MadGraph5_aMC@NLO

Recommendation regarding nlo_mixed_expansion

Asked by Hannes

Hi,

in MG5_aMC v3 there is the parameter nlo_mixed_expansion, which is True by default.

In ATLAS we are in progress of switching from MG5_aMC v2 to v3 but sometimes have trouble due to this parameter, processes that worked before don't work anymore (e.g. p p > t b~ j $$ w+ w- [QCD]).
Now we are wondering in which cases we can expect differences (for NLO QCD calculations) and in which of these cases it should rather be deactivated. I realize this is a fairly vague question but hopefully there is some way to give guidance.

Cheers,
Hannes

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marco zaro Edit question
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marco zaro (marco-zaro) said : 		#1

Dear Hannes,
thanks for your question.
Let me explain this a bit.
In v2, the syntax
generate p p > t t~ [QCD]
*strictly speaking* did not mean "Do NLO QCD for ttbar production". Rather, it means "compute QCD-type loops and real emission for ttbar production". The difference between the two is subtle; indeed, for most processes (ttbar is one of them) there is actually no difference.
However, take VBF. in v2, loops that involve non-colored particles (aka the pentagons) are always discarded, because they are not considered as QCD loops. However, they are a (very small) NLO correction at O(alpha_s) relative to the Born. Hence, they are now included in v3. Single-top is another example (keep in mind that typically, but not always, the discarded contributions are either very small or zero e.g. due to color conservation).
While the v3 behaviour is the most consistent, it is however not always the most convenient practical. Also, one may (rightfully) want to have the same behaviour in the new version.
In general, processes for which there is an effect are those with t-channel vector bosons at the LO (VBF/VBS/single-top/etc)
Please, let me know if this helps, in case we can iterate (I will also link this question to a FAQ for easier tracking)

Cheers,

Marco

Revision history for this message
marco zaro (marco-zaro) said : 		#2

marco zaro suggests this article as an answer to your question:
FAQ #3399: “The effect of the flag nlo_mixed_expansion”.

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