# Return matrix element squared of all terms

Dear experts,

I am generating the following process: g g > h1 h1 [QCD] using the 2HDM_NLO model from the feynrules database. It consists of three types of diagrams, let's call them M1, M2 and M3. These diagrams include one loop of fermions. We want to have separately the contribution for each diagram when only top or bottom quarks are allowed to run in this loop. Therefore, each diagram can be seen as a sum of two: M1= M1t+ M1b, M2=M2t+ M2b,M3= M3t+ M3b, where t and b indicate the fermion contributing to the loop. The total matrix element is then given by:

|M|² =|M1t+M1b+

My question is the following: is there a way of generating the entire process at once and then access the value of the matrix element squared for each of the terms described above?

Thank you,

Ana Luisa

## Question information

- Language:
- English Edit question

- Status:
- Open

- Assignee:
- Valentin Hirschi Edit question

- Last query:
- 2018-09-28

- Last reply:
- 2018-09-28

Hi,

This would require 21 different call to the loop reduction algorithm and therefore slow down the computation by a factor 21. So no this is not possible in one go.

Cheers,

Olivier

> On 27 Sep 2018, at 13:57, Ana Luisa Moreira de Carvalho <email address hidden> wrote:

>

> New question #674456 on MadGraph5_aMC@NLO:

> https:/

>

> Dear experts,

>

> I am generating the following process: g g > h1 h1 [QCD] using the 2HDM_NLO model from the feynrules database. It consists of three types of diagrams, let's call them M1, M2 and M3. These diagrams include one loop of fermions. We want to have separately the contribution for each diagram when only top or bottom quarks are allowed to run in this loop. Therefore, each diagram can be seen as a sum of two: M1= M1t+ M1b, M2=M2t+ M2b,M3= M3t+ M3b, where t and b indicate the fermion contributing to the loop. The total matrix element is then given by:

>

> |M|² =|M1t+M1b+

>

> My question is the following: is there a way of generating the entire process at once and then access the value of the matrix element squared for each of the terms described above?

>

> Thank you,

> Ana Luisa

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

Hello,

Thank you for the reply.

That means that if I want to have access to the individual matrix elements I need to generate the corresponding diagrams separately right? That I know how to do so it should be ok. But then how can I access the matrix element instead of the cross section?

Cheers,

Ana Luísa

-------- Mensagem original --------

De:Olivier Mattelaer

Enviado:Fri, 28 Sep 2018 08:13:44 +0100

Para:Ana Carvalho

Assunto:Re: [Question #674456]: Return matrix element squared of all terms

Your question #674456 on MadGraph5_aMC@NLO changed:

https:/

Status: Open => Answered

Olivier Mattelaer proposed the following answer:

Hi,

This would require 21 different call to the loop reduction algorithm and

therefore slow down the computation by a factor 21. So no this is not

possible in one go.

Cheers,

Olivier

> On 27 Sep 2018, at 13:57, Ana Luisa Moreira de Carvalho <email address hidden> wrote:

>

> New question #674456 on MadGraph5_aMC@NLO:

> https:/

>

> Dear experts,

>

> I am generating the following process: g g > h1 h1 [QCD] using the 2HDM_NLO model from the feynrules database. It consists of three types of diagrams, let's call them M1, M2 and M3. These diagrams include one loop of fermions. We want to have separately the contribution for each diagram when only top or bottom quarks are allowed to run in this loop. Therefore, each diagram can be seen as a sum of two: M1= M1t+ M1b, M2=M2t+ M2b,M3= M3t+ M3b, where t and b indicate the fermion contributing to the loop. The total matrix element is then given by:

>

> |M|² =|M1t+M1b+

>

> My question is the following: is there a way of generating the entire process at once and then access the value of the matrix element squared for each of the terms described above?

>

> Thank you,

> Ana Luisa

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

--

If this answers your question, please go to the following page to let us

know that it is solved:

https:/

If you still need help, you can reply to this email or go to the

following page to enter your feedback:

https:/

You received this question notification because you asked the question.

Hi,

You have to generate the process like this:

generate g g > h1 h1 [virt=QCD]

output

Then you have a standalone code for the evaluation of the matrix-element and nothing else.

Cheers,

Olivier

> On 28 Sep 2018, at 10:43, Ana Luisa Moreira de Carvalho <email address hidden> wrote:

>

> Question #674456 on MadGraph5_aMC@NLO changed:

> https:/

>

> Status: Answered => Open

>

> Ana Luisa Moreira de Carvalho is still having a problem:

> Hello,

>

> Thank you for the reply.

>

> That means that if I want to have access to the individual matrix

> elements I need to generate the corresponding diagrams separately right?

> That I know how to do so it should be ok. But then how can I access the

> matrix element instead of the cross section?

>

> Cheers,

> Ana Luísa

>

> -------- Mensagem original --------

> De:Olivier Mattelaer

> Enviado:Fri, 28 Sep 2018 08:13:44 +0100

> Para:Ana Carvalho

> Assunto:Re: [Question #674456]: Return matrix element squared of all terms

>

> Your question #674456 on MadGraph5_aMC@NLO changed:

> https:/

>

> Status: Open => Answered

>

> Olivier Mattelaer proposed the following answer:

> Hi,

>

> This would require 21 different call to the loop reduction algorithm and

> therefore slow down the computation by a factor 21. So no this is not

> possible in one go.

>

> Cheers,

>

> Olivier

>

>> On 27 Sep 2018, at 13:57, Ana Luisa Moreira de Carvalho <email address hidden> wrote:

>>

>> New question #674456 on MadGraph5_aMC@NLO:

>> https:/

>>

>> Dear experts,

>>

>> I am generating the following process: g g > h1 h1 [QCD] using the 2HDM_NLO model from the feynrules database. It consists of three types of diagrams, let's call them M1, M2 and M3. These diagrams include one loop of fermions. We want to have separately the contribution for each diagram when only top or bottom quarks are allowed to run in this loop. Therefore, each diagram can be seen as a sum of two: M1= M1t+ M1b, M2=M2t+ M2b,M3= M3t+ M3b, where t and b indicate the fermion contributing to the loop. The total matrix element is then given by:

>>

>> |M|² =|M1t+M1b+

>>

>> My question is the following: is there a way of generating the entire process at once and then access the value of the matrix element squared for each of the terms described above?

>>

>> Thank you,

>> Ana Luisa

>>

>> --

>> You received this question notification because you are an answer

>> contact for MadGraph5_aMC@NLO.

>

> --

> If this answers your question, please go to the following page to let us

> know that it is solved:

> https:/

>

> If you still need help, you can reply to this email or go to the

> following page to enter your feedback:

> https:/

>

> You received this question notification because you asked the question.

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

Hello,

Thank you! Just a final question: I generated just like you suggested and it worked. However, I get the following output:

Unknown numerical stability because MadLoop is in the initialization stage.

MadLoop return code = 130

------

------

This is a loop induced process, so only the

unnormalized finite part is output here. Be aware

that all loops are expected to beUV-finite as no

renormalization prescription is considered.

I am not sure what this MadLoop return code means. Does this message mean that some terms are not being taken into account ?

## Can you help with this problem?

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