# Strong interaction
The strong interaction is one of the fundamental forces of the [Standard Model](https://www.edgepedia.org/standard-model) of particle physics: it binds quarks together into protons, neutrons, and other hadrons, and, through a residual effect, holds protons and neutrons together inside atomic nuclei[1](https://www.energy.gov/science/doe-explainsthe-strong-force). Its quantum field theory is quantum chromodynamics (QCD), in which the force is carried by massless particles called gluons between quarks that carry a property called color charge[1](https://www.energy.gov/science/doe-explainsthe-strong-force) • [2](https://pdg.lbl.gov/2025/reviews/rpp2025-rev-qcd.pdf).
| Key fact | Detail |
|---|---|
| Carrier | The gluon, a massless boson; there are eight kinds of gluon, each carrying color and anticolor charge[1](https://www.energy.gov/science/doe-explainsthe-strong-force) • [2](https://pdg.lbl.gov/2025/reviews/rpp2025-rev-qcd.pdf) |
| Charge | Quarks carry one of three color charges (conventionally red, green, blue); only color-neutral combinations exist as free particles[3](https://www.fnal.gov/pub/science/inquiring/questions/strong_force.html) |
| Strength | About 100 times stronger than electromagnetism, but effective only over distances about 100,000 times smaller than an atom's diameter[1](https://www.energy.gov/science/doe-explainsthe-strong-force) |
| Theory | Quantum chromodynamics, the SU(3) component of the Standard Model, with coupling constant \( \alpha_s = g_s^2/4\pi \)[2](https://pdg.lbl.gov/2025/reviews/rpp2025-rev-qcd.pdf) |
| Running coupling | \( \alpha_s \sim 0.1 \) for momentum transfers of 0.1–1 TeV; the theory is strongly interacting at scales around and below 1 GeV[2](https://pdg.lbl.gov/2025/reviews/rpp2025-rev-qcd.pdf) |
| Asymptotic freedom | Discovered in 1973 by David Gross, H. David Politzer, and Frank Wilczek; Nobel Prize in Physics 2004[4](https://www.nobelprize.org/prizes/physics/2004/press-release/?related=1) |
| Nuclear binding | The residual strong force between nucleons is described by pion exchange, with a range of about \( 1.5 \times 10^{-15} \) m[5](https://www2.lbl.gov/LBL-Programs/nsd/education/ABC/wallchart/chapters/04/0.html) |
## Color charge and gluons
Quarks carry, besides electric charge, a "strong charge" called color, which comes in three values conventionally named red, green, and blue, with corresponding anticolors[3](https://www.fnal.gov/pub/science/inquiring/questions/strong_force.html). The name is an analogy with ordinary color mixing and has nothing to do with visible light[3](https://www.fnal.gov/pub/science/inquiring/questions/strong_force.html). In QCD, quarks belong to the fundamental representation of the SU(3) color group, and the gluon fields run over \( N_c^2 - 1 = 8 \) independent states, so there are eight kinds of gluon[2](https://pdg.lbl.gov/2025/reviews/rpp2025-rev-qcd.pdf).
**Gluons differ from photons.** The photon, which mediates electromagnetism, is electrically neutral. Gluons are massless like the photon, but each carries a color and a different anticolor, for example "red-anti-blue"[3](https://www.fnal.gov/pub/science/inquiring/questions/strong_force.html). Because gluons themselves carry the charge of the force they transmit, they interact with one another as well as with quarks, and this self-interaction is what gives the strong force its unusual distance behavior[3](https://www.fnal.gov/pub/science/inquiring/questions/strong_force.html).
## Confinement and asymptotic freedom
The strong force behaves in the opposite way from the other forces: at short distances it is weak, and at long distances it is strong[3](https://www.fnal.gov/pub/science/inquiring/questions/strong_force.html). The three quarks inside a proton or neutron behave almost as if they were moving freely inside an elastic container, but when a quark reaches the boundary the force grows stronger the farther it gets from the others[3](https://www.fnal.gov/pub/science/inquiring/questions/strong_force.html).
