MadGraph5_aMC@NLO

What is the connection between integration channels and diagrams in MadGraph 5?

Asked by Yang Liu

Dear MG experts,

Recently I'm trying to find out a way to tell in a specific parameters setting, which diagrams are used (aka, with non-0 xsec) for some subprocesses.

And I find that, the MG5 calculated the xsec for some integrated channels which in theory could help us to give the answer for above question. But the problem I had is to find the correct relation between the channels and the diagrams.

I found some answers about that in https://answers.launchpad.net/mg5amcnlo/+faq/2944. But it can't explain the channels named like "G3.022".

Does anyone know about this?

Many thanks!

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Olivier Mattelaer
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Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

That part is related to a phase-space splitting when you have conflicting resonances.
For example let assume that one diagram is of the type:
 p p > h > z z > 4l (+ other particles,...)

Then you have an issue that you have two contribution that competes:
first the Higgs onshell and one of the Z off-shell
the two Z onshell and the Higgs off-shell
in this case, those contribution are actually of the same order.

We decided to integrate such two contribution separatly and have put a ternary coding information telling which of the propagator of the associated diagram are requested to be onshell/offshell or no constraint on them.
The ternary information is not only diagram specific but also depends of the internal numbering of the various propagator.
The easiest to determine which channel is what is to look at the log file of the channel and you do have some line stating which propagator is requested to be onshell (or forbiden to be onshell)

Cheers,

Olivier

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Yang Liu (littlepaw) said : 		#2

Thanks Olivier Mattelaer, that solved my question.