Matrix element from Madgraph vs analytical

Asked by Laurent Vashmo on 2018-06-07

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_t²/s))*cos²theta + (4*m_t²/s) ]

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
For:
MadGraph5_aMC@NLO Edit question
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

Laurent Vashmo (laurentvashmo) said : #2

Thanks Olivier Mattelaer, that solved my question.