The strong interaction is one of the four fundamental interactions of nature, alongside electromagnetism, the weak interaction, and gravity. It acts directly on quarks and gluons, binding them into composite particles such as the proton and neutron. Its residual effects bind these particles into an atomic nucleus. The fundamental theory of the interaction is quantum chromodynamics (QCD), the strong-interaction component of the Standard Model. Distinguishing this fundamental color interaction from the effective force between nucleons is essential to understanding its properties. (home.web.cern.ch)
Color charge and gluons
The charge associated with the strong interaction is called color charge. Quarks have three possible color states, conventionally named red, green, and blue; these labels have no connection with visible colors. Antiquarks carry corresponding anticolor states. QCD is a gauge theory based on the symmetry group SU(3), which mathematically describes transformations among these color states. (pdg.lbl.gov)
The interaction is mediated by eight gluon fields. Gluons carry color charge themselves and therefore interact with other gluons, unlike the electrically neutral photon that mediates electromagnetic interactions. These self-interactions are a central feature of QCD and distinguish it from quantum electrodynamics. The theory includes both quark–gluon interactions and vertices involving three or four gluons. (pdg.lbl.gov)
Observable strongly interacting composite particles are called hadrons. They are color-neutral combinations of quarks, antiquarks, and gluons. Familiar examples include nucleons and mesons. Color neutrality does not mean that their constituents lack color charge; it means that the complete state transforms as a color singlet. (pdg.lbl.gov)
Asymptotic freedom and confinement
The effective strength of the interaction depends on the scale at which it is measured. At very short distances, corresponding to large momentum transfers, the strong coupling becomes weaker. This property, called asymptotic freedom, allows quarks to behave approximately as weakly interacting particles in sufficiently high-energy processes. It also makes perturbation theory, which expands predictions in powers of a small coupling, useful for many collision calculations. (nobelprize.org)
At larger distances, the interaction becomes strongly coupled. Isolated quarks and gluons have not been observed as freely propagating particles; instead, they appear within color-neutral hadrons. This phenomenon is known as color confinement. In a useful physical picture, separating a quark and an antiquark stretches a tube of gluonic field energy. When sufficient energy is supplied, new quark–antiquark pairs can form, producing additional hadrons rather than liberated individual quarks. (indico.cern.ch)
Consequently, high-energy collisions produce streams of hadrons rather than directly observable free quarks or gluons. These streams, called jets, retain information about the energetic constituents that initiated them. Their formation involves gluon radiation followed by hadronization, the conversion of quarks and gluons into hadrons. Measurements of jet structure test this evolution and the scale dependence of the strong coupling. (home.web.cern.ch)
The residual nuclear force
Protons and neutrons are individually color-neutral, but their internal strong dynamics generate an effective interaction between them. This nuclear force, often called the residual strong interaction, can overcome electromagnetic repulsion between positively charged protons and bind nuclei. It is not a separate fundamental interaction: it emerges from the same underlying QCD dynamics that govern quarks and gluons. (energy.gov)
The nuclear force has a short range, with important effects over distances of roughly a few femtometres, where one femtometre is metre. Its longest-range component is described by exchange of pions, the lightest hadrons. Nuclear interactions are not uniformly attractive: empirical nucleon–nucleon potentials exhibit attraction at intermediate distances and a strongly repulsive core at short distances. They also depend on the particles’ spin configuration. (jlab.org)
The nuclear force’s limited range should therefore not be confused with a simple cutoff in the fundamental quark–gluon interaction. The confinement of colored constituents and the interaction between color-neutral nucleons are different manifestations of strong dynamics, requiring different descriptions. (indico.cern.ch)
Mass and strongly interacting matter
The strong interaction is crucial to the origin of ordinary matter’s mass. The small intrinsic masses of the proton’s constituent quarks account for only a small fraction of its total mass. Most emerges from quark motion and the energy associated with gluonic fields and their interactions. Thus, the proton cannot be understood simply by adding the masses of three constituent elementary particles. Experiments probe how this dynamically generated mass is distributed inside it. (science.osti.gov)
Under sufficiently hot and dense conditions, strongly interacting matter can form a quark–gluon plasma, in which quarks and gluons are no longer confined within individual hadrons. Such matter existed during the early universe and is studied in heavy-ion collisions at facilities including the Large Hadron Collider. Energetic jets lose energy while traversing the plasma; this “jet quenching” provides information about the medium’s properties. (home.web.cern.ch)
Development and methods of study
The nuclear force was initially studied through models of interactions between protons and neutrons. In 1935, Hideki Yukawa proposed that a massive exchanged particle could explain its finite range; the pion subsequently supplied the relevant particle for its longest-range component. The later quark–gluon description established a more fundamental framework beneath these nuclear models. (jlab.org)
In 1973, David Gross, Frank Wilczek, and David Politzer discovered asymptotic freedom in non-Abelian gauge theories, providing a decisive foundation for QCD. They received the 2004 Nobel Prize in Physics for this discovery. (nobelprize.org)
At low energies, where ordinary perturbative expansions become unreliable, lattice QCD evaluates the theory numerically on a discretized spacetime. It supplies a route from quark–gluon dynamics to hadronic properties and nuclear interactions. Experimental scattering, jet measurements, and heavy-ion collisions provide complementary tests across the interaction’s weakly and strongly coupled regimes. (arxiv.org)