How to change the arrow-head size in `feynmp`

Question

Consider the following MWE

\documentclass[11pt]{article}
\usepackage{feynmp-auto}
\begin{document}
\begin{fmffile}{afilename}
\setlength{\unitlength}{0.1cm}
\begin{fmfgraph}(40,25)
    \fmfleftn{i}{2}
    \fmfrightn{o}{2}
    \fmf{dashes,tension=2}{i1,v4}
    \fmf{dashes,tension=2}{o1,v1}
    \fmf{dashes,tension=2}{o2,v2}
    \fmf{dashes,tension=2}{i2,v3}
    \fmf{dbl_plain_arrow}{v3,v2,v1}
    \fmf{fermion}{v1,v4,v3}
    \fmfdotn{v}{4}

\end{fmfgraph}
\end{fmffile}
\end{document}

which produces the output

How can I change the size of the double-lined arrow-head to have the same size as the arrow-head of the single-line? I've looked into the documentation and in table 3 there is an option listed as decoration.size, but I can't figure out how to use it correctly.. If I exchange the line in the code that produces the double-lined arrow with

\fmf{dbl_plain_arrow,decoration.size=2cm}{v3,v2,v1}

nothing changes..

Appreciate any input on this.

Answer 1

I have done several compilations and changes after I have looked at the documentation...and with a lot of humility I think that it is not possible. In fact I have thought to put, for the same cicle v3, v2, v1 both \fmf{dbl_plain}{v3,v2,v1} (double line) and \fmf{phantom_arrow}{v3,v2,v1}. The proof of my hypothesis it that if you prove this code:

\documentclass[11pt]{article}
\usepackage{feynmp-auto}
\begin{document}
\begin{fmffile}{afilename}
\setlength{\unitlength}{0.1cm}
\begin{fmfgraph}(40,25)
    \fmfleftn{i}{2}
    \fmfrightn{o}{2}
    \fmf{dashes,tension=2}{i1,v4}
    \fmf{dashes,tension=2}{o1,v1}
    \fmf{dashes,tension=2}{o2,v2}
    \fmf{dashes,tension=2}{i2,v3}
    \fmf{dbl_plain}{v3,v2,v1}
    \fmf{phantom_arrow}{v3,v2,v1}
    \fmf{fermion}{v1,v4,v3}
    \fmfdotn{v}{4}

\end{fmfgraph}
\end{fmffile}
\end{document}

you have a strange output:

where you have not a rectangle. Hence the correct code is only, dbl_plain_arrow with the big arrow.

Follow-up 2021/07/14: Based on the suggestion/comment of the very nice user @campa using tension=0 into the row

\fmf{phantom_arrow,tension=0}{v3,v2,v1}

---

\documentclass[11pt]{article}
\usepackage{feynmp-auto}
\begin{document}
\begin{fmffile}{afilename}
\setlength{\unitlength}{0.1cm}
\begin{fmfgraph}(40,25)
    \fmfleftn{i}{2}
    \fmfrightn{o}{2}
    \fmf{dashes,tension=2}{i1,v4}
    \fmf{dashes,tension=2}{o1,v1}
    \fmf{dashes,tension=2}{o2,v2}
    \fmf{dashes,tension=2}{i2,v3}
    \fmf{dbl_plain}{v3,v2,v1}
    \fmf{phantom_arrow,tension=0}{v3,v2,v1}
    \fmf{fermion}{v1,v4,v3}
    \fmfdotn{v}{4}  
\end{fmfgraph}
\end{fmffile}
\end{document}

Answer 2

The FeynMF manual describes something similar in section 2.8 (Raw metafont). In particular, it shows how to draw a line with an enlarged arrow. Here we just need the opposite :-)

The standard line styles are defined in the file feynmp.mp, which is the first thing passed to your .mp file when you issue \begin{fmffile}{...}. In feynmp.mp we find

style_def dbl_plain_arrow expr p =
  draw_double_arrow p;
enddef;

so basically dbl_plain_arrow is an alias for double_arrow, which in turn is defined as

style_def double_arrow expr p =
  draw_double p;
  shrink (1.5);
    cfill (arrow p);
  endshrink;
enddef;

The shrink bit is what makes the arrow look larger (because of the factor 1.5). You can then overwrite this definition and use a normal arrow

\documentclass[11pt]{article}
\usepackage{feynmp-auto}
\begin{document}
\begin{fmffile}{afilename}
\fmfcmd{%
 style_def double_arrow expr p =
  draw_double p;
  cfill (arrow p);
 enddef;
}%
\setlength{\unitlength}{0.1cm}%
\begin{fmfgraph}(40,25)
    \fmfleftn{i}{2}
    \fmfrightn{o}{2}
    \fmf{dashes,tension=2}{i1,v4}
    \fmf{dashes,tension=2}{o1,v1}
    \fmf{dashes,tension=2}{o2,v2}
    \fmf{dashes,tension=2}{i2,v3}
    \fmf{dbl_plain_arrow}{v3,v2,v1}
    \fmf{fermion}{v1,v4,v3}
    \fmfdotn{v}{4}

\end{fmfgraph}
\end{fmffile}
\end{document}

Alternatively you could redefine only dbl_plain_arrow, such that you can still have the original behaviour with double_arrow.