# ggf with madloop library

Asked by Andres Mendez on 2018-04-18

I am learning how to use madgraph and for that I’m trying to calculate the production cross section of the Higgs via ggf at LO. Im trying to get this done with the help of the madloop library, but I’m having some difficulties with the appropriate commands that I should use for this purpose.

What command should I use for this?

>> generate p p > h p p [virt=qcd]

Or

>> generate p p > h [virt=qcd]

Which one would me more appropriated if I just want the production cross section to compare it with the ones in the tables? And what’s the difference between these two command lines ?? Maybe I should use the heft model instead?
Also, after launching the code, how can I retrieve the final value of the cross section? I only see the phase-space point.

Any guidance you can give me will be appreciated!
Thanks!

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2018-04-18
 Olivier Mattelaer (olivier-mattelaer) said on 2018-04-18: #1

HI,

The syntax [virt=QCD] is only for producing single phase-space point actually.

If you want to compute the loop-induced contribution you can do

>>> generate p p > h [QCD]

This is the same syntax as for the NLO case, but here the code will realise that you do not
have any tree-level amplitude and it will switch to LO loop-induced case automatically.

>>> generate p p > h p p [virt=qcd]
>
> Or
>
>>> generate p p > h [virt=qcd]

>>

Both syntax are valid technically but they do not represent the same physics.
In madgraph, the process definition that you give is the one of the hard interaction.
(i.e. the incoming quark)
The "p" symbol is just a shortcut to write
"any of the following particle : g u u~ d d~ c c~ s s~ " (and b b~ if they are massless)

So you do not have to write them in the final state, since otherwise they will be understood as
having two jet produce from this high energy interaction.
For this reason it is more common to use the following shortcut in the final state "j" (which has the same meaning as "p" i.e. a shortcut to the gluon or any massless quark)

i.e. it is quite common to see people using madgraph for processes like (tree level here)
generate p p > t t~
generate p p > t t~ j
generate p p > t t~ j j
on the syntax point of view they are equivalent to
generate p p > t t~
generate p p > t t~ p
generate p p > t t~ p p
But typically nobody use that syntax.

Hope this clarify your point,

Cheers,

Olivier

> On 18 Apr 2018, at 03:52, Andres Mendez <email address hidden> wrote:
>
> New question #668044 on MadGraph5_aMC@NLO:
>
> Dear MadGraph team,
>
> I am learning how to use madgraph and for that I’m trying to calculate the production cross section of the Higgs via ggf at LO. Im trying to get this done with the help of the madloop library, but I’m having some difficulties with the appropriate commands that I should use for this purpose.
>
> What command should I use for this?
>
>>> generate p p > h p p [virt=qcd]
>
> Or
>
>>> generate p p > h [virt=qcd]
>
> Which one would me more appropriated if I just want the production cross section to compare it with the ones in the tables? And what’s the difference between these two command lines ?? Maybe I should use the heft model instead?
> Also, after launching the code, how can I retrieve the final value of the cross section? I only see the phase-space point.
>
>
> Any guidance you can give me will be appreciated!
> Thanks!
>
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.