How assign the order of (QED/QCD)

Hi,

Looks like that you are miss-understanding the concept of NLO contribution.
All computation that we are doing rely on perturbation theory. So we rely on the Taylor expansion of the amplitude and of the cross-section.

So you have
\sigma = \sum_i \alpha_S^i \sigma_i
(just make the expansion on alpha_S for simplicity)

A leading order computation consist of taking into account only the first non-zero \sigma_i
which correspond to a fix power in the coupling.
While next to leading order consist to keep the TWO first non-zero \sigma_i.

Therefore if you ask for NLO in QCD, you should indeed have the contribution of two different order.
So in your case to have alpha alpha_s^2 and alpha alpha_s^3.
If you do not want to have the alpha alpha_s^3 this will then be a Leading order computation.
which you can have with the following syntax:
> generate q q~ > a c c~ QED=1 QCD=2

Since those concept are very important in order to make reasonable use of our code, I would strongly suggest that
you attempt a crash course on Field theory/particle physics. When this is done, then I will suggest that you take a look
at this series of lectures: https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Lund2014

Cheers,

Olivier

On Nov 20, 2014, at 3:16 PM, saeedeh <email address hidden> wrote:

> New question #257968 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/257968
>
> Hi
>
> I want calculate cross section of q q~ > a c c~ (where define q = u d s) in NLO
> I need this process in order alpha alpha_s^2
> so I generate q q~ > a c c~ QED=1 QCD=2 [QCD]
> but in subprocess I also see Feynman diagrams containing alpha alpha_s^3.
> Does these diagrams are entered into the calculation of my cross section? If yes, How can I remove these subprocess ( alpha alpha_s^3).
>
> --
> You received this question notification because you are an answer
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