Tikz-feynman: Searching for the perfect tadpole

tikz-feynmantikz-styles

I'm plotting automatically generated and laid-out (by tikz-feynman's default spring layout) QED feynman diagrams.

I'm looking to improve the look of the "tadpoles" i.e the vertices where both fermion arcs are attached to the same vertex.

This image shows the options I'm aware of/have considered (the last one a "work in progress"):

Self-contained code to produce those is below (works with Debian 11's texlive's lualatex).

The first row shows use of loop. Which I'm not sure is actually part of tikz-feynman as its not mentioned in the documentation and I've just seen it used in other people's examples. That can look good when oriented opposite the photon arc, but so far as I know it needs the angles to be set manually… is there any way of automatically getting the loop to be opposite the incoming photon, bearing in mind I'm not controlling the positioning of anything explicitly and leaving it up to tikz-feynman's layout?

The second row shows simply setting the tadpole vertex to an empty circle. That'd actually be pretty good if I could make the circle a bit bigger and put an arrowhead on it somewhere.

The third row is the start of an attempt to do just that, by defining a tadpole style for a vertex.

Here's the code to produce the above plot. What I'm ideally looking for is what I should be filling out the \tikzfeynmanset{tadpole/.style={... with to get a white-filled black circle with no text label, a specified size and an arrowhead somewhere on it. However at this point I'm completely out of my depth with tikz and how this stuff interacts with tikz-feynman. Thanks for any help or pointers.

Code for above plot:

\documentclass[a4paper]{minimal}

\usepackage{tikz}
\usetikzlibrary{shapes,arrows,positioning,decorations,backgrounds,calc,er,patterns}

\usepackage[compat=1.1.0]{tikz-feynman}

% First groping towards a custom "tadpole" style.
\tikzfeynmanset{
  tadpole/.style={
    /tikz/shape=circle,
    /tikz/minimum size=5mm,
    /tikz/draw=black,
    /tikz/fill=white,
    /tikz/decoration={name=none}
  }
}

% Tweak table appearance
\setlength{\tabcolsep}{1cm}
\renewcommand{\arraystretch}{2.0}

\begin{document}

\begin{center}
  \begin{tabular}{ccc}

    \parbox{3cm}{Using \texttt{loop}:\\Looks reasonable, but needs manual orientation.}

    &

    \parbox{3cm}{
      \resizebox{!}{3cm}{
        \begin{tikzpicture}
          \begin{feynman}
            \diagram[small,vertical=out to in]{
              in [desired at={(0,-1)}],
              in -- [fermion] a [dot],
              a [dot] -- [photon] b [dot],
              b [dot] -- [out=315,in=45,loop] b [dot],
              a [dot] -- [fermion] out,
              out [desired at={(0,1)}]
            };
          \end{feynman}
        \end{tikzpicture}
      }
    }
    
    &

    \parbox{3cm}{
      \resizebox{!}{3cm}{
        \begin{tikzpicture}
          \begin{feynman}
            \diagram[small,vertical'=out to in]{
              in [desired at={(0,-1)}],
              in -- [fermion] a [dot],
              a [dot] -- [photon] b [dot],
              b [dot] -- [out=315,in=45,loop] b [dot],
              a [dot] -- [fermion] out,
              out [desired at={(0,1)}]
            };
          \end{feynman}
        \end{tikzpicture}
      }
    }
    
    \\ & & \\

    \parbox{3cm}{Using \texttt{empty dot}:\\Would be better if bigger, and with an arrow.}
    
    &

    \parbox{3cm}{
      \resizebox{!}{3cm}{
        \begin{tikzpicture}
          \begin{feynman}
            \diagram[small,vertical=out to in]{
              in [desired at={(0,-1)}],
              in -- [fermion] a [dot],
              a [dot] -- [photon] b [empty dot],
              a [dot] -- [fermion] out,
              out [desired at={(0,1)}]
            };
          \end{feynman}
        \end{tikzpicture}
      }
    }
    
    &

    \parbox{3cm}{
      \resizebox{!}{3cm}{
        \begin{tikzpicture}
          \begin{feynman}
            \diagram[small,vertical'=out to in]{
              in [desired at={(0,-1)}],
              in -- [fermion] a [dot],
              a [dot] -- [photon] b [empty dot],
              a [dot] -- [fermion] out,
              out [desired at={(0,1)}]
            };
          \end{feynman}
        \end{tikzpicture}
      }
    }

