Massless graviton vertices of FFST and SSST type

Asked by Jiaming Zheng

Hi,

    I am trying to use MG5 as a phase space integrator for gravitational processes. When I search for how massless graviton are implemented in MG5, I was mostly pointed to the reference arXiv:0805.2554 by Aquino, Hagiwara, Li, and Maltoni. This reference provides a list of vertices added to HELAS at that time.

    However, the subroutines for vertices of type FFST, SSST among others are not presented in that reference. These vertices are needed to cancel some IR divergences. I can generate a UFO file with this kind of vertices with FeynRules, and their coupling does affect the result from MG5. So I assume these kinds of vertices are already included in MG5. Is this right? Is there any further documentation about them? I want to know whether they are already tested or are still experimental features.

thanks in advance,
Jiaming Zheng

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Olivier Mattelaer (olivier-mattelaer) said :
#1

Hi,

MG5aMC is fully model agnostic, the full information about the model is the one written in the UFO file.
(This is also true for the SM).

So if your cross-section/shape does not change when changing some coupling it can means multiple stuff:
1) You do not have any dependence of that coupling in the generated Feynman diagram.
--here one solution to check is to have one model without those interaction and one with those interactions and check that the number of Feynman Diagram generated are different)
2) You do have a dependence in that coupling but it is negligeable compare to the rest.
3) something different (a bug is always a possibility)

> These vertices are needed to cancel some IR divergences.

Are you doing NLO computation or LO?
From your statement i would assume that you need such term only for NLO computation but I can be wrong obviously since I do not have that much information/knowledge about what you trying to achieve.

Cheers,

Olivier

> On 22 Jul 2021, at 11:40, Jiaming Zheng <email address hidden> wrote:
>
> New question #698109 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/698109
>
> Hi,
>
> I am trying to use MG5 as a phase space integrator for gravitational processes. When I search for how massless graviton are implemented in MG5, I was mostly pointed to the reference arXiv:0805.2554 by Aquino, Hagiwara, Li, and Maltoni. This reference provides a list of vertices added to HELAS at that time.
>
> However, the subroutines for vertices of type FFST, SSST among others are not presented in that reference. These vertices are needed to cancel some IR divergences. I can generate a UFO file with this kind of vertices with FeynRules, and their coupling does affect the result from MG5. So I assume these kinds of vertices are already included in MG5. Is this right? Is there any further documentation about them? I want to know whether they are already tested or are still experimental features.
>
> thanks in advance,
> Jiaming Zheng
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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Jiaming Zheng (zhengjm3) said :
#2

Thanks for the prompt reply!

Probably my wording was confusing. Changing the coupling of the SSST vertex DOES change the outcome, and MG5 outputs the diagram with such a vertex. I just want to confirm that this vertex is not an experimental feature in HELAS, as it is not presented in any reference I found(and arXiv:0805.2554 states that this vertex was not included in HELAS back then).

The toy process I played with was S->S1+S1+S2+S2 at tree level, with S1-S1-S couple with a cubic interaction and S2 couples to everything else with massless graviton. Everything except the graviton is real singlet scalar. The IR divergence from the graviton propagator when S2 mass ->0 will cancel if I sum up all the diagrams.

The cancellation was checked analytically with the same rules from FeynRules. However, when I generate such a 1>4 process in MADGRAPH, the mass dependence of the width demonstrates a trend of quadratic divergence.

It is very likely to be my mistake somewhere in operating FeynRules and MADGRAPH. I just want to completely rule out the possibility that it was an issue of MADGRAPH/HELAS.

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