Other meanings of Strong interaction
Particle physics
The strong interaction is the fundamental force binding quarks into hadrons and helping hold atomic nuclei together. Described by quantum chromodynamics (QCD), it acts through gluons and has the unusual properties of color charge, confinement, and asymptotic freedom.
The strong interaction binds quarks by exchanging gluons, the gauge bosons of quantum chromodynamics (QCD).1 Quarks carry one of three color charges—red, green, or blue—and gluons themselves carry combinations of color and anticolor, allowing them to interact with one another as well as with quarks. This self-interaction distinguishes QCD from quantum electrodynamics, whose photons have no electric charge. Observable particles must be color-neutral: baryons contain three quarks, while mesons contain a quark and an antiquark. The force is one of the four fundamental interactions and is embedded in the Standard Model of particle physics. Its familiar nuclear effects are not the whole interaction, but a low-energy consequence of quark and gluon dynamics.
QCD becomes weaker at very short distances but prevents isolated quarks and gluons from appearing as ordinary free particles. This paired behavior is called asymptotic freedom and confinement.2 When quarks are pulled apart, the energy stored in the gluon field grows rather than falling in the way an electric field does; eventually it is energetically favorable to create a new quark–antiquark pair, producing separate color-neutral hadrons. At high momentum transfer, by contrast, quarks behave more nearly as weakly interacting constituents inside hadrons. The 1973 discovery of asymptotic freedom by David Gross, Frank Wilczek, and David Politzer provided the key quantitative explanation for deep-inelastic scattering and was recognized with the 2004 Nobel Prize in Physics.2
The force holding quarks inside protons and neutrons is distinct from the residual strong interaction that binds nuclei. Color-neutral nucleons can exchange composite objects, especially pions, producing an attractive force over distances of roughly a femtometre; this residual interaction competes with the electromagnetic repulsion between protons.3 Nuclear binding therefore reflects QCD indirectly, rather than a simple direct exchange of individual gluons between widely separated nucleons. The same physics explains why nuclear forces saturate: each nucleon interacts strongly with nearby neighbors, while the effective force falls rapidly beyond the nuclear scale. Binding energies, nuclear spectra, and scattering observables are calculated with models rooted in QCD, including chiral effective theories and lattice QCD. The balance between attraction, repulsion, and quantum statistics determines which nuclei are stable.
Much of the proton’s mass comes from strong-interaction energy rather than from the small current masses of its quarks. The proton and neutron each contain three valence quarks, but their interior also includes a changing sea of quark–antiquark pairs and gluons. QCD permits unusual hadrons beyond ordinary mesons and baryons, including experimentally studied tetraquark and pentaquark candidates; their internal arrangements remain an active research subject.4 At extreme temperatures and densities, such as those briefly produced in heavy-ion collisions, hadronic matter can become a quark–gluon plasma.5 Lattice calculations place this theory on a discrete spacetime grid, making nonperturbative predictions possible but computationally demanding, especially for dense matter.
The terms strong interaction, strong force, and strong nuclear force are sometimes used broadly; in this entry, the fundamental interaction is distinguished from its residual nuclear force.
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