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 warm-up 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.