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Manuscript Title: Colour: a computer program for QCD colour factor calculations.
Authors: J. Hakkinen, H. Kharraziha
Program title: colour
Catalogue identifier: ADEW_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 100(1997)311
Programming language: C.
Peripherals: drum.
Keywords: Particle physics, Elementary, Feynman diagrams, Qcd colour factors.
Classification: 11.5.

Nature of problem:
In many QCD Feynman diagrams there is a need to calculate the colour factor. This program calculates colour factors fast and accurately.

Solution method:
QCD Feynman diagrams, with no four-gluon vertices, factorize into a colour (group) factor and a kinematical factor. The colour part of any closed QCD Feynman diagram can be transformed into a sum of diagrams including only closed quark (colour) lines. In each such term, the number of quark (colour) loops is counted, giving factors of 3 (or Nc). The sum of these terms is then the desired colour factor. To perform the translation, we need to have rules to interpret gluons and vertices into quark lines. There is in principle only need for three equalities to be able to calculate any closed QCD diagram (not containing four gluon vertices). To achieve good computing performance further equalities have been introduced. The most essential ones are listed in Appendix A.

Four gluon vertices cannot be included directly in diagrams to be calculated. The number of gluons in diagrams to be calculated are limited to 200, but can easily be changed prior to compiling.

Running time:
For typical diagrams, fractions of a second. Complicated diagrams - a few seconds to hours.