This leads to confinement: pulling two quarks apart takes so much energy that the energy spawns two additional quarks, so quarks are always combined with other quarks and are never found in isolation[1](https://www.energy.gov/science/doe-explainsthe-strong-force). How quarks and gluons—together called partons—turn into bound hadrons is known as hadronisation, a process whose character varies with the energy scale: when energies are high, the strong interaction holds quarks together only feebly, but as energies drop, their coupling grows and hadrons emerge.[6](https://home.cern/partons-hadrons/)
The weak binding at short distances is asymptotic freedom: the closer quarks are to each other, the weaker the color force, until at very small separations they behave almost as free particles[4](https://www.nobelprize.org/prizes/physics/2004/press-release/?related=1). David Gross and [Frank Wilczek](https://www.edgepedia.org/frank-wilczek), and independently [H. David Politzer](https://www.edgepedia.org/h-david-politzer), found in 1973 that a non-abelian gauge field theory satisfies this requirement if there are not too many quarks[4](https://www.nobelprize.org/prizes/physics/2004/press-release/?related=1) • [7](https://www.nobelprize.org/prizes/themes/forces/). The discovery opened the way to QCD as the theory of the strong interaction[7](https://www.nobelprize.org/prizes/themes/forces/), and the three theorists received the 2004 [Nobel Prize in Physics](https://www.edgepedia.org/nobel-prize-in-physics) "for the discovery of asymptotic freedom in the theory of the strong interaction"[4](https://www.nobelprize.org/prizes/physics/2004/press-release/?related=1).
The running of the coupling is described by the QCD beta function, whose leading coefficient is \( b_0 = (33 - 2 n_f)/(12\pi) \), where \( n_f \) is the number of light quark flavors; the minus sign in the renormalization group equation is the origin of asymptotic freedom[2](https://pdg.lbl.gov/2025/reviews/rpp2025-rev-qcd.pdf). The coupling is now measured experimentally: the ATLAS collaboration determined \( \alpha_s \) at the Z-boson mass scale to be 0.1183 ± 0.0009 from Z-boson production in proton–proton collisions at 8 TeV, a relative uncertainty of 0.8 percent[8](https://home.cern/atlas-measures-strength-strong-force-record-precision/).
## The residual strong force and nuclei
The force that holds protons and neutrons together in a nucleus is a residual of the stronger force among the quarks, analogous to the van der Waals forces that bind molecules as a residue of electromagnetism[3](https://www.fnal.gov/pub/science/inquiring/questions/strong_force.html) • [9](https://www.fnal.gov/pub/ferminews/ferminews02-03-29/p1.html). Quarks confined in one hadron do exchange gluons with quarks in neighboring hadrons, but the gluon exchanges within a single hadron largely cancel, so the residual force falls off quickly with distance[9](https://www.fnal.gov/pub/ferminews/ferminews02-03-29/p1.html).
A successful model of this residual force is the exchange of pions, an idea dating back to [Hideki Yukawa](https://www.edgepedia.org/hideki-yukawa)'s work in the 1930s; it works well except at very short distances, and the transition where the pion-exchange picture breaks down is thought to occur around momentum transfers of 1 GeV/c[5](https://www2.lbl.gov/LBL-Programs/nsd/education/ABC/wallchart/chapters/04/0.html). Quantitatively, the strong force binds two protons in a nucleus with about 25 to 35 MeV of energy, while electromagnetic repulsion works against it, so roughly 1 MeV of energy is needed to separate two protons; by comparison, an electron is bound to a proton in hydrogen by only a few electron-volts[3](https://www.fnal.gov/pub/science/inquiring/questions/strong_force.html).
## Mass in the proton
Most of the mass of the proton emerges from intricate quark dynamics, including their motion, and is governed by the strong force mediated by gluons[10](https://www.jlab.org/scientists_locate_missing_mass_proton). A 2023 measurement at the [Thomas Jefferson National Accelerator Facility](https://www.edgepedia.org/thomas-jefferson-national-accelerator-facility), the J/Ψ-007 experiment, used a small color dipole (the J/psi particle) to locate the gluon-generated mass: it found a spherical region concentrated in the proton's central area[10](https://www.jlab.org/scientists_locate_missing_mass_proton).
