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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.

  • 1
    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
3

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}

enter image description here

1

Alternative to Henri Menke's nice answer: follow the particle lines.

\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}

enter image description here

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