How do I put a label inside a Feynman diagram circle?

Question

I have this simple Feynman diagram with a circle:

I would like to put the number "1" in the middle of that circle.

I've tried these:

• \fmfiv{lab=1}{c}
• \fmfiv{lab=1}{.5[nw,se])
• \fmfiv{lab=1}{(.4999w, .5h)}
• \fmfiv{lab=1}{(.5001w, .5h)}

but whatever constants I put, latex refuses to put it INSIDE the circle.

The latex:

\documentclass[24pt]{article}
\usepackage{amsmath,amsfonts,epsf}
\usepackage{amssymb}
\usepackage[pdftex]{graphicx}
\usepackage{grffile}
\usepackage{feynmp-auto}

\begin{document}

    \begin{fmffile}{2ptcorrection1}
    \begin{fmfgraph*}(220,12)
    \fmfpen{thick}
    \fmfleft{i1}
    \fmfright{o1}
    \fmf{plain}{i1,v1}
    \fmf{dashes, left=1, tension=0.3}{v1,v2}
    \fmf{plain, right=1, tension=0.3}{v1,v2}
    \fmf{plain}{v2,o1}
    \end{fmfgraph*}
    \end{fmffile}

\end{document}

btw, the coordinates are: "c" for center, sw, se, ne, nw.

Update: thank you hftf for the answer.

Answer 1

I was able to make the label appear to be in the center of the circle by using a phantom (invisible) edge across the circle and setting the label.dist to 0 to place the label in the middle of that edge.

\documentclass{minimal}
\usepackage{grffile}
\usepackage{feynmp-auto}

\begin{document}

    \begin{fmffile}{2ptcorrection1}
    \begin{fmfgraph*}(220,12)
    \fmfpen{thick}
    \fmfleft{i1}
    \fmfright{o1}
    \fmf{plain}{i1,v1}
    \fmf{dashes, left=1, tension=0.3}{v1,v2}
    \fmf{plain, right=1, tension=0.3}{v1,v2}
    \fmf{plain}{v2,o1}

    % This is where the magic happens:
    \fmf{phantom,label.dist=0,label=1}{v1,v2}

    \end{fmfgraph*}
    \end{fmffile}

\end{document}

And this is what it looks like:

Answer 2

This can be done also with the new TikZ-Feynman package (see also the project page).

Inside the feynman environment, first you draw the actual diagram within the \diagram macro, then you can use the usual TikZ commands to put labels (the syntax used there requires the calc library, which is loaded b default by TikZ-Feynman). You have to compile with lualatex in order to take advantage of the automatic positioning of the vertices.

\documentclass[tikz]{standalone}
\usepackage{tikz-feynman}
\tikzfeynmanset{compat=1.0.0}
\begin{document}
\begin{tikzpicture}
  \begin{feynman}
    \diagram[layered layout, horizontal=a to d] {
      a --  b,
      b -- [scalar, half left] c -- [half left] b,
      c -- d,
    };
    \node [red] at ($(b)!0.5!(c)$) {1};
  \end{feynman}
\end{tikzpicture}
\end{document}