# Balance length of lines in Feynman diagram (feynmp)

How can one make the Feynman diagram more symmetric? I want the 'photon/Z propagator' to be much shorter, and if possible, some more space in the right most part of my diagram, where the 'charginos' and 'Ws' decay.

I have tried to search for how phantom and fmffreez works, but I can't find out how it can help me.

% DIRECT PRODUCTION OF CHARGINO/NEUTRALINO
\unitlength=1mm
\begin{figure}
\begin{center}
\begin{fmffile}{wz}
\begin{fmfgraph*}(90,30)
% INCOMING + OUTGOING PARTICLES
\fmfleft{d1,d2,e1,d3,d4,e2,d5,d6} % incoming particles, from below
\fmfright{l1,n,X01,X02,q,qbar} % outgoing particles, from below
\fmf{phantom}{d1,v0}
\fmf{phantom}{d2,v0}
\fmf{phantom}{d3,v0}
\fmf{phantom}{d4,v0}
\fmf{phantom}{d5,v0}
\fmf{phantom}{d6,v0}
\fmf{plain}{e1,v0} % incoming electron
\fmf{plain}{e2,v0} % incoming positron
\fmf{photon,label=$\gamma/Z$}{v0,v1}

% DRAW STUFF
\fmf{photon,label=\textcolor{red} {$\tilde{\chi}_1^{\pm}$},label.side=right,foreground=red}{v1,v2} % chargino, lower
\fmf{photon,label=\textcolor{red}{$\tilde{\chi}_1^{\pm}$},label.side=left,foreground=red}{v1,v3}  % chargino, upper
\fmf{photon,label=$W^{\pm}$,label.side=right,label.dist=0.3}{v2,v4} % lower W
\fmf{photon,label=$W^{\pm}$,label.side=left,label.dist=0.2}{v3,v5} % upper W
% decay of upper W
\fmf{plain}{l1,v4,n}
% decay of lower W
\fmf{plain}{q,v5,qbar}
% neutralinos in final state
\fmf{photon,foreground=red}{v2,X01} % upper
\fmf{photon,foreground=red}{v3,X02} % lower
% additional lines for SUSY particles
\fmf{plain,foreground=red}{v3,X02}
\fmf{plain,foreground=red}{v1,v3}
\fmf{plain,foreground=red}{v1,v2}
\fmf{plain,foreground=red}{v2,X01}

% LABELLING
\fmflabel{$e^+$}{e2} % incoming proton (first, from top)
\fmflabel{$e^-$}{e1} % incoming proton (second)
\fmflabel{$\bar{q}$}{qbar} % lepton in final state (first) in Z decay
\fmflabel{$q$}{q} %  lepton in final state (second) in Z decay
\fmflabel{$\nu$}{n} % neutralino in final state from W decay
\fmflabel{$\ell^{\pm}$}{l1} % lepton in final state from W decay
\fmflabel{\textcolor{red}{$\tilde{\chi}_1^0$}}{X01} % final state neutralino
\fmflabel{\textcolor{red}{$\tilde{\chi}_1^0$}}{X02} % final state neutralino
\end{fmfgraph*}
\end{fmffile}
\end{center}
\hspace{1cm}
\end{figure}


Add some tension to the long photon.

\fmf{photon,tension=3,label=$\gamma/Z$}{v0,v1}


looks about right on my system.

Each line you draw has a default tension of 1 and a unit weight. Because you have so many extra lines on the right, they stretch the photon in the middle. Adding tension to the photon balances the pull from the right.

• Thank you so much! This was exactly what I needed. I have never quite understood what the tension command does, but things got clearer now. Commented Sep 19, 2016 at 8:05

Although you mention using feynmf/feynmp, I'll provide an answer that uses another package: TikZ-Feynman (CTAN). Don't feel obliged to accept this answer if you need to use feynmf/feynmp.

Using the default underlying algorithm to place all the vertices, this can be done with:

\feynmandiagram [horizontal=z1 to z2] {
%% Incoming electrons
e1 [particle=$$e^{-}$$]
-- [fermion] z1
-- [fermion] e2 [particle=$$e^{+}$$],

%% gamma/Z propagator
z1 -- [boson, edge label=$$\gamma / Z$$] z2,

%% Supersymmetric particles
{[edges={plain, boson}]
z2 -- [edge label'=$$\tilde \chi_{1}^{\pm}$$] x1
-- x3 [particle=$$\tilde \chi_{1}^{0}$$],
z2 -- [edge label=$$\tilde \chi_{1}^{\pm}$$] x2
-- x4 [particle=$$\tilde \chi_{1}^{0}$$],
},
%% We don't want the \chi to be too far apart
x1 -- [draw=none] x2,

x1 -- [boson, edge label'=$$W^{\pm}$$] w1,
q11 [particle=$$\overline q$$] -- [fermion] w1 -- [fermion] q12 [particle=$$q$$],
x2 -- [boson, edge label=$$W^{\pm}$$] w2,
q21 [particle=$$\overline q$$] -- [fermion] w2 -- [fermion] q22 [particle=$$q$$],

%% Finally, the algorithms wants to fan out the particles as much as possible,
%% so to counter that we tie them together
{[edges={draw=none}]
q11 -- q12 -- x3 -- x4 -- q21 -- q22,
},
};


which may suit you, but if you want finer control, then you'll have to use manual or relative placement of vertices. The code gets a little more complicated, but nothing unmanageable:

\begin{tikzpicture}
\begin{feynman}
%% e e -> Z
\vertex (z1);
\vertex [above left=1.5cm and 1cm of z1] (e1) {$$e^{-}$$};
\vertex [below left=1.5cm and 1cm of z1] (e2) {$$e^{+}$$};
\vertex [right=2cm of z1] (z2);

%% Z -> X X
\vertex [above right=1cm of z2] (x1);
\vertex [below right=1cm of z2] (x2);

%% We now add the decays of the supersymmetric particles
\vertex [right=2cm of x1] (x3) {$$\tilde \chi_{1}^{0}$$};
\vertex [above right=1cm of x1] (w1);
\vertex [right=1cm of w1] (q11) {$$\overline q$$};
\vertex [above right=0.5cm and 1cm of w1] (q12) {$$q$$};
%% Repeat with the second particle
\vertex [right=2cm of x2] (x4) {$$\tilde \chi_{1}^{0}$$};
\vertex [below right=1cm of x2] (w2);
\vertex [right=1cm of w2] (q21) {$$\overline q$$};
\vertex [below right=0.5cm and 1cm of w2] (q22) {$$q$$};

%% Now connect the dots (literally)
\diagram* {
(e1) -- [fermion] (z1) -- [fermion] (e2),
(z1) -- [boson, edge label=$$\gamma / Z$$] (z2),

{[edges={plain, boson}]
(z2) -- [edge label=$$\tilde \chi_{1}^{\pm}$$] (x1) -- (x3),
(z2) -- [edge label'=$$\tilde \chi_{1}^{\pm}$$] (x2) -- (x4),
},

(x1) -- [boson, edge label=$$W^{\pm}$$] (w1),
(x2) -- [boson, edge label'=$$W^{\pm}$$] (w2),

(q11) -- [fermion] (w1) -- [fermion] (q12),
(q21) -- [fermion] (w2) -- [fermion] (q22),
};
\end{feynman}
\end{tikzpicture}


For each of them, the code was enclosed in:

\documentclass[tikz]{standalone}

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

\begin{document}
%% <code above>
\end{document}

• Thank you so much for fixing the problem, but I think I want to hold on to the feynmp package for now. I'll remember the tikz option for later:) Commented Sep 19, 2016 at 8:06