I've been doing Feynman diagrams with TikZ for quite some time now, but I have never needed to make loops. Now I'm forced to deal with them, and here comes the issue. I've played around a little and have been inspired by other questions to use the calc package in order to draw nice loops.
Here's the best I can do. Please note how in the first diagram, the circle is messed up, while in the second (without the additional labels) it's fine. Ideally, the semicircles should carry one individual label each, the position of which should be adjustable (or should auto-adjust, depending on the orientation of the spline).
\documentclass{article}
\usepackage{pgfplots}
\usetikzlibrary{positioning,arrows,patterns}
\usetikzlibrary{decorations.markings}
\usetikzlibrary{calc}
\tikzset{
photon/.style={decorate, decoration={snake}, draw=black},
fermion/.style={draw=black, postaction={decorate},decoration={markings,mark=at position .55 with {\arrow{>}}}},
vertex/.style={draw,shape=circle,fill=black,minimum size=3pt,inner sep=0pt},
}
\newcommand{\semiloop}[4][]{%
\draw[#1] let \p1 = ($(#3)-(#2)$) in (#3) arc (#4:({#4+180}):({0.5*veclen(\x1,\y1)});)
}
\begin{document}
\begin{tikzpicture}[node distance=1cm and 1cm]
\coordinate[vertex] (v1);
\coordinate[vertex, right=of v1] (v2);
\coordinate[vertex, right=of v2] (v3);
\coordinate[vertex, right=of v3] (v4);
\coordinate[above right=of v4,label=right:$f$] (f1);
\coordinate[below right=of v4,label=right:$f$] (f2);
\coordinate[above left =of v1,label=left :$e^+$] (e1);
\coordinate[below left =of v1,label=left :$e^-$] (e2);
\draw[fermion] (e1) -- (v1);
\draw[fermion] (v1) -- (e2);
\draw[photon] (v1) -- (v2) node[midway,above=0.1cm] {$Z$};
\draw[photon] (v3) -- (v4) node[midway,above=0.1cm] {$Z$};
\draw[fermion] (f1) -- (v4);
\draw[fermion] (v4) -- (f2);
% the next two lines are causing the mess
\semiloop[fermion]{v2}{v3}{0} node[midway,above=0.1cm] {$f$};
\semiloop[fermion]{v3}{v2}{180} node[midway,above=0.1cm] {$\bar{f}$};
\end{tikzpicture}
\begin{tikzpicture}[node distance=1cm and 1cm]
\coordinate[vertex] (v1);
\coordinate[vertex, right=of v1] (v2);
\coordinate[vertex, right=of v2] (v3);
\coordinate[vertex, right=of v3] (v4);
\coordinate[above right=of v4,label=right:$f$] (f1);
\coordinate[below right=of v4,label=right:$f$] (f2);
\coordinate[above left =of v1,label=left :$e^+$] (e1);
\coordinate[below left =of v1,label=left :$e^-$] (e2);
\draw[fermion] (e1) -- (v1);
\draw[fermion] (v1) -- (e2);
\draw[photon] (v1) -- (v2) node[midway,above=0.1cm] {$Z$};
\draw[photon] (v3) -- (v4) node[midway,above=0.1cm] {$Z$};
\draw[fermion] (f1) -- (v4);
\draw[fermion] (v4) -- (f2);
% the next two lines behave fine, but don't have any labels
\semiloop[fermion]{v2}{v3}{0};
\semiloop[fermion]{v3}{v2}{180};
\end{tikzpicture}
\end{document}
If anybody can tell me what's going wrong here or can show me a more elegant way of drawing circles through two points while being able to use predefined line styles, I would be very happy.
