Quarks
Quarks are fundamental particles — one of the two basic classes of matter particles in the Standard Model, alongside leptons — that combine in groups of two or three to form composite particles called hadrons (including protons and neutrons), and that are never observed in isolation due to a property called confinement.
Curious
For decades after protons and neutrons were discovered, physicists treated them as fundamental, indivisible particles. By the early 1960s, however, a proliferation of newly discovered particles at accelerator experiments (collectively called hadrons) suggested a deeper, simpler underlying structure — leading Murray Gell-Mann and George Zweig, independently, to propose in 1964 that protons, neutrons, and all similar particles are actually built from smaller constituents, which Gell-Mann whimsically named "quarks" (borrowing the word from a line in James Joyce's novel Finnegans Wake).
A proton is made of two up quarks and one down quark; a neutron is made of one up quark and two down quarks. Quarks carry fractional electric charge (a genuinely startling proposal at the time, since all previously known charged particles carried whole-number multiples of the electron's charge) and are bound together by the strong force, mediated by particles called gluons.
Exploring
| Quark | Symbol | Charge (in units of e) | Approx. mass |
|---|---|---|---|
| Up | u | +2/3 | ~2.2 MeV/c² |
| Down | d | −1/3 | ~4.7 MeV/c² |
| Charm | c | +2/3 | ~1.27 GeV/c² |
| Strange | s | −1/3 | ~95 MeV/c² |
| Top | t | +2/3 | ~173 GeV/c² |
| Bottom | b | −1/3 | ~4.18 GeV/c² |
Deep Dive
Gell-Mann's 1964 paper (and Zweig's independent, contemporaneous proposal, which he called "aces" rather than "quarks") organised the growing zoo of known hadrons into simple patterns based on just three hypothetical quark types (up, down, and strange) — a scheme that not only explained existing particles but successfully predicted the existence and properties of a previously unobserved particle, the omega-minus baryon, discovered experimentally in 1964 shortly after the prediction, lending the initially speculative quark model substantial early credibility.
Confinement and asymptotic freedom: Unlike other fundamental particles, quarks are never observed as free, isolated particles — a property called confinement, arising from the peculiar behaviour of the strong force (described by quantum chromodynamics): unlike gravity or electromagnetism, which weaken with distance, the strong force between quarks actually strengthens as they are pulled apart, meaning that attempting to isolate a single quark requires more and more energy the further apart you try to separate it from its partners, until eventually enough energy has been invested to spontaneously create a new quark-antiquark pair from the vacuum rather than achieving actual separation. Conversely, at very short distances, quarks behave as nearly free particles — a property called asymptotic freedom, discovered by David Gross, David Politzer, and Frank Wilczek (earning them the 2004 Nobel Prize in Physics), and essential to explaining the SLAC scattering results.
Six quark "flavours" are now known, organised into three generations of increasing mass (up/down, charm/strange, top/bottom) — mirroring a similar three-generation pattern seen among the leptons, a structural feature of the Standard Model whose deeper origin (why exactly three generations, and why this particular pattern of masses) remains one of several open theoretical questions not explained by the Standard Model itself.
Concept Relationship Map
Requires: Standard Model
Enables: strong force [PENDING FULL ENTRY], quantum chromodynamics [PENDING FULL ENTRY]
Commonly confused with: assuming quark rest mass accounts for most of a proton's mass (it accounts for only about 1%; the rest is binding energy and kinetic energy)
Sources
- Gell-Mann, M. (1964). Physics Letters, 8(3), 214–215.
- Particle Data Group (2024). "Review of Particle Physics." Physical Review D, 110, 030001.
- Griffiths, D.J. (2008). Introduction to Elementary Particles, 2nd ed. Wiley-VCH, Ch. 1.