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Down quark
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{{short description|Type of quark}} {{redirect|Antidown|the EP by Burial|Antidawn{{!}}''Antidawn''}} {{Infobox Particle |bgcolour = |name = Down quark |image = |caption = |num_types = |composition = [[elementary particle]] |statistics = [[fermionic]] |group = [[quark]] |generation = first |interaction = [[Strong interaction|strong]], [[Weak interaction|weak]], [[Electromagnetic interaction|electromagnetic]], [[gravity]] |particle = |antiparticle = down antiquark ({{SubatomicParticle|down antiquark}}) |theorized = [[Murray Gell-Mann]] (1964)<br/>[[George Zweig]] (1964) |discovered = [[SLAC]] (1968) |symbol = {{SubatomicParticle|Down quark}} |mass = {{val|4.7|+0.5|-0.3|ul=MeV/c2}}<ref name="PDG2018"> {{cite journal |author=M. Tanabashi et al. (Particle Data Group) |title=Review of Particle Physics |year= 2018 |doi=10.1103/PhysRevD.98.030001 |volume=98 |issue=3 |pages=1–708 |journal=Physical Review D |pmid=10020536 |url=http://pdglive.lbl.gov/DataBlock.action?node=Q123DM |doi-access=free |bibcode=2018PhRvD..98c0001T |hdl=10044/1/68623 |hdl-access=free }}</ref> |decay_time = |decay_particle = stable or [[up quark]] + [[electron]] + [[electron antineutrino]] |electric_charge = −{{sfrac|1|3}} [[Elementary charge|''e'']] |color_charge = Yes |spin = {{sfrac|1|2}} [[reduced Planck constant|''ħ'']] |num_spin_states = |weak_isospin = {{nowrap|[[Chirality (physics)|LH]]: −{{sfrac|1|2}}, [[Chirality (physics)|RH]]: 0}} |weak_hypercharge= {{nowrap|[[Chirality (physics)|LH]]: +{{sfrac|1|3}}, [[Chirality (physics)|RH]]: −{{sfrac|2|3}}}} }} The '''down quark''' (symbol: d) is a type of [[elementary particle]], and a major constituent of [[matter]]. The down quark is the second-lightest of all [[quark]]s, and combines with other quarks to form [[composite particle]]s called [[hadrons]]. Down quarks are most commonly found in [[atomic nucleus|atomic nuclei]], where it combines with [[up quark]]s to form [[proton]]s and [[neutron]]s. The proton is made of one down quark with two up quarks, and the neutron is made up of two down quarks with one up quark. Because they are found in every single known atom, down quarks are present in all everyday matter that we interact with. The down quark is part of the [[generation (physics)|first generation]] of matter, has an [[electric charge]] of −{{sfrac|1|3}} [[elementary charge|''e'']] and a [[Quark#Mass|bare mass]] of {{val|4.7|+0.5|-0.3|ul=MeV/c2}}.<ref name="PDG2018"/> Like all quarks, the down quark is an elementary [[fermion]] with [[Spin (physics)|spin]] [[spin-1/2|{{sfrac|1|2}}]], and experiences all four [[fundamental interaction]]s: [[gravitation]], [[electromagnetism]], [[weak interaction]]s, and [[strong interaction]]s. The [[antiparticle]] of the down quark is the '''down antiquark''' (sometimes called ''antidown quark'' or simply ''antidown''), which differs from it only in that some of its properties have [[additive inverse|equal magnitude but opposite sign]]. Its existence (along with that of the up and [[strange quark]]s) was postulated in 1964 by [[Murray Gell-Mann]] and [[George Zweig]] to explain the [[Eightfold Way (physics)|Eightfold Way]] classification scheme of [[hadron]]s. The down quark was first observed by experiments at the [[Stanford Linear Accelerator Center]] in 1968. == History == [[File:Murray Gell-Mann at Lection.JPG|thumb|200px|[[Murray Gell-Mann]]]] [[File:George Zweig.jpg|thumb|200px|[[George Zweig]]]] In the beginnings of particle physics (first half of the 20th century), [[hadron]]s such as [[proton]]s, [[neutron]]s, and [[pion]]s were thought to be [[elementary particle]]s. However, as new hadrons were discovered, the '[[particle zoo]]' grew from a few particles in the early 1930s and 1940s to several dozens of them in the 1950s. The relationships between each of them was unclear until 1961, when [[Murray Gell-Mann]]<ref> {{cite book |author=M. Gell-Mann |year=2000 |orig-year=1964 |chapter=The Eightfold