QCD and QED orders

HI,

If you specify an order (QED/QCD or anything else) this is ALWAYS a maximal value.
If you don't specify any order, then MG choose the order which has the less QED order by default since this is, in most of the case ,the most important contribution (i.e. the other contribution are below the theoretical/statistical error) This part is specify in the following FAQ:

FAQ #1942: “How MadGraph assigns the order (QED/QCD/...) automatically?”.

I am confused about this statement

"
If no coupling orders are given, MadGraph will minimize the number of QED couplings and maximize number of QCD couplings, to remove negligible but time-consuming contributions.
"

which you can find by clicking on "Examples/format" on http://madgraph.hep.uiuc.edu/. What does it mean it will "maximize the number of QCD couplings"?

Hi Andrew,

I suggest that you read the following FAQ :https://answers.launchpad.net/mg5amcnlo/+faq/1942
and that you look at the following presentation:
https://cp3.irmp.ucl.ac.be/projects/madgraph/attachment/wiki/MGTalks/13_06_10_tutomg_tasi.pdf

Please come back to me if this doesn’t solve your question.

Cheers,

Olivier

On Feb 18, 2014, at 5:36 PM, Andrew Levin <email address hidden> wrote:

> Question #234568 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/234568
>
> Andrew Levin posted a new comment:
> I am confused about this statement
>
> "
> If no coupling orders are given, MadGraph will minimize the number of QED couplings and maximize number of QCD couplings, to remove negligible but time-consuming contributions.
> "
>
> which you can find by clicking on "Examples/format" on
> http://madgraph.hep.uiuc.edu/. What does it mean it will "maximize the
> number of QCD couplings"?
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Sorry, I looked at the presentation and the FAQ, and I don't see any discussion of what it means

"maximize the number of QCD couplings"

There is no limit to the number of loops or radiated gluons, so I don't understand what this means.

Hi,

Let’s take the example
p p > t t~

you have two independent contribution
p p > t t~ QED=2 QCD=0
p p > t t~ QED=0 QCD=2

if you don’t specify anything, we will take the contribution which maximizes the value of QCD, i.e.
p p > t t~
means
p p > t t~ QED=0 QCD=2

It this more clear?

Cheers,

Olivier

On Feb 18, 2014, at 8:56 PM, Andrew Levin <email address hidden> wrote:

> Question #234568 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/234568
>
> Andrew Levin posted a new comment:
> Sorry, I looked at the presentation and the FAQ, and I don't see any
> discussion of what it means
>
> "maximize the number of QCD couplings"
>
> There is no limit to the number of loops or radiated gluons, so I don't
> understand what this means.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

ok, yes, it is more clear now