# Feynman Diagram (feynmp-auto) : 4-Quark Operator

I modified a sample from this source :

(Look for 5.3 Cross diagram) and my aim is to label every of the "four" fermion propagators with an impuls.

My problem is that displaying a crossed diagram only gives me to propagators to label, but I want to have four. So my questions are : 1. Is there a nice way to label 2 propagators with 4 impulses? 2. Is there a better way to code a four propagator crossed diagram (to display a four-quark operator)?

\begin{fmffile}{Diagram0th}
\begin{fmfgraph*}(100,100)
\fmfleft{i1,i2}
\fmfright{o1,o2}

\fmflabel{$x_3$}{i1}
\fmflabel{$x_1$}{i2}
\fmflabel{$x_4$}{o1}
\fmflabel{$x_2$}{o2}

\fmflabel{z}{v1}

\fmf{fermion, label=$p_1$}{i1,v1,o2}
\fmf{fermion}{i2,v2,o1}
\fmf{photon}{v1,v2}

\fmfv{decoration.shape=square}{v1}
\end{fmfgraph*}
\end{fmffile}


Best regards, Dirk

I don't fully understand your question. You have 5 propagators in the diagram I think you are trying to produce and each can be labelled independently. It's worth remembering that \fmf{type,options...}{v1,v2,v3...} is just shorthand for \fmf{type,options...}{v1,v2} \fmf{type,options...}{v2,v3...} so you can split any groups of propagators and write them separately and then control each one's options independently. For example I think your problem is you want to label i1,v1 and i1,o2 with different messages but currently both of those are combined as v1,i1,o2 but that is just a shorthand so is easy once they are split.

Also it's important to notice that with your code the photon isn't visible as v1 and v2 will end up in the same location. This can be fixed the same way it is in the examples you linked to by drawing with phantom lines and drawing the others either with tension=0 or after an \fmffreeze

My attempt at fixing your code based on what I think you want gives:

                  \fmfleft{i1,i2}
\fmfright{o1,o2}

\fmflabel{$x_3$}{i1}
\fmflabel{$x_1$}{i2}
\fmflabel{$x_4$}{o1}
\fmflabel{$x_2$}{o2}

\fmflabel{z}{v1}

\fmf{fermion, label=$p_2$}{v2,o2}
\fmf{fermion, label=$p_4$}{v1,o1}

\fmf{photon}{v1,v2}
\fmf{phantom}{v1,i1}
\fmf{phantom}{v2,i2}

\fmffreeze

\fmf{fermion}{i1,i1a,v2}
\fmf{fermion,rubout}{i2,i2a,v1}
\fmf{phantom,label=$p_1$,tension=0}{i2,i2a}
\fmf{phantom,label=$p_3$,tension=0}{i1,i1a}
\fmf{phantom,tension=0.5}{i2a,v1}
\fmf{phantom,tension=0.5}{i1a,v2}


which produces the output: