MadGraph5_aMC@NLO

processes with electroweak loops

Asked by andreh

Dear Madgraph and tools authors / experts,

I'm trying to see if I can calculate the decay width of flavour violating processes such as mu+ > e+ a with Madgraph (see e.g. https://picasaweb.google.com/lh/photo/99wNJ_7onRVjZcfB3vyNATGLkad5586e5qyyC6CRl48?feat=directlink for a SM diagram).

For this I took the Standard Model FeynRules model file and essentially applied a PMNS matrix to the lepton sector wherever I found a CKM matrix in the quark sector. I'm not sure I did it right but at least with this model (using FeynRules 2.1) I get a diagram for the lepton flavour violating decay w+ > e+ vm and Madpgraph nicely calculates a decay width for it.

I'm however stuck with asking Madgraph to include diagrams with electroweak loops. I went back to the standard loop_sm model and tried:

     import model loop_sm
     generate e+ e- > e+ e- [QED]

where I get:

     InvalidCmd : Perturbation order QED is not among the perturbation orders allowed for by the loop model.

for g g > g g [QCD] on the other hand, I get plenty of diagrams with QCD loops.

Is there a way of getting QED loops ?

(I'm using MG5_aMC_v2_2_1)

best regards,

Andre

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Olivier Mattelaer
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Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

Electroweak processes are work in progress and not yet availaible.
Actually the computation of the loop is already possible but the phase-space integration and the correct handling of the real divergences are not yet ready (but we are working on that).

Concerning the width computed by MadGraph/FeynRules, I will refer you to the associate paper:arXiv:1402.1178
Where you can see that those computation are leading order only.

Cheers,

Olivier

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andreh (andre-holzner) said : 		#2

Hello Olivier,

thanks a lot for the quick response and for the reference.

In a presentation given by Fabio Maltoni on October 15th I got the impression that EW loops are generally available, but now that I look at it again, I see that this only applies to one specific process (arxiv:1407.0823). Nevertheless, I'm happy to hear that this will be available in the future and I'm looking forward to trying the version which will support it.

thanks !

Andre

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andreh (andre-holzner) said : 		#3

Thanks Olivier Mattelaer, that solved my question.