Interaction order of NP vs QED

Asked by Nishita on 2018-06-19

Hi Olivier and MG5 team,

I have a question about how Madgraph calculates diagrams for models implemented via Feynrules. We have a model with a new physics interaction proportional to lambda * vev^4 where lambda has InteractionOrder -> {NP, 1}. However, I find a double counting of Feynman diagrams where one corresponds NP=1 QED=0 and another to NP=0 QED=-2. The interaction order hierarchy is set as

 M$InteractionOrderHierarchy = {
 {QCD, 1},
 {NP, 1},
 {QED, 2}
};

How can I prevent the double counting?

Thanks,
Nishita.

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2018-06-19
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2018-06-19

Hi,

Are you sure that you have a double counting?
Having the same display for a diagram does not mean that it is the same diagram.

Now if this is not a typo, it seems that you have negative coupling order.
In such case, it's hard for me to see your model as perturbative model (if your model is correct_
In such scenario, I doubt that we double count some diagram, but the notion of coupling order
should not be trust and all your Feynman diagram should put a limit to 99 for all such coupling
(so in your case, you should put --at least-- QED<=99 for all your process definition.).

I'm pretty sure that FeynRules raises an error (at least it did in the past) when such issue arise.)
The typical issue here is that vev is set by default to QED^-1 in the SM of Feynrules.
because it allows to says that HHH vertex is at the order of QED=1 and HHHH at the order of QED=2.
You should check the FR documentation on how they suggest to fix such issue.

Cheers,

Olivier

> On 19 Jun 2018, at 15:57, Nishita <email address hidden> wrote:
>
> New question #670316 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/670316
>
> Hi Olivier and MG5 team,
>
> I have a question about how Madgraph calculates diagrams for models implemented via Feynrules. We have a model with a new physics interaction proportional to lambda * vev^4 where lambda has InteractionOrder -> {NP, 1}. However, I find a double counting of Feynman diagrams where one corresponds NP=1 QED=0 and another to NP=0 QED=-2. The interaction order hierarchy is set as
>
> M$InteractionOrderHierarchy = {
> {QCD, 1},
> {NP, 1},
> {QED, 2}
> };
>
> How can I prevent the double counting?
>
> Thanks,
> Nishita.
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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