Feynman Diagrams
Graphical bookkeeping for perturbative quantum field theory amplitudes.
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Feynman Diagrams
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Purpose
Feynman diagrams are not literal pictures of particle trajectories. They are bookkeeping devices for terms in a perturbative expansion of a scattering amplitude.
Basic Structure
External lines represent incoming and outgoing particles. Internal lines represent propagators. Vertices represent interaction terms in the Lagrangian.
Amplitude
The observable cross section is built from the squared amplitude, schematically
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