Published 2006-11-15 |
Author:
Kjell Magne Fauske

I saw this Feynman diagram in Edward Tufte’s book Beautiful evidence (you can also find it in this thread). It was relatively easy to recreate using trees and decorations.

**Update**: Rewritten using PGF 2.0 features.

Download as: [PDF] [TEX] • [Open in writeLaTeX]

Do you have a question regarding this example, TikZ or LaTeX in general? Just ask in the
**LaTeX Forum**.

Oder frag auf Deutsch auf TeXwelt.de.
En français: TeXnique.fr.

```
% Feynman diagram
% Requires PGF >= 2.0
\documentclass{article}
\usepackage[latin1]{inputenc}
\usepackage{tikz}
\usetikzlibrary{trees}
\usetikzlibrary{decorations.pathmorphing}
\usetikzlibrary{decorations.markings}
\begin{document}
% Define styles for the different kind of edges in a Feynman diagram
\tikzset{
photon/.style={decorate, decoration={snake}, draw=red},
electron/.style={draw=blue, postaction={decorate},
decoration={markings,mark=at position .55 with {\arrow[draw=blue]{>}}}},
gluon/.style={decorate, draw=magenta,
decoration={coil,amplitude=4pt, segment length=5pt}}
}
\begin{tikzpicture}[
thick,
% Set the overall layout of the tree
level/.style={level distance=1.5cm},
level 2/.style={sibling distance=2.6cm},
level 3/.style={sibling distance=2cm}
]
\coordinate
child[grow=left]{
child {
node {$g$}
% The 'edge from parent' is actually not needed because it is
% implicitly added.
edge from parent [gluon]
}
child {
node {$g$}
edge from parent [gluon]
}
edge from parent [gluon] node [above=3pt] {$g$}
}
% I have to insert a dummy child to get the tree to grow
% correctly to the right.
child[grow=right, level distance=0pt] {
child {
child {
child {
node {$\bar{d}$}
edge from parent [electron]
}
child {
node {$u$}
edge from parent [electron]
}
edge from parent [photon]
}
child {
node {$b$}
edge from parent [electron]
}
edge from parent [electron]
node [below] {$t$}
}
child {
child {
node {$\bar{b}$}
edge from parent [electron]
}
child {
child {
node {$\bar{v}$}
edge from parent [electron]
}
child {
node {$e^{-}$}
edge from parent [electron]
}
edge from parent [photon]
}
edge from parent [electron]
node [above] {$\bar{t}$}
}
};
\end{tikzpicture}
\end{document}
```

## Comments

The arrows on tbar and bbar should be reversed.

Actually, all of the antifermion arrows need to be reversed!

I know very little about Feynman diagrams, so you are probably right. The diagram is based on a diagram in Edward Tufte’s book Beautiful evidence. If my version is wrong then the original is wrong as well.

If the anti-fermion arrows are reversed, should the particle then not be called a tbar and bbar and just t and b. Is it not that a t particle is a tbar travelling backwards in time, and so vice versa. It depends if the diagram is showing progress through space or through time.

is there an example about loop diagrams (fermion, fermion-boson, gluions)

Feynman diagrams are not pictures of particles flying in space or time. The vertices, internal lines, and external lines all represent specific factors in the calculation of the scattering amplitudes.

A vertex with two arrows pointing into it and a wavy line going out would represent two fermions annihilating into a vector (eg a photon). This process is not allowed in the standard model, and such vertices would cause eg proton decay.

With the arrows correctly drawn the diagram represents the ttbar production and subsequent decay into the single electron channel, on of the most important discovery channel for the top-quark at the Tevatron.

I find it ironic that while Feynman and Tufte apparently knew each other well (from serving on the Challenger commission), Tufte's example of Feynman's brilliant maths-in-pictures turns out total gibberish.

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