I try to draw this Feynman diagram with feynmf
in LaTeX
:
Here is my code:
\begin{fmffile}{edm_neutrino}
\begin{fmfchar*}(50,40)
\fmfkeep{vlm1}
\fmfleft{o}\fmflabel{$\nu_\tau$}{o}
\fmfright{i}\fmflabel{$\nu_\tau$}{i}
\fmftop{g}\fmflabel{$\gamma_\mu$}{g}
\fmf{plain, tension=1.5}{i,v1} \fmflabel{$Y_{ib}$}{v1}
\fmf{phantom, label=$\tau_j$, tension=0.8, left=1.0,tag=1}{v2,v1}
\fmf{dashes_arrow, label=$W^+$, left=-1.0, tension=0.4}{v2,v1}
\fmf{plain, tension=2.5}{v2,o}
\fmfposition
\fmfipath{p[]}
\fmfiset{p1}{vpath1(__v2,__v1)}
\fmfi{fermion}{subpath (0,2length(p1)/3) of p1}
\fmfi{fermion}{subpath (2length(p1)/3,length(p1)) of p1}
\fmfi{photon}{point length(p1)/3 of p1 -- point 2length(p1)/3 of p1}
%should go to g
\end{fmfchar*}
\end{fmffile}
But I don't know how to connect the point on the W^-
line to the external point g
. Is there any possibility to write something like that?
\fmfi{photon}{point 2length(p1)/3 of p1 -- point g}
This line is not correct, I know. But how to fix?
Thank you very much for reading!
matthias
Best Answer
You are beyond the powers of the
feynmf
automatic layout routine so you need to use "immediate" mode to place your nodes.However at that point, it usually become easier to draw the diagram directly in Metapost, using the
feynmp
macros. Like this:(Note that a
fullcircle
path has 8 points on it running anticlockwise from the 3 o'clock position.)The interface is quite neat:
feynmp
defines a number of macros as path decorators, that take a path as the right argument and return a decorated path. For examplefermion
above. Unfortunately the set of decorators is not quite complete, so for the dashed arrow we have to call thedraw_dashes_arrow
macro directly. The only documentation that I've ever found is the Metapost source file, butfmfman.pdf
is still useful.