Negative distributions with MC@NLO
Dear Madgraph authors,
I'm running into issues trying to do something unusual with the MC@NLO machinery. I want to use the real correction events at fixed order NLO without a shower. I obviously cannot use the H events directly because the same phase space receives contributions from the S events with a single shower emission. I thus have to add a single shower emission to the S events. My process is the following:
- I generate p p > t t~ [QCD]
- Do launch -p and select PYTHIA8 as the shower option
- Write a Pythia program that uses a user hook to only add a single shower emission to the S events. I load in the cmdn card I find in the PYTHIA81 directory in MG5 to ensure I'm using the correct settings.
When I then plot some distributions of the resulting events I find that some go negative. In particular, infrared-sensitive quantities (like the top+gluon invariant mass) go negative at small values. It seems that this would originate mostly from S events with negative weight that do a small-scale shower branching, because the H events have no enhancement in the IR region.
I'm wondering if my problem is technical or conceptual. My understanding is that the events I obtain after this procedure should be physically sensible, but maybe I'm missing something.
Thanks,
Rob
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- Rikkert Frederix Edit question
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