# Tikz Feynman Delbruck Scattering

I'd like to make a Delbruck scattering diagram with Tikz-feynman but for some reason my code wont compile, its blowing up my CPU and eventually memory. Have I written something wrong, is there a better way to do this? I mostly just copied the box diagram from the documentation.

\feynmandiagram [layered layout, horizontal=a to b] {
% Draw the top and bottom lines
i1
-- [photon, edge label = $$\gamma$$] a
-- [fermion, edge label=$$e$$] b
-- [photon, edge label $$\gamma$$] f1 ,
i2
-- [photon, edge label $$\gamma$$]  c
-- [anti fermion, edge label=$$e$$]  d
-- [photon, edge label $$\gamma$$]  f2,
% Draw the two internal fermion lines
{ [same layer] a -- [anti fermion, edge label =$$e$$] c },
{ [same layer] b -- [fermion, edge label=$$e$$] d},
};

• Welcome to TeX.SE! Please make your code snippet compilable. – Kurt Feb 1 '18 at 1:59
• Welcome to TeX.SX! Please make your code compilable (if possible), or at least complete it with \documentclass{...}, the required \usepackage's, \begin{document}, and \end{document}. That may seem tedious to you, but think of the extra work it represents for TeX.SX users willing to give you a hand. Help them help you: remove that one hurdle between you and a solution to your problem. – Martin Schröder Feb 6 '18 at 22:39

## 1 Answer

You simply forgot three times to put = signs after label.

\documentclass{article}
\usepackage{tikz-feynman}
\begin{document}
\feynmandiagram[layered layout, horizontal=a to b] {
% Draw the top and bottom lines
i1
-- [photon, edge label = $$\gamma$$] a
-- [fermion, edge label=$$e$$] b
-- [photon, edge label=$$\gamma$$] f1 ,
i2
-- [photon, edge label=$$\gamma$$]  c
-- [anti fermion, edge label=$$e$$]  d
-- [photon, edge label=$$\gamma$$]  f2,
%Draw the two internal fermion lines
{ [same layer] a -- [anti fermion, edge label =$$e$$] c },
{ [same layer] b -- [fermion, edge label=$$e$$] d},
};
\end{document}