MadGraph5_aMC@NLO

C++ code for standalone Matrix element calculator for loop events

Asked by Samyukta

Hello!

We are using standalone the MadLoop library to evaluate matrix elements for the events in our processes. I saw that we can generate a C++ output by using 'output standalone_cpp PATH' here: https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/FAQ-General-4

However, in the case of MadLoop I am getting this error Command "output standalone_cpp cpp_test_madloop" interrupted with error:
PhysicsObjectError : Usage of get_color_amplitudes is not allowed in a LoopHelas MatrixElement.
Please let us know if there is some way to fix this or generate the C++ code some other way.

Thank you so much for your time. We appreciate your help.

Best,
Sam

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

Indeed MadLoop does not allow to have a pure C++ output. No one is working in that direction so do not expect this soon.
(Obviously you can do it/use automated tools to make the translation/ link fortran to C code/...)

Cheers,

Olivier

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Samyukta (skrishnamurt) said : 		#2

Thanks Olivier Mattelaer, that solved my question.

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Samyukta (skrishnamurt) said : 		#3

Dear Olivier Mattelaer,

As a part of our project described in the above question, I have been following this link:
https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/MadLoopStandaloneLibrary to evaluate matrix elements for each phase-space point using the program check_sa.f.
We also ran into another program check_sa_loop_induced.f.
This seems different than the check_sa.f file. How are the calculations different? Do we have to use the loop induced one for our processes if they are one or two loop?

As always, thank you for your time.
Best,
Sam

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Samyukta (skrishnamurt) said : 		#4

Just to clarify, we are calculating Matrix elements for gg > H > ZZ, with the Zs decaying to leptons and also the same process with an additional jet.

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Olivier Mattelaer (olivier-mattelaer) said : 		#5

Hi,

Can you be more specific?
When I generate g g > h [sqrvirt=QCD]
I have only one check_sa.f

I indeed found a file
madgraph/iolibs/template_files/loop/check_sa_loop_induced.inc
But this is a template on how to write check_sa.f
So I do not think this is what you refer to.

Cheers,

Olivier

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Samyukta (skrishnamurt) said : 		#6

Hi!

Apologies for not being specific enough, yes, I did also find the check_sa_loop_induced.inc and you are right, I cannot seem to find check_sa_loop_induced.f. I am sorry for the confusion.

So, if check_sa_loop_induced.inc is just a template to write check_sa.f does this mean all the check_sa.f in the various directories within SubProcess are the same? or are the calculations somehow different?

Thank you for your response.
Best,
Sam

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Olivier Mattelaer (olivier-mattelaer) said : 		#7

> So, if check_sa_loop_induced.inc is just a template to write check_sa.f
> does this mean all the check_sa.f in the various directories within
> SubProcess are the same?

The point is that they are different.

> or are the calculations somehow different?

But the computation is basically the same for all file
(they differ due to the number of particle in the inital/final state, about the name of the function call/...)

Cheers,

Olivier

> On 4 Mar 2020, at 19:16, Samyukta <email address hidden> wrote:
>
> Question #688982 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/688982
>
> Samyukta gave more information on the question:
> Hi!
>
> Apologies for not being specific enough, yes, I did also find the
> check_sa_loop_induced.inc and you are right, I cannot seem to find
> check_sa_loop_induced.f. I am sorry for the confusion.
>
> So, if check_sa_loop_induced.inc is just a template to write check_sa.f
> does this mean all the check_sa.f in the various directories within
> SubProcess are the same? or are the calculations somehow different?
>
> Thank you for your response.
> Best,
> Sam
>
> --
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