[Tex/LaTex] Feynman Diagrams with tikz

feynmantikz-pgf

I am trying to find some example code for some very simple Feynman diagrams. Something similar to any or all of the pictures below

Simple Feynman diagrams

I was able to find some example code on texample.net, but being new to TikZ I am not really able to tweak it to produce something simple like the examples above. I would appreciate any help :).

EDIT: I managed to get the code down to a smaller example that is close to what I want:

\documentclass[a4paper,12pt]{article}
\usepackage{tikz}
\usetikzlibrary{decorations.pathmorphing,decorations.markings,trees}
\begin{document}

    \tikzset{
    photon/.style={decorate, decoration={snake}, draw=red},
    particle/.style={draw=blue, postaction={decorate},
        decoration={markings,mark=at position .5 with {\arrow[draw=blue]{>}}}},
    antiparticle/.style={draw=blue, postaction={decorate},
        decoration={markings,mark=at position .5 with {\arrow[draw=blue]{<}}}},
    gluon/.style={decorate, draw=black,
        decoration={snake,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=3.5cm},
    ]
    \coordinate
        child[grow=left]{
            edge from parent [gluon]
            child {
                edge from parent [particle]
                node [above] {$g$}
            }
            child {
                edge from parent [antiparticle]
                node [below] {$g$}
            }
            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 {
                edge from parent [antiparticle]
                node [above] {$g$}
            }
            child {
                edge from parent [particle]
                node [below] {$g$}
            }
        };
\end{tikzpicture}
\end{document}

However, if instead of the coil decoration for the gluon, I want a straight dashed line, I change the line

gluon/.style={decorate, draw=black,
        decoration={snake,amplitude=4pt, segment length=5pt}}

for

gluon/.style={draw=black,dashed}

However, this makes the two particles to the left of the gluon become dashed as well. Can someone suggest a way to fix this?

Best Answer

As far as I understand it, the "dashed" style option is a shorthand that sets /tikz/dash pattern to a dashed style (PGF manual v2.1, p157). That then stays in effect for the rest of the path, until you change the dash pattern again. Because the other styles don't specify the dash pattern, they'll keep using whatever was in effect. Try adding "solid" to the definitions of (anti-)particle and gluon, and it should work.

Related Solutions

[Tex/LaTex] Rotate a node but not its content: the case of the ellipse decoration

The source of the difficulty is that ellipses are constructed in a particular way in TikZ. They are paths that start from the x-axis and proceed counter-clockwise around their centre. The vast majority of the time, the exact parametrisation doesn't matter. You appear to have found the one situation where it does!

In the actual question, you only want to be able to mirror the ellipse, and so draw it starting from the negative x-axis (the title of the question suggests a more flexible approach). That's actually not too hard since we can exploit the symmetry of the ellipse. The key is to provide it with a negative x-radius, since then it will start from the negative x-axis (and proceed clockwise, but we could correct for that by negating the y-radius as well). To do this, we interrupt the call from the node shape to the drawing command and flip the sign of the x-radius. The simplest way to do this is to redefine the \pgfpathellipse macro to do the negation and then call the original macro. The following code does this.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations,shapes,decorations.markings}
\makeatletter
\let\origpgfpathellipse=\pgfpathellipse
\def\revpgfpathellipse#1#2#3{%
  #2%
  \pgf@xa=-\pgf@x
  \origpgfpathellipse{#1}{\pgfqpoint{\pgf@xa}{0pt}}{#3}}
\makeatother

\tikzset{
  reversed ellipse/.style={
    ellipse,
    reverse the ellipse%
  },
  reverse the ellipse/.code={
    \let\pgfpathellipse=\revpgfpathellipse
  }
}
\begin{document}
\begin{tikzpicture}
\node[ellipse,
  draw,
  postaction={
    decorate,
    decoration={
      markings,
      mark=at position 1 with {
        \arrow[line width=5pt,blue]{>}
      }
    }
  }
] at (0,0) {hello world};

\node[reversed ellipse,
  draw,
  postaction={
    decorate,
    decoration={
      markings,
      mark=at position 1 with {
        \arrow[line width=5pt,blue]{>}
      }
    }
  }
] at (0,-2) {hello world};
\end{tikzpicture}

\end{document}

Here's the result:

rotated ellipse

(the arrow got clipped, but you can see where it lies)

[Tex/LaTex] Tikz-Tree: Arrow-style depending on parent node

You can set an option that only applies to the children, if you use the every child style.

The no edge from this parent style is setup in a way that the children of the child it is used on doesn’t draw the edge.

Instead of draw one you can set any other option an edge can have. This does not include a \draw command!

For edges in a tree there is a special key that sets the path: edge from parent path.

The default path is

(\tikzparentnode\tikzparentanchor) -- (\tikzchildnode\tikzchildanchor)

Code

\documentclass[tikz,convert=false]{standalone}
\begin{document}
\begin{tikzpicture}[
  no edge from this parent/.style={
    every child/.append style={
      edge from parent/.style={draw=none}}},
  a/.style={circle, draw=black},
  b/.style={rectangle, draw=red},
]
\node[a] {}
  child {node[a] {}}
  child[no edge from this parent] {node[b] {no}
    child {node[a] {no}}
    child {node[b] {no}}
  }
  child {node[a] {}}
;
\end{tikzpicture}
\end{document}

Output

enter image description here