# Unable to reproduce SM result for g g > t t~

Hello,

I am probably missing something really obvious, but I have spent a few hours trying to pin down the problem, so please forgive me if this is a dumb question.

I am using MG5 to cross-check a computation, specifically the process g g > t t~ in the SM. I have computed the cross section by hand and get precisely the result given in eq. (50.21) here:

http://

Now, the number I obtain for the total cross section does not at all match the number that MG gives me.

Since my number is larger than the one from MG, I suppose I might be missing a cut. However, I have switched off all cuts in the run_card (to my knowledge).

To obtain my number, I use the values for the couplings taken from MG's param_card. I am also setting the top-width to zero and am fixing the RG scale to the Z mass.

Furthermore, generating the process with g g > t t~ / t, I get a cross-section from MG for purely the s-channel diagram. This number agrees perfectly with my analytic computation. Vice versa, if I exclude the s-channel diagram, the result is completely different.

I have checked g g > u u~ and agree perfectly with MGs numbers, including cuts etc.

Are there any cuts/other subtleties I might have missed here? I'd be very grateful for any hints!

Kindly,

Matthias

## Question information

- Language:
- English Edit question

- Status:
- Solved

- Assignee:
- No assignee Edit question

- Solved by:
- Matthias König

- Solved:
- 2018-10-04

- Last query:
- 2018-10-04

- Last reply:
- 2018-10-03

Hi,

What is the script that you are using?

I would use a script like that one:

(See https:/

generate g g > t t~

output

launch

set no_parton_cut

set wt 0

set fixed_scale 91.188

set lpp 0

set ebeam 1000

I have run that script with both the latest and with some quite old version (validated both by ATLAS and CMS)

and they all give the same number. So yes I expect something on your side but without a script like the above one, it is difficult to know what you are missing.

Note that it might be more efficient to not compare the cross-section but the amplitude square directly.

For that you can do "output standalone". This will at least simplify the comparison since you bypass the phase-space integration.

Cheers,

Olivier

> On 3 Oct 2018, at 16:02, Matthias König <email address hidden> wrote:

>

> New question #674631 on MadGraph5_aMC@NLO:

> https:/

>

> Hello,

>

> I am probably missing something really obvious, but I have spent a few hours trying to pin down the problem, so please forgive me if this is a dumb question.

>

> I am using MG5 to cross-check a computation, specifically the process g g > t t~ in the SM. I have computed the cross section by hand and get precisely the result given in eq. (50.21) here:

>

> http://

>

> Now, the number I obtain for the total cross section does not at all match the number that MG gives me.

>

> Since my number is larger than the one from MG, I suppose I might be missing a cut. However, I have switched off all cuts in the run_card (to my knowledge).

>

> To obtain my number, I use the values for the couplings taken from MG's param_card. I am also setting the top-width to zero and am fixing the RG scale to the Z mass.

>

> Furthermore, generating the process with g g > t t~ / t, I get a cross-section from MG for purely the s-channel diagram. This number agrees perfectly with my analytic computation. Vice versa, if I exclude the s-channel diagram, the result is completely different.

>

> I have checked g g > u u~ and agree perfectly with MGs numbers, including cuts etc.

>

> Are there any cuts/other subtleties I might have missed here? I'd be very grateful for any hints!

>

> Kindly,

> Matthias

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

Very nice, I did not know about the option of checking the matrix element directly.!

I was able to pin down the problem - it was indeed on my side. Thanks for the time and sorry for the trouble!

Cheers,

Matthias