TeX - LaTeX Asked on September 30, 2020
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
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:
prologues := 3;
outputtemplate := "%j%c.eps";
input feynmp;
beginfig(1);
path ring; ring = fullcircle scaled 78;
z1 = -z2 = 78 left; z3 = z2 rotated -24;
label.top(btex $tau_j$ etex, point 2 of ring);
label.bot(btex $W^+$ etex, point 6 of ring);
label.top(btex $nu_tau$ etex, z1);
label.top(btex $nu_tau$ etex, z2);
label.rt(btex $gamma_mu$ etex, z3);
draw fermion z1 -- point 4 of ring;
draw fermion subpath(4,0) of ring;
draw fermion point 0 of ring -- z2;
draw_dashes_arrow subpath(4,8) of ring;
draw photon origin -- z3 cutbefore ring;
endfig;
end.
(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 example fermion
above. Unfortunately the set of decorators is not quite complete, so for the dashed arrow we have to call the draw_dashes_arrow
macro directly. The only documentation that I've ever found is the Metapost source file, but fmfman.pdf
is still useful.
Answered by Thruston on September 30, 2020
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