    \\ & & \\

    \parbox{3cm}{Attempting to use a custom \texttt{tadpole} style:\\I've idea how to control the size and suppress label.}
    
    &

    \parbox{3cm}{
      \resizebox{!}{3cm}{
        \begin{tikzpicture}
          \begin{feynman}
            \diagram[small,vertical=out to in]{
              in [desired at={(0,-1)}],
              in -- [fermion] a [dot],
              a [dot] -- [photon] b [tadpole],
              a [dot] -- [fermion] out,
              out [desired at={(0,1)}]
            };
          \end{feynman}
        \end{tikzpicture}
      }
    }
    
    &

    \parbox{3cm}{
      \resizebox{!}{3cm}{
        \begin{tikzpicture}
          \begin{feynman}
            \diagram[small,vertical'=out to in]{
              in [desired at={(0,-1)}],
              in -- [fermion] a [dot],
              a [dot] -- [photon] b [tadpole],
              a [dot] -- [fermion] out,
              out [desired at={(0,1)}]
            };
          \end{feynman}
        \end{tikzpicture}
      }
    }

  \end{tabular}
\end{center}

\end{document}

Best Answer

\documentclass[tikz, border=1cm]{standalone}
\usepackage[compat=1.1.0]{tikz-feynman}
\usetikzlibrary{arrows.meta}

\newcommand{\tadradius}{3mm}
\newcommand{\tadangle}{90}
\tikzset{
tadpole/.style={
circle,
minimum size=2*\tadradius,
inner sep=0,
append after command={
\pgfextra
\draw[-{Stealth[length=4pt, sep=0pt -1]}] (\tikzlastnode) +(\tadangle:\tadradius) arc[start angle=\tadangle, end angle=\tadangle-360, radius=\tadradius];
\endpgfextra
}}}

\begin{document}

\begin{tikzpicture}
\begin{feynman}
\diagram[small,vertical=out to in]{
in [desired at={(0,-1)}],
in -- [fermion] a [dot],
a [dot] -- [photon] b [tadpole],
a [dot] -- [fermion] out,
out [desired at={(0,1)}]
};
\end{feynman}
\end{tikzpicture}

\begin{tikzpicture}
\begin{feynman}
\diagram[small,vertical'=out to in]{
in [desired at={(0,-1)}],
in -- [fermion] a [dot],
a [dot] -- [photon] b [tadpole],
a [dot] -- [fermion] out,
out [desired at={(0,1)}]
};
\end{feynman}
\end{tikzpicture}

\end{document}

Edit:

If the loop is a fermion, you should change the arrow tip to Triangle to mimic tikz-feynman arrows. But why not use the packages own notation like this:

\documentclass[tikz, border=1cm]{standalone}
\usepackage[compat=1.1.0]{tikz-feynman}

\begin{document}

\begin{tikzpicture}
\begin{feynman}
\diagram[small, vertical=out to in]{
in [desired at={(0,-1)}],
in -- [fermion] a [dot],
a [dot] -- [photon] b  -- [half left] bm -- [fermion, half left] b,
a [dot] -- [fermion] out,
out [desired at={(0,1)}]
};
\end{feynman}
\end{tikzpicture}
  
\end{document}

Edit: For completeness, here is the tadpole style without label:

\documentclass[tikz, border=1cm]{standalone}
\usepackage[compat=1.1.0]{tikz-feynman}
\usetikzlibrary{arrows.meta, bending}

\newcommand{\tadradius}{3mm}
\newcommand{\tadangle}{90}
\tikzset{
tadpole/.style={
particle={},
circle,
minimum size=2*\tadradius,
inner sep=0,
append after command={
\pgfextra
\draw[-{Triangle[width=3pt, length=4pt, sep=0pt -1]}] (\tikzlastnode) +(\tadangle+10:\tadradius) arc[start angle=\tadangle+10, end angle=\tadangle-360, radius=\tadradius];
\endpgfextra
}}}

\begin{document}

\begin{tikzpicture}
\begin{feynman}
\diagram[small,vertical=out to in]{
in [desired at={(0,-1)}],
in -- [fermion] a [dot],
a [dot] -- [photon] b [tadpole],
a [dot] -- [fermion] out,
out [desired at={(0,1)}]
};
\end{feynman}
\end{tikzpicture}

\end{document}

Related Solutions

[Tex/LaTex] Installing Tikz-Feynman

Thanks for your interest in TikZ-Feynman. Since the package is available on CTAN, all you really need to do is make sure your distribution is up to date which is well described in this question.

