The Chopper

TheChopper-pl is a perl script that removes subprocesses of which the sum contributes less then a given percentage to the total cross section. It can be found in the

./madevent/bin/

directory of the gridpack and should be run from the main ./madevent/ directory.

Given an percentage as input (e.g. 0.01 for 1%) it reads all the contributions from all the subprocess and groups the subprocess for which the contribution is the smallest. It continues as long as the total sum of the grouped contributions is less than the given percentage from the total cross section. After this a script

chop_procs

is created. Executing this script will add to the names of all the directories in =./SubProcesses= that can be removed " {{{ XXX }}} " and remove the corresponding directories from the {{{ subproc.mg }}}. This should prevent these directories/subprocesses from being executed. Of course, the directories starting with {{{ XXX }}} can be removed to reduce the size of the gridpack.

For processes with lots of subprocesses there are, in general, many that do not contribute substantially to the total cross section. Using The Chopper reduces the size of the gridpack, without affecting the cross section and generated events much: a better than 1% integration error cannot be expected anyway.

• This is very similar as to what Alpgen does. In Alpgen only contributions with maximally one light quark line are included, as it is known that in processes with many legs, subprocesses with external gluons dominate.
• Removing these subprocesses does not decrease the generation time of the events with the gridpack. The subprocesses that have a very small contribution to the total cross section are almost never executed anyway.

-- Main.RikkertFrederix - 02 Mar 2009