MadGraph5_aMC@NLO

how interference is included in individual sub-processes cross-sections

Asked by Alexander

Hello,

I evaluate sub-process cross-section from LHE event weights summing up the weights of the events for a given sub-process. For the same initial and final state there are several sub-processes with different intermediate particle. They should interfere with each other. How in my procedure the interference are taken into account ? For example I have h125 and h+ as an intermediate particle for the same initial and final states. If I want to calculate cross-section with h+ or with h125 involved in my way described above how do I know the part of the cross-section associated with interference.

Thanks, Sasha.

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

From the lhe file, you can not evaluate such interference term. The fact that we wrote intermediate particles inside the lhe file is only done when particles are onshell where the interference typically plays a small role.
Actually the reason of those particles written in the lhe file, is in order to allow the parton-shower to shower the particles in a way that preserve such resonances (which is obviously important for agreement with data). It is not suppose to be used for anything else.

If you want the contribution of various type of diagram (and/or interference) you have to compute them one by one.
(either via dedicated phase-space integration or via re-weighting --likely not working for resonances--)

Cheers,

Olivier

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