# Difference between nominal xs and avg event weight for interference calculation

Hi,

we observe a difference between the cross-section reported by MG and the

Cross-section : 8.587e-05 +- 4.837e-06 pb

and the actual average event weight from the LHE file, which is

9.93e-05

and thus not compatible within uncertainties.

And actually, the cross-section should be zero, I think, this being the interference of the SM with a CP-odd contribution - but it's not, quite significantly.

Since this comes up in a interference calculation

generate p p > mu+ mu- e+ ve NP<=1 NP^2==1 NPcWtil^2==1

my initial suspicion is that this is related to the known limitation of the integration of interference (we have been warned). Is that the origin?

Is it possible how large the integration error is (if that is indeed the origin)? Calculating the full process (NP<=1) is of course an option but this way one needs a lot of events to determine the interference with reasonable statistical uncertainty.

Cheers,

Hannes

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