and down components of isospin* which they carry. !trange quarks were i/en their names because they were disco/ered to be components of the strane particles disco/ered in cosmic rays years before the quark model was proposed0 these particles were deemed strange+ because they had unusually lon lifetimes. Glashow* who co proposed harm quark with 59orken* is quoted as sayin*I Je called our construct the >charme /uark’ * for we were fascinated and pleased by the symmetry it brouht to the !ub-nuclear worl I. The names )bottom+ and )top+* coined by 'arari* were chosen because they are loical partners for up and down quarksI. Dn the past* bottom and top quarks were sometimes referred to as )beauty+ and )truth+ respecti/ely. Therefore accelerator comple2es de/oted to massi/e production of bottom quarks are sometimes called )beauty factories+ .