# Matrix element from Madgraph vs analytical

Hi everyone,

I am quite new using Madgraph and so I have a lot of doubts, therefore I would like to say sorry in advance in case I ask very silly things..

I want to compare my results for the process q qbar -> t tbar, e.g. u ubar -> t tbar, being the q massless and the t massive. So, I got by hand the squared matrix element as follows:

M² = (2/9)*g_s²*[ 1 + (1-(4*m_

So I use the standalone code in Madgraph as follows:

>> generate u u~ > t t~

>> output standalone uubarttbar

>> launch

and I get

-------

n E px py pz m

1 0.5000000E+03 0.0000000E+00 0.0000000E+00 0.5000000E+03 0.0000000E+00

2 0.5000000E+03 0.0000000E+00 0.0000000E+00 -0.5000000E+03 0.0000000E+00

3 0.5000000E+03 0.1040730E+03 0.4173556E+03 -0.1872274E+03 0.1730000E+03

4 0.5000000E+03 -0.1040730E+03 -0.4173556E+03 0.1872274E+03 0.1730000E+03

------

Matrix element = 0.61562818665255248 GeV^ 0

------

from that I infere that theta = 112°, and that using the alpha_s = 0.118 and m_top = 173 GeV that is using Madgraph, by entering those variables in my expression above, it should yield

M² = 0.4096722

which nothing has to do with the resulting matrix element from the output of Madgraph... I would appreciate really much if someone could shed some light on which one is the problem.. or if you know somewhere I can look on.. I have searched for some similar problems but couldn't find a solution.

Thank you in advance!

## Question information

- Language:
- English Edit question

- Status:
- Solved

- Assignee:
- No assignee Edit question

- Solved by:
- Olivier Mattelaer

- Solved:
- 2018-06-07

- Last query:
- 2018-06-07

- Last reply:
- 2018-06-07

>which nothing has to do with the resulting matrix element from the output of Madgraph...

Well, it seems that you are a factor 2/3 wrong. This is not what I call nothing to do with the result.

Now, I'm not going to check your theoretical formula (but if it is coming from a book (like Peskin/...)

Cheers,

Olivier

Laura Moreno (laumova) said : | #2 |

Thanks Olivier Mattelaer, that solved my question.