MadGraph5_aMC@NLO

Question about negative weights when using DIM6^2 syntax with dim6top_LO_UFO model

Asked by Kelci Mohrman

Hello MG experts,

I have a question about using the dim6top_LO_UFO model; I am trying to use the "DIM6^2<=2" syntax in the process card to include the interference between SM diagrams and diagrams involving two EFT vertices. I am using MG version 2.6.5. In the process card, I have:

generate p p > t t~ h FCNC=0 DIM6^2<=2

However I've noticed that some of the generated events actually seem to have negative weights. This was surprising to me since this is a LO sample. I am wondering if negative weights are expected when using the DIM6^2 syntax? Or if I am making a mistake somewhere?

Thanks very much! - Kelci

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Kelci Mohrman
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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Dear Kelci,

I do not know the model and the convention that they used.
However the syntax DIM6^2<=2
means that you apply a restriction on the amplitude square then it is therefore possible that you select the SM contribution, the interference term but not the New physics squared diagram.
In that case your matrix-element squared is not definite possitive anymore and it is indeed possible to have negative events (especially if your EFT theory is not in the perturbative regime).

Now even with definite positive amplitude, some people are using NLO (or NNLO) PDF for LO computation, such type of PDF are not definite positive and therefore can also lead to negative events. In principle you should not use those for LO computation.

Cheers,

Olivier

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Kelci Mohrman (kmohrman) said : 		#2

Thank you very much for the answer, Olivier! - Kelci