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Feynman Diagrams


Originally conceived to describe quantum electrodynamic (QED) interactions, they were extended to show the interactions associated with the weak nuclear forces and chromodynamic (QCD), the interactions of the strong nuclear forces. See History

Feynman diagrams represent interactions between matter particles (fermions, including quarks and leptons) and their corresponding force carriers (bosons).

They consist of points, called vertices, which represent the interactions and lines attached to the vertices which represent the fermions and bosons involved in the interaction. Reading from left to right, the horizontal axis represents time and the vertical axis represents space. Thus particles in the initial state enter the vertex from the left and the particles in the final state leave the vertex towards the right.

 

They are represented in Feynman diagrams as follows:

Feynman Electron initial stateFeynman Vertex

A fermion such as an electron or a quark in the initial state is represented by a solid line with an arrow pointing toward the vertex (shown simply as a dot in these examples).

Feynman VertexFeynman Electron initial state

A fermion in the final state is represented by a line with an arrow pointing away from the vertex.

Feynman Positron final state)Feynman Vertex

An antiparticle such as a positron or an antiquark in the initial state is represented by a solid line with an arrow pointing away from the vertex.

They appear as though moving backward along the time axis though this is not the case.

Feynman VertexFeynman Positron final state)

An antiparticle in the final state is represented by a line with an arrow pointing towards the vertex.

Sinewave (Photon)

Bosons such as the photon mediating the electromagnetic force and the W and Z bosons mediating the weak force are represented in the initial and the final states by a wavy lines.

Helix (Gluon)

Gluons, the bosons mediating the strong force are represented by a helical lines.

A vertex always 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.

Feynman Vertex: The Interaction Point

Example Interactions or Events

Electron Positron Anihilation

An electron collides with a positron and on coming into contact annihilate eachother creating a photon.

Electron Positron Pair Creation

The spontaneous transformation of a photon producing an electron and a positron.

Feynman Diagram of Beta Decay

The weak nuclear force and the process of beta decay.


The neutron (n) decays into a proton (p) with the emission of a heavy, short lived W- boson which quickly decays further into an electron (beta particle) (e-) and an anti neutrino ve.

 

The diagrams may also illustrate multi-stage interactions involving a sequence of events with the propagation of the interactions through either bosons or fermions. In such cases two sequential interactions may be illustrated by connecting the two vertices representing the individual events directly with a corresponding bosonic or fermionic propagator.

The diagram opposite involves both QED and QCD interactions.

An electron and a positron annihilate eachother in the first event (QED) producing a photon which spontaneously transforms into a quark-antiquark pair (QCD), after which the antiquark radiates a gluon (QCD).

Feynman Diagram Showing QED and QCD Interactions

 

Feynman diagrams may look like strings, but don't confuse them with "string theory" which is something quite different.


 

 

 

 

 

 

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