Way: A theory of strong interaction symmetry |editor=M. Gell-Mann, Y. Ne'eman |title=The Eightfold Way |page=11 |publisher=[[Westview Press]] |isbn=978-0-7382-0299-0 }}<br/> Original: {{cite journal |author=M. Gell-Mann |year=1961 |title=The Eightfold Way: A theory of strong interaction symmetry |journal=Synchrotron Laboratory Report CTSL-20 |publisher=[[California Institute of Technology]] }}</ref> and [[Yuval Ne'eman]]<ref> {{cite book |author=Y. Ne'eman |year=2000 |orig-year=1964 |chapter=Derivation of strong interactions from gauge invariance |editor=M. Gell-Mann, Y. Ne'eman |title=The Eightfold Way |publisher=[[Westview Press]] |isbn=978-0-7382-0299-0 }}<br/>Original {{cite journal |author=Y. Ne'eman |year=1961 |title=Derivation of strong interactions from gauge invariance |journal=[[Nuclear Physics (journal)|Nuclear Physics]] |volume=26 |pages=222–229 |doi=10.1016/0029-5582(61)90134-1 |bibcode = 1961NucPh..26..222N |issue=2 }}</ref> (independently of each other) proposed a hadron classification scheme called the [[Eightfold Way (physics)|Eightfold Way]], or in more technical terms, [[SU(3)]] [[flavor symmetry]]. This classification scheme organized the hadrons into [[isospin|isospin multiplet]]s, but the physical basis behind it was still unclear. In 1964, Gell-Mann<ref name="Gell-Man1964"> {{cite journal |author=M. Gell-Mann |title=A Schematic Model of Baryons and Mesons |journal=[[Physics Letters]] |volume=8 |issue=3 |pages=214–215 |year=1964 |doi=10.1016/S0031-9163(64)92001-3 |bibcode=1964PhL.....8..214G }}</ref> and [[George Zweig]]<ref name="Zweig1964a"> {{cite journal |author=G. Zweig |title=An SU(3) Model for Strong Interaction Symmetry and its Breaking |journal=CERN Report No.8181/Th 8419 |year=1964 }}</ref><ref name="Zweig1964b"> {{cite journal |author=G. Zweig |title=An SU(3) Model for Strong Interaction Symmetry and its Breaking: II |journal=CERN Report No.8419/Th 8412 |year=1964 }}</ref> (independently of each other) proposed the [[quark model]], then consisting only of [[up quark|up]], down, and [[strange quark|strange]] quarks.<ref name="Carithers"> {{cite journal |author=B. Carithers, P. Grannis |title=Discovery of the Top Quark |url=http://www.slac.stanford.edu/pubs/beamline/25/3/25-3-carithers.pdf |journal=[[Beam Line (journal)|Beam Line]] |volume=25 |issue=3 |pages=4–16 |year=1995 |access-date=2008-09-23 }}</ref> However, while the quark model explained the Eightfold Way, no direct evidence of the existence of quarks was found until 1968 at the [[Stanford Linear Accelerator Center]].<ref name="Bloom"> {{cite journal |author=E. D. Bloom|title=High-Energy Inelastic ''e''–''p'' Scattering at 6° and 10° |journal=[[Physical Review Letters]] |volume=23 |issue=16 |pages=930–934 |year=1969 |doi=10.1103/PhysRevLett.23.930 |bibcode=1969PhRvL..23..930B |display-authors=etal |doi-access=free }}</ref><ref name="Breidenbach"> {{cite journal |author=M. Breidenbach |s2cid=2575595 |title=Observed Behavior of Highly Inelastic Electron–Proton Scattering |journal=[[Physical Review Letters]] |volume=23 |issue=16 |pages=935–939 |year=1969 |doi=10.1103/PhysRevLett.23.935 |bibcode=1969PhRvL..23..935B |osti=1444731 |display-authors=etal|url=https://www.slac.stanford.edu/cgi-bin/getdoc/slac-pub-0650.pdf }}</ref> [[Deep inelastic scattering]] experiments indicated that protons had substructure, and that protons made of three more-fundamental particles explained the data (thus confirming the quark model).