The strong interaction is the color interaction described by quantum chromodynamics. Quarks carry color charge, gluons mediate the interaction and also carry color themselves, and physical low-energy states organize into color-singlet hadrons.
At short distances, asymptotic freedom makes a perturbative treatment useful. At hadronic distances, confinement and strong collective dynamics require other methods. The binding of quarks into a proton and the residual interaction between nucleons in an atomic nucleus are related effects at different descriptive levels. The latter can be modeled using hadrons rather than pretending that a free gluon simply travels between two structureless nucleons. Scattering resolves the short-distance constituents, while hadron and nuclear spectra constrain their low-energy collective behavior.
Hadronic processes with disconnected quark-flavour lines are often suppressed relative to connected quark rearrangement. A predominantly strange phi meson can decay to kaons through light-pair creation while its original strange lines survive, whereas a purely pionic final state requires strange-pair annihilation. The rule is an approximate dynamical pattern, not an exact conservation law.
An interaction among outgoing hadrons that changes the observed channel amplitude or its phase after the short-distance production vertex. Rescattering can mix channels with the same conserved quantum numbers. Consequently identical CKM suppression orders do not imply equal branching fractions, and a free-quark spectator diagram need not determine the ordering of exclusive hadronic decay rates.
A phase in a hadronic matrix element generated by strong interaction dynamics, including rescattering of final states. It does not change sign under conjugation of the weak coupling constant. Interference measurements depend on relative strong phases, rather than their common overall value.
A quark-line diagram can show how existing valence quarks are regrouped into outgoing hadrons while gluons exchange momentum and colour. Alternatively, a quark-antiquark pair can annihilate and create a different flavour pair with zero net flavour. Such drawings enforce global charge, baryon number and flavour constraints; they are not controlled perturbative calculations of an exclusive low-energy hadronic amplitude because binding is nonperturbative.
Scattering with one meson and one baryon in the initial and final states. The strong interaction conserves electric charge, baryon number, net strangeness, four-momentum, total angular momentum, and parity. Approximate isospin symmetry imposes additional relations among charge-channel amplitudes.
The channel has baryon number one, strangeness zero and , matching the ordinary Delta baryon. The channel has strangeness plus one, which no three-light-quark baryon can have because each strange quark contributes minus one. A resonance in that channel would require exotic content such as four quarks and an antiquark; the ordinary low-energy qqq multiplets therefore explain the contrast.
For an initial pion-nucleon state and outgoing meson octet-baryon octet pair, conservation of baryon number and strangeness allows , , , and , subject to electric charge and production thresholds. Total isospin is or ; the and channels contain only .
