Feynmp: Circle with five vertices

Question

I am trying to draw a Feynman diagram by using feynmp package at one-loop level contains two different diagrams:

1. one-loop with five legs (what I draw is nice, but the problem is the loop gets disordered and it is out of shape)
2. How can I attach a tree-level vertex to one-loop? (reducible diagram). Imagine I have one-loop with four external legs (photons) but one graviton emerges from on of these legs... which actually is like attaching a tree-level vertex to the loop... Any idea? I appreciate any help.

Thanks, Naser

Answer 1

As you can read in the feynmf documentation, the lines of a diagram are like rubber bands, they stretch as you attach them to other bands (they even have a tension parameter defined). However, the command \fmffreeze literally freeze the previous draw.

So, I'd draw the loop

\begin{fmffile}{diagram}
\begin{fmfgraph}(40,40)
%%% Draw the loop
\fmfleft{i1,i2}
\fmfright{o1,o2}
\fmf{phantom}{o1,v1}
\fmf{phantom}{v2,o2}
\fmf{phantom}{i2,v3}
\fmf{phantom}{i1,v4}
\fmf{fermion}{v1,v2,v3,v4,v1}
\fmffreeze

\end{fmfgraph}
\end{fmffile} 

And them draw the legs

\begin{fmffile}{diagram}
\begin{fmfgraph}(40,40)
%%% Draw the loop
\fmfleft{i1,i2}
\fmfright{o1,o2}
\fmf{phantom}{o1,v1}
\fmf{phantom}{v2,o2}
\fmf{phantom}{i2,v3}
\fmf{phantom}{i1,v4}
\fmf{fermion}{v1,v2,v3,v4,v1}
\fmffreeze

%%% Draw the photons
\fmf{photon}{o1,v1}
\fmf{photon}{v2,o2}
\fmf{photon}{i2,v3}
\fmf{photon}{i1,v4}

\end{fmfgraph}
\end{fmffile} 

However, if you have an extra decay, one of the final photons must end into an internal vertex (say v5), and you'd need an extra final vertex (say o3)

\begin{fmffile}{diagram}
\begin{fmfgraph}(40,40)
%%% Draw the loop
\fmfleft{i1,i2}
\fmfright{o1,o2,o3}
\fmf{phantom}{o1,v1}
\fmf{phantom}{v2,o3}
\fmf{phantom}{i2,v3}
\fmf{phantom}{i1,v4}
\fmf{fermion}{v1,v2,v3,v4,v1}
\fmffreeze

%%% Draw the photons
\fmf{photon}{o1,v1}
\fmf{photon}{v2,v5}
\fmf{phantom}{v5,o3}
\fmf{photon}{i2,v3}
\fmf{photon}{i1,v4}
\fmffreeze

%%% The extra decay
\fmf{dbl_wiggly}{v5,o2}
\fmf{photon}{v5,o3}

\end{fmfgraph}
\end{fmffile} 

I obtained this

Cheers