Same Feynman rules lead to different results and cross sections

Asked by Jan H on 2020-09-15

Dear Madgraph experts,

I wrote a custom UFO model according to https://arxiv.org/pdf/1310.1921.pdf
The model should eventually describe the production of dark photons in a bremsstrahlung reaction (e- + nucleus -> e- + nucleus + dark photon).
The dark photon couples via a U1 symmetry to the electron.

I have the following problem:
If I run the simulation with a dark photon model without any mass terms in the Lagrangian, I get different results compared to ordinary Bremsstrahlung(the two additional diagrams for this case have been discarded).
I checked the corresponding vertices, couplings and Feynman rules. They are identical.

What could be the reason for this strange phenomenon?
Any help on this will be greatly appreciated.

Thank you
Jan

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2020-09-24
Last query:
2020-09-24
Last reply:
2020-09-16

Hi,

They are typically two potential issue:
1) your matrix-element is not identical ( looks like you checked that already)
I would however suggest to run
generate e- nucleus -> e- nucleus darkphoton
output standalone
launch

for each model and check the value of the matrix-element for a single phase-space point.

2) a phase-space integration issue. One potential difference in this case might be related to the mass of the particles which modifiies the phase-space boundaries. Otherwise, you can compare the value of each channel of integration to see which channel is different (you have an html page for that in
HTML/run_01 directory .

Cheers,

Olivier

> On 15 Sep 2020, at 17:25, Jan H <email address hidden> wrote:
>
> New question #692923 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/692923
>
> Dear Madgraph experts,
>
> I wrote a custom UFO model according to https://arxiv.org/pdf/1310.1921.pdf
> The model should eventually describe the production of dark photons in a bremsstrahlung reaction (e- + nucleus -> e- + nucleus + dark photon).
> The dark photon couples via a U1 symmetry to the electron.
>
> I have the following problem:
> If I run the simulation with a dark photon model without any mass terms in the Lagrangian, I get different results compared to ordinary Bremsstrahlung(the two additional diagrams for this case have been discarded).
> I checked the corresponding vertices, couplings and Feynman rules. They are identical.
>
> What could be the reason for this strange phenomenon?
> Any help on this will be greatly appreciated.
>
> Thank you
> Jan
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Jan H (janhein) said : #2

Thank you for your quick reply.

I checked the matrix elements according to proposal 1. They are identical.
However the resulting cross sections differ significantly for contributing channels.
So it does seem to be case 2.

Is there a good way to solve the phase space integration issue?

Cheers
Jan

HI,

Can you create a bug report for this?
(and attach to your bug report the two UFO model, I will need to see the full log of both process to be able to assess the situation.)

Unfortunately, questions do not allow to attach file so we can not continue in this thread.

Cheers,

Olivier

> On 16 Sep 2020, at 15:20, Jan H <email address hidden> wrote:
>
> Question #692923 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/692923
>
> Status: Answered => Open
>
> Jan H is still having a problem:
> Thank you for your quick reply.
>
> I checked the matrix elements according to proposal 1. They are identical.
> However the resulting cross sections differ significantly for contributing channels.
> So it does seem to be case 2.
>
> Is there a good way to solve the phase space integration issue?
>
> Cheers
> Jan
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Jan H (janhein) said : #4

Thanks Olivier Mattelaer, that solved my question.