# Matrix element of mirror process

Hi,

I am trying to compute the total cross section for the process pp > htj, but I am having some troubles, so I decided to study the different subprocesses in a more detailed way.

My doubt comes when studying the processes u b > h t d and b u > htd, i.e. just the mirror process so to say. When I use the standalone output to get the matrix elements and compute both processes, I obtain different results. As information, I write here the code I use.

>> import model loop_sm-no_b_mass

>> generate u b > h t d

>> output standalone ub_htd_st

>> launch

and the same for the other process.

For u b > h t d, I get:

Matrix element = 6.3550723077195

whereas for the b u > h t d:

Matrix element = 2.9000254780896

when, given that it is exactly the same process, it should yield exactly the same result. I already checked that the set of momenta used in both of them is the same, so the problem is not here. So I am not so sure where the problem can be. Am I computing something wrong? Or missing something that I did not take into account?

Also, when then - instead of using the standalone output, but the normal one - I get different values for the cross section, slightly different, but enough to be not in agreement between them.

Thank you in advanced!

## Question information

- Language:
- English Edit question

- Status:
- Open

- Assignee:
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- Last query:
- 2018-10-08

- Last reply:
- 2018-10-08

Hi,

In order to have the same matrix-element, you should not use the same phase-space point for the comparison but the mirror one.

Cheers,

Olivier

> On 7 Oct 2018, at 16:32, Laurent Vashmo <email address hidden> wrote:

>

> New question #674807 on MadGraph5_aMC@NLO:

> https:/

>

> Hi,

>

> I am trying to compute the total cross section for the process pp > htj, but I am having some troubles, so I decided to study the different subprocesses in a more detailed way.

>

> My doubt comes when studying the processes u b > h t d and b u > htd, i.e. just the mirror process so to say. When I use the standalone output to get the matrix elements and compute both processes, I obtain different results. As information, I write here the code I use.

>

>>> import model loop_sm-no_b_mass

>>> generate u b > h t d

>>> output standalone ub_htd_st

>>> launch

>

> and the same for the other process.

>

> For u b > h t d, I get:

> Matrix element = 6.3550723077195

>

> whereas for the b u > h t d:

> Matrix element = 2.9000254780896

>

> when, given that it is exactly the same process, it should yield exactly the same result. I already checked that the set of momenta used in both of them is the same, so the problem is not here. So I am not so sure where the problem can be. Am I computing something wrong? Or missing something that I did not take into account?

>

> Also, when then - instead of using the standalone output, but the normal one - I get different values for the cross section, slightly different, but enough to be not in agreement between them.

>

> Thank you in advanced!

>

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

Laura Moreno (laumova) said : | #2 |

Hi Olivier,

thanks, that solved my problem.

But still there is something that I don't understand. Now I get the same matrix elements, but when I integrate over the whole phase space in order to get the total cross section (using in this case the normal output, modifying the run_card.dat, and generating the events) I get different values of the cross section for both values.

With nevents = 10^6 and a scale of (m_higgs + m_top)/4, using LHAPDF set 21000 and fixed scaled, I get:

cross-section u b > h t d = 16.670 +- 0.005 fb

cross-section b u > h t d = 16.256 +- 0.005 fb

which is not in agreement. But I am applying no cuts, so they should give the same... I also get the same problem when changing the scale, for example, I tried also with scale = (m_higgs + m_top)/2 and the results are (also with nevents = 10^6):

cross-section u b > h t d = 17.566 +- 0.006 fb

cross-section b u > h t d = 17.116 +- 0.006 fb

I also created a code that integrates over the phase space and over the pdfs using VEGAS, and for both processes I get the same result using the matrix elements from Madgraph, so the problem must be somewhere in my run from Madgraph, but I have no idea where, and I checked the run card and in both processes I have the same.

Do you have an idea of where the problem could be? Thank you so much!

Best,

L.

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