Chapter 16 Technical details – Advanced Spells
16.1 Efficiency and tuning
Since massless fermions and vector bosons (or almost massless states
in a certain approximation) lead to restrictive selection rules for
allowed helicity combinations in the initial and final state. To make
use of this fact for the efficiency of the WHIZARD program, we are
applying some sort of heuristics: WHIZARD dices events into all
combinatorially possible helicity configuration during a warmup
phase. The user can specify a helicity threshold which sets the number
of zeros WHIZARD should have got back from a specific helicity
combination in order to ignore that combination from now on. By that
mechanism, typically half up to more than three quarters of all
helicity combinations are discarded (and hence the corresponding
number of matrix element calls). This reduces calculation time up to
more than one order of magnitude. WHIZARD shows at the end of the
integration those helicity combinations which finally contributed to
the process matrix element. Note that this list – due to the numerical heuristics – might very
well depend on the number of calls for the matrix elements per
iteration, and also on the corresponding random number seed.
