MadGraph5_aMC@NLO

Unexpected other initial state appears in QCD NLO calculation

Asked by teddym

Hi:
   We want to calculate the process:
  g g > c c~ [QCD]

we understand that only including this process in not physical, but we just want to calculated only gluon initiated process to cross check with some hard calculation.

But when we generate this process in MG5, in the real emission category, we will also find processes with g c/g d etc. initial states.
So, why will we get these processes, and how can we get rid of those processes and just calculate gluon initiated process only (We don't even need to set PDF for the initial states)

Best

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

Those processes are part of the NLO correction and are the precise reason why your above syntax does not make sense. Since those processes have additional divergencies compensated by loop process associated to born that you did not include in your syntax.

If you just care about the hard part of the NLO correction and only associated to the gluon should not you ask to generate
g g > c c~ g at LO (which seems to be what you want?)
You can do an output standalone in order to have only the matrix-element.

Cheers,

Olivier

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