A name and a proposal
By the 1950s and 1960s accelerators had produced hundreds of new particles, and physicists needed a way to order them. In 1961 Murray Gell-Mann and Yuval Ne'eman classified them by an SU(3) symmetry, the "Eightfold Way". In 1964 Gell-Mann and George Zweig independently proposed that protons and neutrons are built from smaller constituents; Gell-Mann took the name "quark" from the line "Three quarks for Muster Mark!" in James Joyce's Finnegans Wake. Because the proposed charges were fractions of the electron's, many at first treated quarks as a bookkeeping device rather than real objects.
Discovery and the six flavours
In 1968 deep inelastic scattering experiments at SLAC fired electrons at protons and found them bouncing off tiny point-like constituents inside (Nobel Prize 1990). In November 1974 teams at Brookhaven and SLAC simultaneously discovered the J/ψ particle containing a charm quark — the "November Revolution". The bottom quark followed in 1977, and the heaviest, the top quark, was found at Fermilab in 1995, completing the set. Quarks and leptons form the matter particles of the Standard Model, each quark has an antimatter partner, and part of their mass comes from interaction with the Higgs field.
| Quark | Charge | Mass | Proposed/found |
|---|---|---|---|
| Up | +2/3 | ~2.2 MeV | 1964 |
| Down | −1/3 | ~4.7 MeV | 1964 |
| Charm | +2/3 | ~1.27 GeV | 1974 |
| Strange | −1/3 | ~95 MeV | 1964 |
| Top | +2/3 | ~173 GeV | 1995 |
| Bottom | −1/3 | ~4.18 GeV | 1977 |
Colour charge and confinement
Besides electric charge, quarks carry "colour charge" (red, green or blue — nothing to do with visible colour) and are bound by exchanging gluons in the strong interaction. The theory of this, quantum chromodynamics (QCD), was completed by David Gross, Frank Wilczek and David Politzer's 1973 discovery of asymptotic freedom: the force weakens as quarks get closer (Nobel Prize 2004). Conversely, it grows as they separate, so pulling a quark out creates a new quark–antiquark pair, and quarks are never observed alone — confinement. Most of a proton's mass comes from this binding energy rather than the quarks themselves. Just after the Big Bang quarks and gluons roamed free as a quark–gluon plasma, recreated in heavy-ion collisions at RHIC and the LHC in the 2000s.
FAQ
- What quarks is a proton made of?
- Two up quarks and one down quark (uud). A neutron is one up and two down (udd); their charges add to +1 and 0 respectively.
- Is there anything smaller than a quark?
- At the precision of current experiments quarks look like point particles with no internal structure. Some theories propose deeper constituents, but there is no evidence for them.
Sources reviewed: 2026-10-26
References
- Gell-Mann, M. (1964). A schematic model of baryons and mesons. Physics Letters, 8(3), 214–215.
- Zweig, G. (1964). An SU(3) model for strong interaction symmetry and its breaking. CERN Report 8182/TH.401.
- Bloom, E. D., et al. (1969). High-energy inelastic e–p scattering at 6° and 10°. Physical Review Letters, 23(16), 930–934.
- Gross, D. J., & Wilczek, F. (1973). Ultraviolet behavior of non-abelian gauge theories. Physical Review Letters, 30(26), 1343–1346.
- Politzer, H. D. (1973). Reliable perturbative results for strong interactions? Physical Review Letters, 30(26), 1346–1349.
- Aubert, J. J., et al. (1974). Experimental observation of a heavy particle J. Physical Review Letters, 33(23), 1404–1406.
- CDF Collaboration (Abe, F., et al.). (1995). Observation of top quark production in p̄p collisions. Physical Review Letters, 74(14), 2626–2631.
- Joyce, J. (1939). Finnegans Wake. London: Faber and Faber.
- Particle Data Group (Navas, S., et al.). (2024). Review of Particle Physics. Physical Review D, 110, 030001.