## Quark-gluon plasma
QCD predicts that at very high temperatures and densities, quarks and gluons should no longer be confined inside composite particles but exist in a state of matter called quark-gluon plasma[11](https://cms.cern/physics/how-did-matter-form). The transition should occur above a temperature around 2000 billion degrees, about 100,000 times hotter than the core of the Sun; for a few millionths of a second after the [Big Bang](https://www.edgepedia.org/big-bang) the whole universe was in this state, before cooling and condensing into protons and neutrons[11](https://cms.cern/physics/how-did-matter-form).
Experiments at CERN's Super Proton Synchrotron reported evidence for the plasma in 2000, and the [Relativistic Heavy Ion Collider](https://www.edgepedia.org/relativistic-heavy-ion-collider) at [Brookhaven National Laboratory](https://www.edgepedia.org/brookhaven-national-laboratory) has since pursued an extensive program[11](https://cms.cern/physics/how-did-matter-form). At the [Large Hadron Collider](https://www.edgepedia.org/large-hadron-collider), heavy lead nuclei are collided at near-light speed to recreate the plasma; within the fleeting droplet, quarks and gluons flow collectively in ways that resemble an ultrahot liquid more than a gas[12](https://www.livescience.com/physics-mathematics/particle-physics/physicists-recreated-the-first-millisecond-after-the-big-bang-and-found-it-was-surprisingly-soupy).
## Testing QCD
Because quarks and gluons cannot be accelerated or detected directly, QCD is tested through the particles they form. A hallmark at the LHC is the production of collimated jets when quarks and gluons scatter at high energies; decades of jet measurements have probed QCD across a wide range of energy scales[13](https://atlas-public.web.cern.ch/updates/briefing/precision-jet-substructure-lund-jet-plane). Early support came quickly: the SLAC deep-inelastic scattering results, the color-factor evidence from electron-positron collisions at ADONE in the early 1970s, and the first three-jet events at PETRA in Hamburg in the late 1970s, interpreted as one gluon and two quarks[14](https://www.nobelprize.org/uploads/2018/06/advanced-physicsprize2004-2.pdf).
## References
1. [DOE Explains...The Strong Force, U.S. Department of Energy](https://www.energy.gov/science/doe-explainsthe-strong-force)
2. [Quantum Chromodynamics, Review of Particle Physics (Particle Data Group, 2025)](https://pdg.lbl.gov/2025/reviews/rpp2025-rev-qcd.pdf)
3. [How strong is the strong force?, Fermilab Inquiring Minds](https://www.fnal.gov/pub/science/inquiring/questions/strong_force.html)
4. [Press release: The 2004 Nobel Prize in Physics, NobelPrize.org](https://www.nobelprize.org/prizes/physics/2004/press-release/?related=1)
5. [Four Fundamental Interactions, Nuclear Science Wall Chart, Lawrence Berkeley National Laboratory](https://www2.lbl.gov/LBL-Programs/nsd/education/ABC/wallchart/chapters/04/0.html)
6. [From partons to hadrons, CERN](https://home.cern/partons-hadrons/)
7. [Forces, NobelPrize.org](https://www.nobelprize.org/prizes/themes/forces/)
8. [ATLAS measures strength of the strong force with record precision, CERN](https://home.cern/atlas-measures-strength-strong-force-record-precision/)
9. [FermiNews, March 29, 2002, Fermilab](https://www.fnal.gov/pub/ferminews/ferminews02-03-29/p1.html)
10. [Scientists Locate the Missing Mass Inside the Proton, Jefferson Lab](https://www.jlab.org/scientists_locate_missing_mass_proton)
11. [How did Matter form?, CMS Experiment, CERN](https://cms.cern/physics/how-did-matter-form)
12. [Physicists recreated the first millisecond after the Big Bang, Live Science](https://www.livescience.com/physics-mathematics/particle-physics/physicists-recreated-the-first-millisecond-after-the-big-bang-and-found-it-was-surprisingly-soupy)
13. [Novel probes of the strong force: precision jet substructure and the Lund jet plane, ATLAS Collaboration, CERN](https://atlas-public.web.cern.ch/updates/briefing/precision-jet-substructure-lund-jet-plane)
14. [Advanced information on the Nobel Prize in Physics 2004, NobelPrize.org](https://www.nobelprize.org/uploads/2018/06/advanced-physicsprize2004-2.pdf)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Particles and nuclei › Particle physics › Standard Model particle content › Gauge bosons and the Higgs sector › Gluon*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license