费曼图中的平行线

Question

我正在尝试绘制这个费曼图

有点棘手的部分是图表左侧和右侧的平行线。该怎么做？

我尝试在feynmf手册中搜索。我看到了类似的图表，它使用

\fmffreeze
\fmfi{plain}{vpath (__i1,__b) shifted (thick*(0,2))}
\fmfi{plain}{vpath (__i1,__b) shifted (thick*(1,-2))}

但就我而言，它仅在左侧产生 2 条线...

我的代码是

\documentclass{article}
\usepackage{feynmp-auto}
\unitlength=1mm
\begin{document}
\begin{fmffile}{ElasticScattering}
\fmfframe(1,7)(1,7){
  \begin{fmfgraph}(40,25)
            \fmfleft{i1,i2}
      \fmfright{o1,o2}
      \fmfbottom{b}
      \fmf{fermion}{i2,v1,o2}
      \fmf{photon}{v1,b}
      \fmfblob{.15w}{b}
      \fmf{phantom}{i1,b,o1}
      \fmffreeze
      \fmfi{plain}{vpath (__i1,__b) shifted (thick*(0,2))}
      \fmfi{plain}{vpath (__i1,__b) shifted (thick*(1,-2))}
  \end{fmfgraph}}
\end{fmffile}
\end{document}

我的结果是

Answer 1

\documentclass{article}
\usepackage{feynmp-auto}
\unitlength=1mm
\begin{document}
\begin{fmffile}{ElasticScattering}
  \begin{fmfgraph*}(40,25)
    % External vertices on the left
    \fmfleft{i1,i2}
    % External vertices on the right
    \fmfright{o1,o2}
    % Labels on external vertices
    \fmflabel{\(k\)}{i2}
    \fmflabel{\(p\)}{i1}
    \fmflabel{\(k'\)}{o2}
    \fmflabel{\(p'\)}{o1}
    % Add a fermion line between `i2', new internal vertex `v' and `o2'
    \fmf{fermion}{i2,v,o2}
    % Add a single plain line between `i1', new internal vertex `b' and `o1'
    \fmf{plain}{i1,b,o1}
    % Add photon line, with label, between `v' and `b'
    \fmf{photon,label=\(q\)}{v,b}
    % Insert a blob at `b'
    \fmfblob{.15w}{b}
    \fmffreeze
    % Add two plain lines between `i1' and `b', one shifted above and the
    % other below the already present plain line
    \fmfi{plain}{vpath (__i1,__b) shifted (thick*(-0.5,2))}
    \fmfi{plain}{vpath (__i1,__b) shifted (thick*(0.5,-2))}
    % Same as above, between `b' and `o1'
    \fmfi{plain}{vpath (__b,__o1) shifted (thick*(0.5,2))}
    \fmfi{plain}{vpath (__b,__o1) shifted (thick*(-0.5,-2))}
  \end{fmfgraph*}
\end{fmffile}
\end{document}

如果您想要顶点处有一个空圆b（如原始示例所示）而不是阴影圆，请\fmfblob{.15w}{b}用 替换\fmfv{decor.shape=circle,decor.filled=empty,decor.size=.15w}{b}。