% Read in by FEYNMANDOC:  FEYNMANDOC4H.  Called by FD4H.COM
%\chapter{Advanced Features con't Links, Stems and Caps: CAPS}
%
\subsection{Capped Gluons}

Gluons drawn in the styles \bs REG, \bs FLIPPED, \bs CURLY, \bs FLIPPEDCURLY,
\bs FLAT, \bs FLIPPEDFLAT and \bs SQUASHED commence and conclude on one
{\em edge} of the particle line as opposed to \bs CENTRAL gluons which begin
and end near the middle of the line.  This can be seen clearly in section
2.10.1.  On occasion one wishes to draw a gluon which is central on one side
and non-central and the other.  To centralize a gluon the \bs{\it gluoncap}
modifier is invoked.  Similarly gluon verticies may be capped via
\bs{\it vertexcap} and \bs{\it vertexcaps}.

\subsubsection{Capped Lines}

The \bs{\it gluoncap} statement is similar to the \bs gluonlink statement.
Only the rear of the gluon is ever capped.  If both ends need to be 
centralized it is best to use a \bs CENTRAL gluon.  Any horizontal or vertical
gluon may be capped but \bs CENTRAL and \bs FLIPPEDCENTRAL gluons will receive
caps of size zero.  Diagonal gluons may be stemmed but not capped.  Examples
are:
\begin{verbatim}

\drawline\gluon[\W\FLAT](8000,0)[5]\gluoncap 
\drawline\gluon[\E\SQUASHED](10000,0)[5]\gluoncap 

\end{verbatim}

\begin{picture}(15000,5000)
\drawline\gluon[\W\FLAT](8000,2500)[5]\gluoncap 
\drawline\gluon[\E\SQUASHED](10000,2500)[5]\gluoncap 
\end{picture}
\vskip 0.2in

Caps may be emboldened and are not printed in phantom mode.
The length of the stem of the cap is \bs stemlength and this may be re-set as
described in the section on stems (4.3.2).
Unlike the case of links both \bs gluonbackx,y and
\bs pbackx,y will refer to the end of the cap.  The details of 
the uncapped gluon may be stored prior to issuing the \bs gluoncap command.
(and ditto for
\bs gluonlengthx,y \etc).  As an exercise try to reproduce the capped example
at the conclusion of section 2.10.2 (note that this picture is better drawn
using stemmed CENTRAL gluons).

\subsubsection{Capped Vertices}

Gluon caps find their greatest use when appended to \ddrawvertex-generated
vertices.  One does not have a choice of which gluon type is produced when
\ddrawvertex\bs gluon is employed.  Capped vertices may be seen in the lower
left of the cover illustration.  In this case gluons of types \bs CURLY and
\bs FLIPPEDCURLY are capped.  The syntax is
\begin{verbatim}

\vertexcap<n>\drawvertex\gluon...
\vertexcaps\drawvertex\gluon...

\end{verbatim}
Where, once again, \verb@<n>@ is the gluon line to be capped.  \bs vertexcaps
caps all lines which can be capped.  Examples would be
\begin{verbatim}

\vertexcap1\vertexcap4\drawvertex\gluon[\S 4](0,0)[6]
\vertexcaps\drawvertex\gluon[\W 3](0,0)[3]

\end{verbatim}
In the latter case no caps would be drawn since all three lines are of a 
CENTRAL variety.  \bs vertexcap may be used in conjunction with 
\bs vertexlink and \bs stemvertex.

Exercise:  Produce

\begin{picture}(20000,21000)
% Work our way from the upper left to lower right.
%
\global\stemlength=1500
\backstemmed\drawline\photon[\SE\FLIPPED](0,20000)[8]
\global\advance\pbackx by 1000
\global\advance\pbacky by -1000
\THICKLINES\put(\pbackx,\pbacky){\circle{2830}}\THINLINES   % The circle (thick)
\global\advance\pbackx by 1000
\global\advance\pbacky by -1000
\global\stemlength=1500  
% The extra length on this vertex line is from one loop plus a link.
% Note that the line is both stemmed and linked.
\frontstemmed\drawline\gluon[\SE\FLIPPED](\pbackx,\pbacky)[1]\gluonlink
\vertexcap2\vertexcap3\drawvertex\gluon[\SE 3](\gluonbackx,\gluonbacky)[3]
% Alternately we could have \vertexlink1\vertexcap2\vertexcap3\drawvertex...
% Now draw the fermions, arrows and labels:
\drawline\fermion[\SW\REG](\vertexthreex,\vertexthreey)[2000]
\drawarrow[\SW\ATBASE](\pmidx,\pmidy)
\global\advance\fermionbackx by -700
\global\advance\fermionbacky by -450
\put(\fermionbackx,\fermionbacky){$q$}
\drawline\fermion[\SE\REG](\vertexthreex,\vertexthreey)[2000]
\drawarrow[\NW\ATBASE](\pmidx,\pmidy)
\global\advance\fermionbackx by 50
\global\advance\fermionbacky by -450
\put(\fermionbackx,\fermionbacky){$\bar q$}
\drawline\fermion[\NE\REG](\vertextwox,\vertextwoy)[2000]
\drawarrow[\NE\ATBASE](\pmidx,\pmidy)
\global\advance\fermionbackx by 50
\global\advance\fermionbacky by -250
\put(\fermionbackx,\fermionbacky){$q$}
\drawline\fermion[\SE\REG](\vertextwox,\vertextwoy)[2000]
\drawarrow[\NW\ATBASE](\pmidx,\pmidy)
\global\advance\fermionbackx by 50
\global\advance\fermionbacky by -750
\put(\fermionbackx,\fermionbacky){$\bar q$}
\end{picture}

\vskip 0.2in
In the above the circle is of diameter $1000\sqrt2$ and the `stems' attached 
to it of length 1500.  Try to work from the upper left to the lower right.
