What Are Gluons, and Why Can’t We See a Single Quark?

Protons and neutrons are made of quarks. That is one of the best-established facts in physics. Yet in more than half a century of searching, no detector has ever recorded a single, isolated quark. Quarks only ever turn up in groups. The reason lies with the particles that hold them together, the gluons, and with a property of the strong force that runs exactly opposite to gravity and electromagnetism. Our tee Quarkbinders is three quarks who cannot leave each other.

What are quarks?

In 1964 Murray Gell-Mann proposed that the zoo of strongly interacting particles could be explained if they were built from a few smaller constituents, which he called quarks (Gell-Mann 1964^[1]). Today the Standard Model has six quarks: up, down, strange, charm, bottom and top (CERN^[2]). A proton is two up quarks and a down; a neutron is two downs and an up.

What are gluons?

Each of the fundamental forces in the Standard Model has a carrier particle. For electromagnetism it is the photon; for the strong force it is the gluon (CERN^[2]). The theory of the strong force is quantum chromodynamics (QCD). In it, quarks carry a kind of charge called colour, which comes in three types, conventionally called red, green and blue. They have nothing to do with visible colour; it is just a label for a three-valued charge (Particle Data Group^[3]).

The twist: gluons carry colour too

Photons carry no electric charge, so light does not attract light. Gluons are different: they carry colour charge themselves, so gluons interact with other gluons. In QCD there are eight types of gluon. This self-interaction is what makes the strong force so unlike electromagnetism, and it is the root of everything below.

Why does the strong force get stronger with distance?

Asymptotic freedom (Nobel 2004)

In 1973 David Gross and Frank Wilczek (Gross & Wilczek 1973^[4]), and independently David Politzer (Politzer 1973^[5]), showed that in theories like QCD the effective strength of the interaction falls at short distances. They shared the 2004 Nobel Prize in Physics “for the discovery of asymptotic freedom in the theory of the strong interaction” (NobelPrize.org^[6]).

The Nobel press release spells out both halves: the closer quarks are to each other, the weaker the colour charge, so that when they are really close they behave almost as free particles; and “the converse is true when the quarks move apart: the force becomes stronger when the distance increases.” It compares this to a rubber band: the more you stretch it, the stronger the pull (Nobel press release^[7]).

The strength of the interaction is measured by the coupling α_s, which really does change with energy; the Particle Data Group review collects the measurements and shows it falling as the energy (and so the resolving power) rises (PDG^[3]).

So why can’t we pull a single quark out?

Try to drag one quark away from the others and the energy stored in the gluon field between them keeps rising with distance. Long before you get a free quark, there is enough energy in that field to create a new quark–antiquark pair out of it (E = mc² at work). The new quarks pair up with the old ones, and instead of one free quark you get extra hadrons: protons, pions and other composite particles. Pulling harder only makes more of them. This is colour confinement: only colour-neutral combinations, like three quarks or a quark and an antiquark, are ever seen on their own.

If quarks are never free, how do we know they exist?

Jets

When an electron and a positron annihilate at high energy, they can produce a quark and an antiquark flying apart. Each turns into a narrow spray of hadrons, a jet. In 1979 experiments at the PETRA collider in Hamburg saw events with three jets, the third coming from a gluon radiated by one of the quarks. The TASSO collaboration reported “evidence for planar events” (TASSO 1979^[8]), and the Mark J collaboration reported the discovery of three-jet events as a test of QCD (Mark J 1979^[9]); other PETRA experiments reported similar results.

Quark–gluon plasma

Confinement can also be loosened. At CERN and elsewhere, collisions of heavy nuclei such as lead and gold briefly create a quark–gluon plasma, a state so hot and dense that quarks and gluons are bound only weakly and move on their own, as they did for a few millionths of a second after the Big Bang (CERN^[10]). Even then, the plasma cools within moments and every quark ends up inside a hadron again.

Has confinement been proven? (open question)

Not mathematically. Confinement is observed in every experiment and reproduced by large computer simulations of QCD on a lattice, which also calculate the proton’s mass from quarks and gluons (Dürr et al. 2008^[11]). But nobody has derived it rigorously from the theory. A closely related problem, proving that quantum Yang–Mills theory has a “mass gap”, is one of the Clay Mathematics Institute’s Millennium Prize Problems; Clay notes that experiment and simulations suggest the mass gap exists, “but no proof of this property is known” (Clay Mathematics Institute^[12]). We label this open.

What does this have to do with the nucleus?

The force that holds protons and neutrons together in a nucleus is a leftover of the colour force between their quarks, often called the residual strong force. Our explainer What Holds the Nucleus Together? picks up from here, and How Did the First Elements Form? explains how quarks first settled into protons and neutrons after the Big Bang.

Wear the strong force

Pick up Quarkbinders or the Hadron Brawler tee, or browse the Forces collection.

FAQ

Has anyone seen a free quark?

No. Quarks have only ever been observed bound inside hadrons, or briefly in a quark–gluon plasma.

What is the colour of a quark?

A label for the strong-force charge, which comes in three types. It has nothing to do with visible colour.

How many gluons are there?

Eight types in QCD. Unlike photons, gluons carry the charge of their own force.

Why did Gross, Politzer and Wilczek win the Nobel Prize?

For discovering asymptotic freedom: the strong force weakens at short distances and grows as quarks separate.

References

1. M. Gell-Mann (1964), “A schematic model of baryons and mesons,” Phys. Lett. 8, 214. https://doi.org/10.1016/S0031-9163(64)92001-3
2. CERN, “The Standard Model”. https://home.cern/science/physics/standard-model
3. Particle Data Group (2024), Review of Particle Physics: Quantum Chromodynamics. https://pdg.lbl.gov/2024/reviews/rpp2024-rev-qcd.pdf
4. D. J. Gross & F. Wilczek (1973), “Ultraviolet Behavior of Non-Abelian Gauge Theories,” Phys. Rev. Lett. 30, 1343. https://doi.org/10.1103/PhysRevLett.30.1343
5. H. D. Politzer (1973), “Reliable Perturbative Results for Strong Interactions?” Phys. Rev. Lett. 30, 1346. https://doi.org/10.1103/PhysRevLett.30.1346
6. NobelPrize.org, The Nobel Prize in Physics 2004 (asymptotic freedom). https://www.nobelprize.org/prizes/physics/2004/summary/
7. NobelPrize.org, Press release: The Nobel Prize in Physics 2004. https://www.nobelprize.org/prizes/physics/2004/press-release/
8. TASSO Collaboration, R. Brandelik et al. (1979), “Evidence for planar events in e⁺e⁻ annihilation at high energies,” Phys. Lett. B 86, 243. https://doi.org/10.1016/0370-2693(79)90830-X
9. Mark J Collaboration, D. P. Barber et al. (1979), “Discovery of Three-Jet Events and a Test of Quantum Chromodynamics at PETRA,” Phys. Rev. Lett. 43, 830. https://doi.org/10.1103/PhysRevLett.43.830
10. CERN, “Heavy ions and quark-gluon plasma”. https://home.cern/science/physics/heavy-ions-and-quark-gluon-plasma
11. Dürr et al. (2008), “Ab Initio Determination of Light Hadron Masses,” Science 322, 1224, arXiv:0906.3599. https://arxiv.org/abs/0906.3599
12. Clay Mathematics Institute, Millennium Problems, “Yang–Mills & the Mass Gap”. https://www.claymath.org/millennium/yang-mills-the-maths-gap/

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.