# How to draw Feynman diagram for Majorana particles?

This is what I got. Here is an example. Can you help me with the second one? I am able to do all but not the cross in the center.

$0\nu\beta\beta$

\usepackage{feynmp}

\usepackage{ifpdf}
\ifpdf
\DeclareGraphicsRule{*}{mps}{*}{}
\fi

\makeatletter
\def\endfmffile{%
\fmfcmd{\p@rcent\space the end.^^J%
end.^^J%
endinput;}%
\if@fmfio
\immediate\closeout\@outfmf
\fi
\IfFileExists{\thefmffile.mp}{\immediate\write18{mpost \thefmffile}}{}
\let\thefmffile\relax
}
\makeatother

\begin{fmffile}{0nu2beta}
\begin{fmfgraph}(200,150)
\fmfpen{thick}
\fmfleft{i1,i4}
\fmfright{o1,o2,o3,o4}
\fmf{fermion,tension=1}{i1,v1}
\fmf{fermion,tension=0.5}{v1,o1}
\fmf{fermion,tension=1}{i4,v4}
\fmf{fermion,tension=0.5}{v4,o4}
\fmf{fermion,tension=.35}{v2,o2}
\fmf{fermion,tension=.35}{v3,o3}
\fmf{fermion,tension=0.20}{v2,v3}
\fmf{fermion,tension=0.20}{v3,v2}
%\fmf{crossed}{v2,v3}
\fmf{photon,tension=0.5}{v1,v2}
\fmf{photon,tension=0.5}{v4,v3}
\fmfdot{v1,v2,v3,v4}
\end{fmfgraph}
\end{fmffile}'


crossed is not working

(P.S. I know I am not using this copied code but cross should work and its not working with \fmf{crossed}{v2,v3} why? and how to solve it?

\fmfcmd{%
style_def majorana expr p =
cdraw p;
cfill (harrow (reverse p, .5));
cfill (harrow (p, .5))
enddef;
style_def alt_majorana expr p =
cdraw p;
cfill (tarrow (reverse p, .55));
cfill (tarrow (p, .55))
enddef;})

• Welcome! Show us what you've done! The best way to do that is to provide a minimal document that we can compile, a MWE. Feb 15, 2016 at 20:09
• Please be aware that not everybody is familiar with particle physics! (A majorana particle is a fermion that is its own anti-particle)
– user31729
Feb 15, 2016 at 20:15
• With so little in the way of information, I would suggest you look at this link and see if it solves your issue. If so, let us know. tex.stackexchange.com/questions/86588/… Feb 15, 2016 at 20:16

Instead of feynmp, there is a new package called tikz-feynman (project page) which also allows you to draw Feynman diagram. In particular, it has support for Majorana particles and mass insertions:

\documentclass[convert]{standalone}

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

\begin{document}
\begin{tikzpicture}
\begin{feynman}
\vertex (b);
\vertex [below=of b] (c);
\vertex [below left=1cm and 0.4cm of c] (d);
\vertex [above left=1cm and 0.4cm of b] (a);
\vertex [left=of a] (i1);
\vertex [left=of d] (i2);
\vertex[above right of=a] (f1);
\vertex[right of=b] (f2);
\vertex[right of=c] (f3);
\vertex[below right of=d] (f4);

\diagram* {
(a) -- [photon] (b) -- [majorana, insertion=0.5] (c) -- [photon] (d),
(i1) -- [fermion] (a),
(i2) -- [fermion] (d),
(a) -- [fermion] (f1),
(b) -- [fermion] (f2),
(c) -- [fermion] (f3),
(d) -- [fermion] (f4),
};
\end{feynman}
\end{tikzpicture}
\end{document}


I just paste the code for feynmp.

\begin{fmfgraph*}(50,50)
\fmfleft{i0,i1,i2,i3}
\fmfright{f0,f1,f2,f3}
\fmf{fermion,label=$n$,label.side=right}{i3,v1}
\fmf{fermion,label=$p$,label.side=right}{v1,f3}
\fmf{boson,label=$W$,label.side=right,tension=0.3}{v1,v2}
\fmf{fermion,label=$e^-$,label.side=right}{v2,f2}
\fmf{phantom}{i2,v2}
\fmf{plain}{v2,v3,v4}
\fmf{phantom}{i1,v4}
\fmf{fermion,label=$e^-$,label.side=right}{v4,f1}
\fmf{boson,label=$W$,label.side=right,tension=0.3}{v4,v5}
\fmf{fermion,label=$n$,label.side=right}{i0,v5}
\fmf{fermion,label=$p$,label.side=right}{v5,f0}
\fmfdot{v1,v2,v4,v5}
\fmfv{decor.shape=cross,decor.size=.1w}{v3}
\end{fmfgraph*}


I added a cross on a dummy vertex and now the code for neutrinoless double beta decay is: [P.S. Use Astrek (with fmfgraphs) to use labels!]

    \documentclass[paper=a4, fontsize=11pt]{article}
\usepackage{feynmp}
%\usepackage{feynmf}
\usepackage{ifpdf}
\ifpdf
\DeclareGraphicsRule{*}{mps}{*}{}
\fi
\makeatletter
\def\endfmffile{%
\fmfcmd{\p@rcent\space the end.^^J%
end.^^J%
endinput;}%
\if@fmfio
\immediate\closeout\@outfmf
\fi
\IfFileExists{\thefmffile.mp}{\immediate\write18{mpost \thefmffile}}{}
\let\thefmffile\relax
}
\makeatother
\begin{document}
\begin{center}
\begin{fmffile}{0nu2beta}
\begin{fmfgraph*}(200,200)
%\fmfpen{thick}
\fmfleft{i1,i4}
\fmfright{o1,o2,o3,o4}
\fmf{fermion,tension=.8}{i1,v1}
\fmf{fermion,tension=0.5}{v1,o1}
\fmf{fermion,tension=.8}{i4,v4}
\fmf{fermion,tension=0.5}{v4,o4}
\fmf{fermion,tension=.4}{v2,o2}
\fmf{fermion,tension=.4}{v3,o3}
\fmf{fermion,tension=1}{v5,v2}
\fmf{fermion,tension=1}{v5,v3}
\fmfv{decor.shape=cross}{v5} % This is the point where I used cross on a dummy vertex
\fmf{photon,tension=.65,label=$W_L$}{v1,v2}
\fmf{photon,tension=.65,label=$W_L$}{v4,v3}
%\fmf{phantom, tension=2}{o1,o2,o3,o4,i1}
\fmfdot{v1,v2,v3,v4}
\fmflabel{$v_1$}{v5}
\fmflabel{u}{o1}
\fmflabel{$e_L$}{o2}
\fmflabel{$e_L$}{o3}
\fmflabel{u}{o4}
\fmflabel{d}{i1}
\fmflabel{d}{i4}
\end{fmfgraph*}
\end{fmffile}

\end{center}
\end {document}


There is an interesting link I found to help you with Feynman Diagrams: Blog for installing feynmf and feynmp packages to Miktex (with some good ref. for using these packages too)!

• Are you aware that feynmp-auto package avoids you 99% of your preamble? Feb 15, 2016 at 23:14