missing fermion loop in EW NLO

Asked by yoshi on 2016-03-10

Dear all,

I have tried

import model loop_qcd_qed_sm
generate e+ e- > e+ e- a [QED]

using MG5_aMC_v2.3.3.

However, I noticed that there are missing diagrams which contains a fermion loop with three photons being attached to it (two propagator photons and 1 final state photon).

I think those diagrams are part of NLO corrections to "e+ e- > e+ e- a" process. Do you have any idea why those loop diagrams are absent?

Yoshitaro Takaesu

Question information

Language:
English Edit question
Status:
Open
For:
MadGraph5_aMC@NLO Edit question
Assignee:
Valentin Hirschi Edit question
Last query:
2016-03-10
Last reply:
2016-03-10

Hi,

Those are actually zero due to Furry theorem.

Cheers,

Olivier

> On Mar 10, 2016, at 08:02, yoshi <email address hidden> wrote:
>
> New question #288401 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/288401
>
> Dear all,
>
> I have tried
>
> import model loop_qcd_qed_sm
> generate e+ e- > e+ e- a [QED]
>
> using MG5_aMC_v2.3.3.
>
> However, I noticed that there are missing diagrams which contains a fermion loop with three photons being attached to it (two propagator photons and 1 final state photon).
>
> I think those diagrams are part of NLO corrections to "e+ e- > e+ e- a" process. Do you have any idea why those loop diagrams are absent?
>
> Yoshitaro Takaesu
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

yoshi (useakat) said : #2

Hi Olivier,

Thanks!
Can I ask a question further?

I also tried

e+ e- > e+ e- a a [QED] .

In this case, the fermion loop diagram with four photons being attached also seems to be absent. Do you know why they do not exist?

Cheers,
Yoshitaro

Hi,

That's sounds weird especially since those loop are present for
a a > e+ e- mu+mu-
and for
a a > a a

Maybe I miss an argument so I let Valentin answer that poin.

Cheers,

Olivier

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