If, for any reason, you can't update your distribution, you can still install the package in your local TEXMF tree. Firsty, you'll need to obtain the package files from either project page or CTAN. Next, you'll need to locate your local TEXMF tree, and there are instructions for that in other questions (Linux / Unix, Windows).

In the end, you should have all the files in:

$TEXMFHOME/tex/latex/tikz-feynman/<package files>

Note that TikZ-Feynman contains a few extra files in addition to the usual .cls files and just to be safe, you should copy them all.

[Tex/LaTex] tadpole diagram using tikz-feynman

According to the manual, \diagram* should be kind of a last resort, meaning that you need to do all yourself. Here is a proposal.

\documentclass{beamer}
\usetheme{Warsaw}
\usepackage{mathrsfs}
\usepackage{braket}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{tikz-feynman} 
\usetikzlibrary{calc}
\usepackage{cancel}
\begin{document}
\begin{frame}
\frametitle{A manually adjusted Feynman diagram}
\begin{overlayarea}{\textwidth}{\textheight}
\begin{tikzpicture}
\begin{feynman}
\vertex (a){\(1\)};
\vertex[right=2 cm of a ](b){\(2\)};
\vertex[below=2 cm of b] (c){\(B\)};
\vertex[left=2 cm of c](d){\(A\)};
\vertex[above=1cm of d](e);
\vertex[left=1cm of e](h);
\diagram*{
    (d)--(a), (e)--[out=135,in=90,min distance=0.5cm](h), 
    (e)--[out=-135,in=-90,min distance=0.5cm](h), (c)--(b)
};
\end{feynman}
\end{tikzpicture}
\begin{tikzpicture}
\begin{feynman}
\vertex (a){\(1\)};
\vertex[right=2 cm of a ](b){\(2\)};
\vertex[below=2 cm of b] (c){\(B\)};
\vertex[left=2 cm of c](d){\(A\)};
\vertex[above=1cm of c](e);
\vertex[right=1cm of e](h);
\diagram*{
    (d)--(a), (e)--[out=45,in=90,min distance=0.5cm](h), 
    (e)--[out=-45,in=-90,min distance=0.5cm](h), (c)--(b)
};
\end{feynman}
\end{tikzpicture}
\begin{tikzpicture}
\begin{feynman}
\vertex (a){\(1\)};
\vertex[right=2 cm of a ](b){\(2\)};
\vertex[below=2 cm of b] (c){\(B\)};
\vertex[left=2 cm of c](d){\(A\)};
\path (a)--(c) coordinate[pos=0.25] (e);
\coordinate (h) at ($(e)+(-{1/sqrt(2)},-{1/sqrt(2)})$);
\diagram*{
        (e)--[out=-180,in=135,min distance=0.5cm](h), 
    (e)--[out=-90,in=-45,min distance=0.5cm](h), (c)--(a),
    (d)--[draw=white,double=black,very thick](b)
};
\end{feynman}
\end{tikzpicture}
\begin{tikzpicture}
\begin{feynman}
\vertex (a){\(1\)};
\vertex[right=2 cm of a ](b){\(2\)};
\vertex[below=2 cm of b] (c){\(B\)};
\vertex[left=2 cm of c](d){\(A\)};
\path (b)--(d) coordinate[pos=0.25] (e);
\coordinate (h) at ($(e)+({1/sqrt(2)},-{1/sqrt(2)})$);
\diagram*{
        (e)--[out=0,in=45,min distance=0.5cm](h), 
    (e)--[out=-90,in=-135,min distance=0.5cm](h),
    (d)--(b), (c)--[draw=white,double=black,very thick](a)
};
\end{feynman}
\end{tikzpicture}

\end{overlayarea}
\end{frame}
\end{document}

As you see, you can get acceptable results this way, but it does require some effort.

As for tikzfeynhand, I never heard about this before, but you may download it from here. Looking at its manual it was rather amusing to see that this package also supports "Anti-Majorana fermions", which I previously thought were unique to tikz-feynman. This is a very funny oxymoron, but probably should not be part of any serious package that has not been released on April 1.

Best Answer

Related Solutions

[Tex/LaTex] Installing Tikz-Feynman

[Tex/LaTex] tadpole diagram using tikz-feynman

Related Question