MadGraph5_aMC@NLO

Excluding gluon initiated real processes in aMC@NLO

Asked by Florian Lyonnet

Hi all,

I am trying to calculate p p > z > t t~ at NLO using aMC@NLO within MadGraph 5.

I defined the multiparticle p as :

define p = u d s c b u~ d~ s~ c~ b~ i.e. excluding the Gluon from the proton.

However when I generate the process above using : p p >z > t t~[QCD]

I get some gluon-quark initiated real processes that I would like to get rid off.

For instance I obtain g d > z > t t~ .

Is this behaviour expected and is there a way of removing these contributions ?

Thank you in advance ,

Best,

Florian

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
marco zaro Edit question
Solved by:
marco zaro
Solved:
Last query:
Last reply:
Revision history for this message
Best marco zaro (marco-zaro) said : 		#1

Dear Florian,
thanks for your question.
Why do you need only the qq-initiated process?
The gluon-quark initiated real emissions have to be taken into account in order to have a consistent NLO computation. What a quark or a gluon is depends on the facorization scale you use, therefore the results you will get will not make much sense, in my opinion.

aMC@NLO looks for all the possible splittings in the model, ignoring the fact that you have defined the proton without the gluon (this is actually used only to get the LO processes, and the code should warn you about this).
If you really want not to include gq (being aware that the results are not much physical), one possible workaround would be to set the gluon PDF to zero.
You can do this looking inside the Source/PDF folder files, in particular the pdg2pdf (or pdg2pdf_lhapdf if you are linking lhapdf).

Let me know if you need further help.
Best,

Marco

Revision history for this message
Florian Lyonnet (dibus2) said : 		#2

Dear Marco,

thank you for your quick reply.

I am interested in this configuration because I would like to compare it with another calculation.

The problem is that in the qg channel there re some QED divergences (one initial quark emits a photon that splits into ttbar) I would like to isolate. I will actually proceed as you suggested.

However could you provide me with information concerning how the QED divergences are treated in MadGraph 5 or point me towards the relevant literature ?

Thanks a lot,

Best,

Florian

Revision history for this message
marco zaro (marco-zaro) said : 		#3

Dear Florian,
for the moment, QED divergences are not considered at all, which means, you have to apply proper cuts in order not to have a divergent cross-section (e.g. cuts on the pt of final state particles).
Please let me know if you need more on this.
Best,

Marco

On 14 Oct 2013, at 12:56, Florian Lyonnet wrote:

> Question #237291 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/237291
>
> Status: Answered => Open
>
> Florian Lyonnet is still having a problem:
> Dear Marco,
>
> thank you for your quick reply.
>
> I am interested in this configuration because I would like to compare it
> with another calculation.
>
> The problem is that in the qg channel there re some QED divergences (one
> initial quark emits a photon that splits into ttbar) I would like to
> isolate. I will actually proceed as you suggested.
>
> However could you provide me with information concerning how the QED
> divergences are treated in MadGraph 5 or point me towards the relevant
> literature ?
>
> Thanks a lot,
>
> Best,
>
> Florian
>
> --
> You received this question notification because you are a direct
> subscriber of the question.

Revision history for this message
Florian Lyonnet (dibus2) said : 		#4

Thanks marco zaro, that solved my question.