MadGraph5_aMC@NLO

Electroweak ttbar at NLO

Asked by Baptiste Ravina

Dear experts,

After reading https://answers.launchpad.net/mg5amcnlo/+faq/3399
I was under the impression that the change of the meaning of [QCD] would manifest itself in adding more diagrams virtual diagrams at NLO. As stated in that FAQ, there is no effect on p p > t t~ [QCD] between the two versions I tried, MG v2.9.2 and v.3.5.0.
However, if I ask for p p > t t~ QED=2 [QCD] in v2 and p p > t t~ aEW=2 [QCD] in v3, I get rather different diagrams:

- there is no g g > t t~ channel in v3
- I get a q > t t~ and q q > t t~ a contributions in v3 (where q is a quark)
- in the q g > t t~ g channels, I effectively lose the 5 born-level diagrams containing only gluon propagators

The behaviour of the v3 syntax is however not equivalent to p p > t t~ QED=2 QCD=0 [QCD] in v2, since I do get the 3 born-level diagrams for q q~ > t t~, with photon/Z/gluon propagators.

Finally, if I generate p p > t t~ aEW=2 aS=2 [QCD] in v3, I get the gluon diagrams back, and it looks like a complete (?) NLO QCD+EW process.

Could you clarify the expected use of the syntax, and give a recommendation as to how to generate meaningful NLO+PS events including some EW terms? The v2 syntax above includes LO photon/Z propagators, and only adds QCD-type corrections to such diagrams at NLO. Is this still achievable in v3, or not recommended for some physical reason?

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
marco zaro Edit question
Last query:
Last reply:
Revision history for this message
marco zaro (marco-zaro) said : 		#1

Hi,
thanks for inquiring.
The generation of events to be showered is not possible when EW corrections are included.
Then, depending on the two versions, unset orders may be constrained differently, which may explain the differences you see.
Also, when ew corrections are computed, only squared-amplitude order constraints (so QED^2=... or aEW=...) should be imposed. Any other syntax is prone to give bogus results.

Finally, my understanding is that you want to generate the purely EW contribution to t t~, with NLO QCD corrections on top of it, and possibly shower it. In this case, you start from the alpha^2 born, and want to reach alpha^2 alpha_s at NLO. However, the same coupling combination can be obtained if you start from the alpha alpha_s born, and do EW corrections on top of it. Indeed, in general it is not possible to separate the two contributions, because both are needed in order to have an IR-finite cross section. There may be cases (e.g. VBS) where you can assume that only QCD-type corrections arise, but as said this is in general not true, and it is not ture for the specific case of ttbar production.

Let me knkow if this answers, if not please feel free to re-iterate.

Best wishes,

Marco Zaro

Can you help with this problem?

Provide an answer of your own, or ask Baptiste Ravina for more information if necessary.

To post a message you must log in.