Positron - Photon feynman diagram

I made a simple Feynman diagram of the channel S=(k1 + k2)^2 in the Positron - Photon scattering as follows

\feynmandiagram[horizontal=a to b]{
i1 [particle=$$\gamma$$]
-- [photon, momentum=$$\vec{k}_1$$] a -- [fermion, momentum'=$$\vec{k}_2$$] i2 [particle=$$e^{+}$$],
a -- [anti fermion, edge label'=$$(\vec{k}_1+\vec{k}_2)$$] b,
f1 [particle=$$\gamma$$]
-- [photon, momentum'=$$\vec{k}_3$$] b -- [anti fermion, momentum=$$\vec{k}_4$$] f2 [particle=$$e^{+}$$],
};


The problem is that the k2 and k3 arrows are not in the correct direction. How may I do it to get the correct one? I do not get how to build this S channel properly.

• 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. – Henri Menke Jan 17 '18 at 22:43

Use reversed momentum (or rmomentum) as described in the manual.

\documentclass{article}
\usepackage{tikz-feynman}
\begin{document}

\begin{tikzpicture}
\feynmandiagram[horizontal=a to b]{
i1 [particle=$$\gamma$$]
-- [photon, momentum=$$\vec{k}_1$$] a -- [fermion, reversed momentum'=$$\vec{k}_2$$] i2 [particle=$$e^{+}$$],
a -- [anti fermion, edge label'=$$(\vec{k}_1+\vec{k}_2)$$] b,
f1 [particle=$$\gamma$$]
-- [photon, reversed momentum'=$$\vec{k}_3$$] b -- [anti fermion, momentum=$$\vec{k}_4$$] f2 [particle=$$e^{+}$$],
};
\end{tikzpicture}

\end{document}


\documentclass{article}
\usepackage{tikz-feynman}
\begin{document}
\begin{tikzpicture}

\feynmandiagram[horizontal=a to b]{
i1 [particle=$$\gamma$$]
-- [photon, momentum=$$\vec{k}_1$$] a,
i2 [particle=$$e^{+}$$]  -- [anti fermion, momentum=$$\vec{k}_2$$] a,
a -- [anti fermion, momentum'=$$(\vec{k}_1+\vec{k}_2)$$] b,
b -- [anti fermion, momentum=$$\vec{k}_4$$] f2 [particle=$$e^{+}$$],
b -- [photon, momentum=$$\vec{k}_3$$]   f1 [particle=$$\gamma$$]
};
\end{tikzpicture}
\end{document}