Difference between cross section from gridpack and event generation
Hi experts,
I've produced a gridpack for p p > e- ve~ mu+ mu- j j, with minimal cuts (essentially just mjj > 100 and mll > 60). At the gridpack generation time, I get a cross section of 0.0006804 +- 1.076e-06 pb. After generating 100 events from the gridpack, the output cross section, and the one written to the LHE file is 0.64498281176E-03 =- 0.11946674812E-04. For other combinations of events/seed, I get different results. For example, with 5000 events, I see 6.700e-04 +/- 2.732e-06. In general the results are not in statistical agreement.
I think this issue is well-known and seems to be related to processes with difficult phase space. My real question is, why not just use the gridpack cross section every time? I thought for NLO this was the case.
Thanks,
Kenneth
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