Feynman diagram

A graph used to represent terms in perturbative quantum field theory. Its lines and vertices encode mathematical expressions for particle interactions and scattering amplitudes.

Feynman rules: A set of prescriptions assigning mathematical factors to the lines, vertices, and other elements of a Feynman diagram. They turn each line and vertex into factors that can be combined into an amplitude.
Quantum field theory: A framework describing particles and their interactions through quantum fields distributed across spacetime. Feynman diagrams are a notation for calculations within this framework.
Richard Feynman: An American theoretical physicist who developed path-integral methods and graphical techniques for quantum electrodynamics. He introduced the diagrammatic method that bears his name.
Feynman–Kac formula: A theorem connecting solutions of certain differential equations with expectations over stochastic processes. Despite the shared name, it is a distinct mathematical tool, not a particle-interaction diagram.
Feynman diagram interpretation: The question of how diagrammatic elements relate to physical particles, histories, or mathematical terms. A diagram is not generally a literal picture of particles traveling along observable paths.
Propagator: A function describing how a quantum field excitation contributes between two points or interactions. Internal lines represent propagators connecting interaction vertices.
Perturbation theory: A method that approximates a difficult problem by expanding its solution in powers of a small parameter. Diagrams organize successive terms in expansions in interaction strength.
Quantum electrodynamics: The relativistic quantum field theory of electromagnetic interactions between charged particles and photons. The earliest and best-known diagrams were developed for its calculations.
Quantum chromodynamics: The quantum field theory describing strong interactions among quarks and gluons. Its perturbative calculations use diagrams with quark, gluon, and ghost lines.
Nonperturbative method: A method that does not rely on an expansion in a small interaction parameter. Ordinary Feynman expansions can fail when interactions are too strong for perturbation theory.

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