Feynman rules

Feynman rules assign mathematical factors to the elements of particle-interaction diagrams, turning each diagram into a contribution to a scattering amplitude.

Feynman diagram: A diagrammatic representation of terms in a perturbative calculation of particle interactions. The rules assign expressions to its lines and vertices.
Quantum field theory: A framework describing particles and their interactions through quantized fields. Feynman rules are calculational prescriptions derived from its dynamics.
Quantum electrodynamics: The quantum field theory of electrically charged particles interacting through the electromagnetic field. Its electron-photon vertex and propagators give a canonical rule set.
Richard Feynman: An American theoretical physicist who developed a diagrammatic approach to quantum electrodynamics. His notation and prescriptions gave the rules their name.
Propagator: A function describing the propagation of a particle between interaction points in a quantum field theory. Each internal line contributes a propagator factor.
Lagrangian density: A function of fields and their derivatives that specifies a field theory's dynamics. Its terms determine the propagators and interaction vertices.
Quantum chromodynamics: The quantum field theory of quarks and gluons and their strong interaction. Its color-carrying fields require specialized propagators and vertices.
Julian Schwinger: An American theoretical physicist who developed a powerful operator formulation of quantum electrodynamics. His formulation offered a distinct route to the same QED predictions.
Interaction vertex: A point in a quantum field theory diagram where fields interact, represented by a coupling-dependent factor. Vertex factors encode the interactions specified by the theory.
Perturbation theory: A method that approximates a problem by expanding its solution in a small parameter. The rules organize amplitudes as sums ordered by interaction strength.

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