Why don't I get the number of events I asked for from MadEvent?

Getting significantly fewer events than you ask for usually indicates that you are trying to integrate over a divergence. E.g., if you have gluons in your process and no drjj, pj, mjj or xqcut cuts, you hit the collinear and soft divergencies of QCD. If you have implemented your own model and given some particles zero width although the Feynman rules indicate that they can decay, the you will get a divergence in any process where these particles can occur as on-shell s-channel resonances. And so on.

You can get a hint to where the problem is by looking at the Luminocity numbers for the subprocesses and integration channels. The lowest-luminocity channel is where the problem is worst. The channels are correlated with Feynman diagrams as determined by the file config_subproc_map.inc in the P... directory.

-- Main.JohanAlwall - 2011-09-24