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 were 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 of up,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 thequark model).[10]
Despite being extremely common, thebare mass of the up quark is not well determined, but probably lies between 1.8 and3.0 MeV/c2.[15]Lattice QCD calculations give a more precise value:2.01±0.14 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 each quark (seeMass–energy equivalence). The bare mass of up quarks is so light, 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.
