Thedown quark (symbol: d) is a type ofelementary particle, and a major constituent ofmatter. The down quark is the second-lightest of allquarks, and combines with other quarks to formcomposite particles calledhadrons. Down quarks are most commonly found inatomic nuclei, where it combines withup quarks to formprotons andneutrons. 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.
In the beginnings of particle physics (first half of the 20th century),hadrons such asprotons,neutrons, andpions were thought to beelementary 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, whenMurray Gell-Mann[2] andYuval Ne'eman[3] (independently of each other) proposed a hadron classification scheme called theEightfold Way, or in more technical terms,SU(3)flavor symmetry.
This classification scheme organized the hadrons intoisospin multiplets, but the physical basis behind it was still unclear. In 1964, Gell-Mann[4] andGeorge Zweig[5][6] (independently of each other) proposed thequark model, then consisting only ofup, down, andstrange quarks.[7] However, while the quark model explained the Eightfold Way, no direct evidence of the existence of quarks was found until 1968 at theStanford Linear Accelerator Center.[8][9]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).[10]
Despite being extremely common, thebare mass of the down quark is not well determined, but probably lies between4.5 and 5.3 MeV/c2.[15]Lattice QCD calculations give a more precise value:4.79±0.16 MeV/c2.[16]
When found inmesons (particles made of one quark and oneantiquark) orbaryons (particles made of three quarks), the 'effective mass' (or 'dressed' mass) of quarksbecomes greater because of thebinding energy caused by thegluon field between quarks (seemass–energy equivalence). For example, the effective mass of down quarks in a proton is around300 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.
^M. Gell-Mann (2000) [1964]. "The Eightfold Way: A theory of strong interaction symmetry". In M. Gell-Mann, Y. Ne'eman (ed.).The Eightfold Way.Westview Press. p. 11.ISBN978-0-7382-0299-0. Original:M. Gell-Mann (1961). "The Eightfold Way: A theory of strong interaction symmetry".Synchrotron Laboratory Report CTSL-20.California Institute of Technology.
