Loops diagrams and LO/NLO process

Hi,

The following three model: sm, loop_sm and loop_qcd_qed_sm
are the same (the standard model)
and differ just by the fact that

sm is only able to do tree level computation
loop_sm is able to do tree level computation and one loop computation in QCD (so usefull for NLO in QCD)
loop_qcd_qed_sm is the most complex model, which can do tree-level, one loop computation in QCD and electroweak.

madgraph has even method to switch from sm model to loop_sm or to loop_qcd_qed_sm
depending of the syntax requested by the user.
i.e.
if the user does
import model sm
generate p p > t t~ [QCD]
the model used will automatically be loop_sm and not sm

> I already know that [QCD/QED] give me a loop process referent to that interaction but I don't know what is the meaning of [virt/real/Loop/noborn=...]

So [QCD] and [QED] are the syntax for NLO computation.

[noborn=QCD] is the syntax for loop-induced process (so LO computation happening at one loop)
Here some old version of MG5aMC (before version 3) allows to just specify [QCD] and the code will switch automatically to [noborn=QCD] if it does not find a tree level diagram for the requested diagram.

[virt=QCD] is in order to have a code that only compute the loop for (one) phase-space point.
[real=QCD] it generates the MadFKS code not linked to madloop (such that you have to interface it yourself to your favorite OLP provider)
[loop=QCD] is a syntax that does not exists

the list of possible keyword before the "=" are
all real virt sqrvirt tree noborn LOonly

"sqvirt" is the same as virt but for having the square of the loop rather the loop times tree
"LOonly" is a mode that returns a code for LO computation only, it is typically less optimised than the normal LO code but can be usefull for merging sample and/or for some feature that are exclusive to NLO code.

Cheers,

Olivier

> On 13 Dec 2022, at 16:05, Francisco Casalinho <email address hidden> wrote:
>
> New qustion #704113 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/704113
>
> Hello,
>
> I'm a beginner at LO/NLO process in madgraph and change the model, so I have some questions;
>
> How I can know that is the differences between models?
>
> Where I can find some pdf/ tutorial about NLO and the specifications about differents models?
>
> I already know that [QCD/QED] give me a loop process referent to that interaction but I don't know what is the meaning of [virt/real/Loop/noborn=...]
>
> What is the different between this models: loop_sm and loop_qcd_qed_sm?
>
> Cheers,
>
> Francisco
>
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