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Down quark
Type of quark
Type of quark
| Field | Value |
|---|---|
| name | Down quark |
| composition | elementary particle |
| statistics | fermionic |
| group | quark |
| generation | first |
| interaction | strong, weak, electromagnetic, gravity |
| antiparticle | down antiquark () |
| theorized | Murray Gell-Mann (1964) |
| George Zweig (1964) | |
| discovered | SLAC (1968) |
| symbol | |
| mass | |
| 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 |
| decay_particle | stable or up quark + electron + electron antineutrino |
| electric_charge | − *e* |
| color_charge | Yes |
| spin | *ħ* |
| weak_isospin | LH: −, RH: 0 |
| weak_hypercharge | LH: +, RH: − |
George Zweig (1964) |doi-access=free |hdl-access=free
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 quarks, and combines with other quarks to form composite particles called hadrons. Down quarks are most commonly found in atomic nuclei, where they combine with up quarks to form protons and neutrons. 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 first generation of matter, has an electric charge of − e and a bare mass of . Like all quarks, the down quark is an elementary fermion with spin , and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. 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 equal magnitude but opposite sign.
Its existence (along with that of the up and strange quarks) was postulated in 1964 by Murray Gell-Mann and George Zweig to explain the Eightfold Way classification scheme of hadrons. The down quark was first observed by experiments at the Stanford Linear Accelerator Center in 1968.
History
In the beginnings of particle physics (first half of the 20th century), hadrons such as protons, neutrons, and pions were thought to be elementary particles. 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
This classification scheme organized the hadrons into isospin multiplets, but the physical basis behind it was still unclear. In 1964, Gell-Mann |access-date=2008-09-23 |display-authors=etal |doi-access=free |display-authors=etal|url=https://www.slac.stanford.edu/cgi-bin/getdoc/slac-pub-0650.pdf |access-date = 2008-09-29 |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
At first people were reluctant to identify the three-bodies as quarks, instead preferring Richard Feynman's parton description, |url-access=limited |url-access=registration
Mass
Despite being extremely common, the bare mass of the down quark is not well determined, but probably lies between . |access-date=2013-07-23 |display-authors=etal
When found in mesons (particles made of one quark and one antiquark) or baryons (particles made of three quarks), the 'effective mass' (or 'dressed' mass) of quarks becomes greater because of the binding energy caused by the gluon field between quarks (see mass–energy equivalence). For example, the effective mass of down quarks in a proton is around . 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
References
- Cho, Adrian. (April 2010). "Mass of the Common Quark Finally Nailed Down". Science Magazine.
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