Contents of Non-central Loops

Non-central Loops

To draw a circular gluonic loop, or portion thereof, one uses the drawloop command whose syntax is reminiscent of the and commands.

\drawloop<particle type>[<initial direction><extent>](x,y)

where the particle type is currently limited to gluon. The `extent' is the number of eighths of a complete loop which are to be drawn (1-8) with 8 indicating a complete, closed, circular loop. The loop commences from the point (x, y) in the direction of `initial direction' and continues being drawn clockwise until the requested number of eighths have been completed. A number of useful parameters are returned after drawloop has been called.

\loopfrontx,\loopfronty   co-ords of beginning point of loop
\loopbackx,\loopbacky     co-ords of opposite point of loop
\loopmidx,\loopmidy       co-ords of geometric middle point of loop
\pbackx,\pbacky           co-ords of end point of loop
\gluonbackx,\gluonbacky   co-ords of end point of loop (for a gluon loop)

It must be noted that loopbackx,y and loopmidx,y are only assigned values if at least half of a loop has been drawn, that is if the loops `extent' is at least 4. Some examples will illustrate this:

\THICKLINES
\begin{picture}(8000,8000)
\drawline\fermion[\E\REG](0,0)[2000]
\drawloop\gluon[\NE 3](\pbackx,\pbacky)
\drawline\fermion[\E\REG](\pbackx,\pbacky)[2000]
\drawline\fermion[\W\REG](\pbackx,\pbacky)[7000]
\end{picture}

would produce

$\begin{picture}(8000,8000) \drawline\fermion[\E\REG](0,0)[2000] \drawloop\gluon[... ...kx,\pbacky)[2000] \drawline\fermion[\W\REG](\pbackx,\pbacky)[7000] \end{picture}$
whereas

\THICKLINES
\begin{picture}(8000,8000)
\drawline\fermion[\E\REG](0,0)[2000]
\drawloop\gluon[\N 5](\pbackx,\pbacky)
\drawline\fermion[\E\REG](\pbackx,\pbacky)[2000]
\drawline\fermion[\W\REG](\pbackx,\pbacky)[7000]
\end{picture}

would give

$\begin{picture}(8000,8000) \drawline\fermion[\E\REG](0,0)[2000] \drawloop\gluon[... ...kx,\pbacky)[2000] \drawline\fermion[\W\REG](\pbackx,\pbacky)[7000] \end{picture}$

As can be seen THICKLINES works. So does phantom mode. To produce:

$\begin{picture}(8000,8000) \drawline\fermion[\E\REG](0,0)[2000] \drawloop\gluon[... ...[\fermionbackx] \drawline\fermion[\S\REG](\pfrontx,\pfronty)[2000] \end{picture}$
one would enter

\begin{picture}(8000,8000)
\drawline\fermion[\E\REG](0,0)[2000]
\drawloop\gluon[\NE 5](\pbackx,\pbacky)
\negate\gluonbackx
\global\advance\fermionbackx by \gluonbackx
\double\fermionbackx   \multroothalf\fermionbackx
\drawline\fermion[\NW\REG](\pbackx,\pbacky)[\fermionbackx]
\drawline\fermion[\S\REG](\pfrontx,\pfronty)[2000]
\end{picture}

Finally, to point out exactly where loopmidx,y and loopbackx,y are, we try:

$\begin{picture}(8000,8000) \drawloop\gluon[\NE 8](0,0) \drawline\fermion[\W\REG]... ...,\loopmidy)[1000] \drawline\fermion[\S\REG](\pbackx,\pbacky)[2000] \end{picture}$

created via

\drawloop\gluon[\NE 8](0,0)

\drawline\fermion[\W\REG](\loopfrontx,\loopfronty)[2000]
\drawline\fermion[\E\REG](\loopbackx,\loopbacky)[2000]
\drawline\fermion[\E\REG](\loopmidx,\loopmidy)[1000]
\drawline\fermion[\W\REG](\loopmidx,\loopmidy)[1000]
\drawline\fermion[\S\REG](\loopmidx,\loopmidy)[1000]
\drawline\fermion[\N\REG](\loopmidx,\loopmidy)[1000]
\drawline\fermion[\S\REG](\pbackx,\pbacky)[2000]