phase space decays

hello,

at the GENIE neutrino generator http://www.genie-mc.org/ we are using TGenPhaseSpace as part of a KNO-based hadronization model, eg see: http://hepunx.rl.ac.uk/~candreop/generators/GENIE/devel/src/Fragmentation/KNOHadronization.cxx

When I run the generator in a mode producing *unweighted* events
(after selecting multiplicity and final state particles)
I use the rejection method and based on the weight reported by each TGenPhaseSpace::Generate() I try to select an 'unweighted decay'.

The problem is that for events with:
(invariant mass) - (sum of final state particle masses) = (small)
TGenPhaseSpace reports a *huge* maximum weight: many orders of magnitude away from the maximum weight one would *ever* get by running the TGenPhaseSpace::Generate().

This can be seen in
http://hepunx.rl.ac.uk/~candreop/outbox/phaseSpace.gif The red line corresponds to the max weight as reported by TGenPhaseSpace *A macro to reproduce this plot is attached.*

For our generator this means that very few events (in the edges of our 'phase space' but still physical ones) take disproportionally long time to be generated - screwing the overall performance.

I can always search for the maximum weight myself, but I would like to understand better why this happens: Do I use TGenPhaseSpace incorrectly? a bug? a limitation?

cheers,

     Costas

-- 
Dr. Constantinos Andreopoulos, CCLRC - Rutherford Lab.
http://hepunx.rl.ac.uk/~candreop/







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