<ref> {{cite web |author = J. I. Friedman |title = The Road to the Nobel Prize |url = http://www.hueuni.edu.vn/hueuni/en/news_detail.php?NewsID=1606&PHPSESSID=909807ffc5b9c0288cc8d137ff063c72 |publisher = [[Hue University]] |access-date = 2008-09-29 |url-status = dead |archive-url = https://web.archive.org/web/20081225093044/http://www.hueuni.edu.vn/hueuni/en/news_detail.php?NewsID=1606&PHPSESSID=909807ffc5b9c0288cc8d137ff063c72 |archive-date = 2008-12-25 }}</ref> At first people were reluctant to identify the three-bodies as quarks, instead preferring [[Richard Feynman]]'s [[parton (particle physics)|parton]] description,<ref> {{cite journal |author=R. P. Feynman |title=Very High-Energy Collisions of Hadrons |journal=[[Physical Review Letters]] |volume=23 |issue=24 |pages=1415–1417 |year=1969 |doi=10.1103/PhysRevLett.23.1415 |bibcode=1969PhRvL..23.1415F |url=https://authors.library.caltech.edu/3871/1/FEYprl69.pdf }}</ref><ref> {{cite journal |author=S. Kretzer |author2=H. Lai |author3=F. Olness |author4=W. Tung |title=CTEQ6 Parton Distributions with Heavy Quark Mass Effects |journal=[[Physical Review D]] |volume=69 |issue=11 |page=114005 |year=2004 |arxiv=hep-ph/0307022 |doi=10.1103/PhysRevD.69.114005 |bibcode = 2004PhRvD..69k4005K |s2cid=119379329 }}</ref><ref name="Griffiths"> {{cite book |author=D. J. Griffiths |title=Introduction to Elementary Particles |url=https://archive.org/details/introductiontoel00grif_077 |url-access=limited |page=[https://archive.org/details/introductiontoel00grif_077/page/n49 42] |publisher=[[John Wiley & Sons]] |year=1987 |isbn=978-0-471-60386-3 }}</ref> but over time the quark theory became accepted (see ''[[November Revolution (physics)|November Revolution]]'').<ref> {{cite book |author=M. E. Peskin, D. V. Schroeder |year=1995 |title=An introduction to quantum field theory |url=https://archive.org/details/introductiontoqu0000pesk |url-access=registration |page=[https://archive.org/details/introductiontoqu0000pesk/page/556 556] |publisher=[[Addison–Wesley]] |isbn=978-0-201-50397-5 }}</ref> == Mass == Despite being extremely common, the [[Quark#Mass|bare mass]] of the down quark is not well determined, but probably lies between {{val|4.5|and|5.3|ul=MeV/c2}}.<ref name="PDG2013"> {{cite web |author=J. Beringer |collaboration=[[Particle Data Group]] |url=http://pdg.lbl.gov/2013/tables/rpp2013-sum-quarks.pdf |title=PDGLive Particle Summary 'Quarks (u, d, s, c, b, t, b′, t′, Free)' |publisher=[[Particle Data Group]] |year=2013 |access-date=2013-07-23 |display-authors=etal }}</ref> [[Lattice QCD]] calculations give a more precise value: {{val|4.79|0.16|ul=MeV/c2}}.<ref name=lqcd>{{cite web|last=Cho|first=Adrian|title=Mass of the Common Quark Finally Nailed Down|url=http://news.sciencemag.org/sciencenow/2010/04/mass-of-the-common-quark-finally.html|publisher=Science Magazine|date=April 2010|url-status=dead|archive-url=https://web.archive.org/web/20120306065114/http://news.sciencemag.org/sciencenow/2010/04/mass-of-the-common-quark-finally.html|archive-date=2012-03-06}}</ref> When found in [[meson]]s (particles made of one quark and one [[antiparticle|antiquark]]) or [[baryon]]s (particles made of three quarks), the 'effective mass' (or 'dressed' mass) of quarks [[Quark#Mass|becomes greater]] because of the [[Quantum chromodynamics binding energy|binding energy]] caused by the [[gluon|gluon field]] between quarks (see [[mass–energy equivalence]]). For example, the effective mass of down quarks in a proton is around {{val|300|ul=MeV/c2}}. Because the bare mass of down quarks is so small, it cannot be straightforwardly calculated because relativistic effects have to be taken into account. == References == {{reflist|2}} == Further reading == * {{cite journal |author=A. Ali, G. Kramer |year=2011 |title=JETS and QCD: A historical review of the discovery of the quark and gluon jets and its impact on QCD |journal=[[European Physical Journal H]] |volume= 36|issue= 2|pages=245 |doi=10.1140/epjh/e2011-10047-1 |bibcode = 2011EPJH...36..245A |arxiv = 1012.2288 |last2=Kramer |s2cid=54062126 }} * {{cite web |author=R. Nave |title=Quarks |url=http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/quark.html |work=[[HyperPhysics]] |publisher=[[Georgia State University]], Department of Physics and Astronomy |access-date=2008-06-29 }} * {{cite book |author=A. Pickering |title=Constructing Quarks |pages=114–125 |publisher=[[University of Chicago Press]] |year=1984 |isbn=978-0-226-66799-7 }} {{Particles}} {{DEFAULTSORT:Down Quark}} [[Category:Quarks]] [[Category:Elementary particles]]
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