Edit
Following an issue that has arisen from the accepted answer, this is the output of \listfiles
*File List*
article.cls 2007/10/19 v1.4h Standard LaTeX document class
size10.clo 2007/10/19 v1.4h Standard LaTeX file (size option)
pgfplots.sty 2013/10/03 v1.9 Data Visualization (1.9)
tikz.sty 2010/10/13 v2.10 (rcs-revision 1.76)
pgf.sty 2008/01/15 v2.10 (rcs-revision 1.12)
pgfrcs.sty 2010/10/25 v2.10 (rcs-revision 1.24)
everyshi.sty 2001/05/15 v3.00 EveryShipout Package (MS)
pgfrcs.code.tex
pgfcore.sty 2010/04/11 v2.10 (rcs-revision 1.7)
graphicx.sty 1999/02/16 v1.0f Enhanced LaTeX Graphics (DPC,SPQR)
keyval.sty 1999/03/16 v1.13 key=value parser (DPC)
graphics.sty 2009/02/05 v1.0o Standard LaTeX Graphics (DPC,SPQR)
trig.sty 1999/03/16 v1.09 sin cos tan (DPC)
graphics.cfg 2010/04/23 v1.9 graphics configuration of TeX Live
pdftex.def 2011/05/27 v0.06d Graphics/color for pdfTeX
infwarerr.sty 2010/04/08 v1.3 Providing info/warning/error messages (HO)
ltxcmds.sty 2011/11/09 v1.22 LaTeX kernel commands for general use (HO)
pgfsys.sty 2010/06/30 v2.10 (rcs-revision 1.37)
pgfsys.code.tex
pgfsyssoftpath.code.tex 2008/07/18 (rcs-revision 1.7)
pgfsysprotocol.code.tex 2006/10/16 (rcs-revision 1.4)
xcolor.sty 2007/01/21 v2.11 LaTeX color extensions (UK)
color.cfg 2007/01/18 v1.5 color configuration of teTeX/TeXLive
pgfcore.code.tex
pgfcomp-version-0-65.sty 2007/07/03 v2.10 (rcs-revision 1.7)
pgfcomp-version-1-18.sty 2007/07/23 v2.10 (rcs-revision 1.1)
pgffor.sty 2010/03/23 v2.10 (rcs-revision 1.18)
pgfkeys.sty
pgfkeys.code.tex
pgffor.code.tex
tikz.code.tex
xparse.sty 2013/12/31 v4634 L3 Experimental document command parser
expl3.sty 2014/01/07 v4646 L3 Experimental code bundle wrapper
l3names.sty 2014/01/04 v4640 L3 Namespace for primitives
l3bootstrap.sty 2014/01/04 v4640 L3 Experimental bootstrap code
etex.sty 1998/03/26 v2.0 eTeX basic definition package (PEB)
l3basics.sty 2014/01/04 v4642 L3 Basic definitions
l3expan.sty 2014/01/04 v4642 L3 Argument expansion
l3tl.sty 2013/12/27 v4625 L3 Token lists
l3seq.sty 2013/12/14 v4623 L3 Sequences and stacks
l3int.sty 2013/08/02 v4583 L3 Integers
l3quark.sty 2013/12/14 v4623 L3 Quarks
l3prg.sty 2014/01/04 v4642 L3 Control structures
l3clist.sty 2013/07/28 v4581 L3 Comma separated lists
l3token.sty 2013/08/25 v4587 L3 Experimental token manipulation
l3prop.sty 2013/12/14 v4623 L3 Property lists
l3msg.sty 2013/07/28 v4581 L3 Messages
l3file.sty 2013/10/13 v4596 L3 File and I/O operations
l3skip.sty 2013/07/28 v4581 L3 Dimensions and skips
l3keys.sty 2013/12/08 v4614 L3 Experimental key-value interfaces
l3fp.sty 2014/01/04 v4642 L3 Floating points
l3box.sty 2013/07/28 v4581 L3 Experimental boxes
l3coffins.sty 2013/12/14 v4624 L3 Coffin code layer
l3color.sty 2012/08/29 v4156 L3 Experimental color support
l3luatex.sty 2013/07/28 v4581 L3 Experimental LuaTeX-specific functions
l3candidates.sty 2014/01/06 v4643 L3 Experimental additions to l3kernel
ifpdf.sty 2011/01/30 v2.3 Provides the ifpdf switch (HO)
supp-pdf.mkii
pdftexcmds.sty 2011/11/29 v0.20 Utility functions of pdfTeX for LuaTeX (HO)
ifluatex.sty 2010/03/01 v1.3 Provides the ifluatex switch (HO)
epstopdf-base.sty 2010/02/09 v2.5 Base part for package epstopdf
grfext.sty 2010/08/19 v1.1 Manage graphics extensions (HO)
kvdefinekeys.sty 2011/04/07 v1.3 Define keys (HO)
kvoptions.sty 2011/06/30 v3.11 Key value format for package options (HO)
kvsetkeys.sty 2012/04/25 v1.16 Key value parser (HO)
etexcmds.sty 2011/02/16 v1.5 Avoid name clashes with e-TeX commands (HO)
epstopdf-sys.cfg 2010/07/13 v1.3 Configuration of (r)epstopdf for TeX Live
***********
Best Answer
The
nodewon't work because you have used;in your\semiloopdefinition. Actuallywill expand to
i.e.,
nodewithout\(\node). You can redefine the command like this with the help ofxparsepackage:Full code:
It looks like you need an update for your tex distribution. Add
\listfilesand compare yours (you can find it in your log file at the end) with following: