Editing Feynman diagram (section)

== Description ==

[[File:Feynman diagram general properties.svg|350px|thumb|General features of the scattering process A + B → C + D:
<br />• internal lines <span style="color:red;">'''(red)'''</span> for intermediate particles and processes, which has a propagator factor ("prop"), external lines <span style="color:orange;">'''(orange)'''</span> for incoming/outgoing particles to/from vertices '''(black)''',
<br />• at each vertex there is 4-momentum conservation using delta functions, 4-momenta entering the vertex are positive while those leaving are negative, the factors at each vertex and internal line are multiplied in the amplitude integral,
<br />• space {{math|'''x'''}} and time {{mvar|t}} axes are not always shown, directions of external lines correspond to passage of time.

]]

A Feynman diagram represents a perturbative contribution to the amplitude of a quantum transition from some initial quantum state to some final quantum state.

For example, in the process of electron-positron annihilation the initial state is one electron and one positron, while the final state is two photons.

Conventionally, the initial state is at the left of the diagram and the final state at the right (although other layouts are also used).

The particles in the initial state are depicted by lines pointing in the direction of the initial state (e.g., to the left). The particles in the final state are represented by lines pointing in the direction of the final state (e.g., to the right).

[[Quantum electrodynamics|QED]] involves two types of particles: matter particles such as electrons or positrons (called [[fermions]]) and exchange particles (called [[gauge bosons]]). They are represented in Feynman diagrams as follows:
* Electron in the initial state is represented by a solid line, with an arrow indicating the [[Spin (physics)|spin]] of the particle e.g. pointing toward the vertex (→•).
* Electron in the final state is represented by a line, with an arrow indicating the spin of the particle e.g. pointing away from the vertex: (•→).
* Positron in the initial state is represented by a solid line, with an arrow indicating the spin of the particle e.g. pointing away from the vertex: (←•).
* Positron in the final state is represented by a line, with an arrow indicating the spin of the particle e.g. pointing toward the vertex: (•←).
* Virtual Photon in the initial and the final states is represented by a wavy line (<big>~•</big> and <big>•~</big>).

In QED each vertex has three lines attached to it: one bosonic line, one fermionic line with arrow toward the vertex, and one fermionic line with arrow away from the vertex.

Vertices can be connected by a bosonic or fermionic [[propagator]]. A bosonic propagator is represented by a wavy line connecting two vertices (•~•). A fermionic propagator is represented by a solid line with an arrow connecting two vertices, (•←•).

The number of vertices gives the order of the term in the perturbation series expansion of the transition amplitude.

=== Electron–positron annihilation example ===
[[File:Feynman EP Annihilation.svg|class=skin-invert-image|thumb|Feynman diagram of electron/positron annihilation]]

The [[electron–positron annihilation]] interaction:

:e<sup>+</sup> + e<sup>−</sup> → 2γ

has a contribution from the second order Feynman diagram:

In the initial state (at the bottom; early time) there is one electron (e<sup>−</sup>) and one positron (e<sup>+</sup>) and in the final state (at the top; late time) there are two photons (γ).

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