MadGraph5_aMC@NLO

regarding LO switch in the NLO calculation

Asked by Disha Bhatia

Hi,

I want to calculate the k factor for my process.
I am looking at the process p p > t t~.

For this, I generated p p > t t~[QCD] in madgraph.

Then I compute the cross-section using the NLO and LO switch of madgraph.
I got sigma = 6.766e+02 pb [LO]
and sigma = 6.914e+02 pb [NLO]

But I wanted to verify this cross-section using the simple p p > t t~ command in madgraph.

I did that and substituted the same pdf as the one which was used in nlo i.e. nn23nlo
 with pdf label 244600. But this time the cross-section value which I obtained is 463.79 pb.

I was thinking that I will get same answer but this didn't happen.

Can you please tell me if I am missing something over here?
or there is some lapse in my understanding of this issue.

Thank you,
Disha

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Solved by:
Olivier Mattelaer
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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

The default scale choice is not the same for both computation. But i guess that they are something else. (I will look in more details after my hollidays)

Cheers,

Olivier

Revision history for this message
Disha Bhatia (dishabhatia1989) said : 		#2

Ok thank you, I shall wait for your reply.

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#3

I did not reproduce your discripancy.

LO from "p p > t t~" I get 463 (I change the PDF and the scale to use the same as for the second computation)
and LO from "p p > t t~ [QCD]" I get 462

So no issue.

Cheers,

Olivier

Revision history for this message
Disha Bhatia (dishabhatia1989) said : 		#4

Thanks a lot. I had changed the pdf, but how to change the scale?
I had kept it as -1 = dynamical_scale_choice.

On Thu, Jul 30, 2020 at 2:15 AM Olivier Mattelaer <
<email address hidden>> wrote:

> Your question #691949 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/691949
>
> Status: Open => Answered
>
> Olivier Mattelaer proposed the following answer:
> I did not reproduce your discripancy.
>
> LO from "p p > t t~" I get 463 (I change the PDF and the scale to use the
> same as for the second computation)
> and LO from "p p > t t~ [QCD]" I get 462
>
> So no issue.
>
> Cheers,
>
> Olivier
>
> --
> If this answers your question, please go to the following page to let us
> know that it is solved:
>
> https://answers.launchpad.net/mg5amcnlo/+question/691949/+confirm?answer_id=2
>
> If you still need help, you can reply to this email or go to the
> following page to enter your feedback:
> https://answers.launchpad.net/mg5amcnlo/+question/691949
>
> You received this question notification because you asked the question.
>

--
Regards,
Disha Bhatia

Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said : 		#5

I put that value to "3" which means HT/2

Cheers,

Olivier

Revision history for this message
Disha Bhatia (dishabhatia1989) said : 		#6

Thanks a lot for remembering and answering the question.

However, I want to ask one more thing. I had changed the pdf in the run card , but how to change the scale?

I had kept it as default -1 = dynamical_scale_choice! for both cases

Thanks,
Disha

Revision history for this message
Disha Bhatia (dishabhatia1989) said : 		#7

Thank you for the clarification.

On Thu, Jul 30, 2020 at 2:45 AM Olivier Mattelaer <
<email address hidden>> wrote:

> Your question #691949 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/691949
>
> Status: Open => Answered
>
> Olivier Mattelaer proposed the following answer:
> I put that value to "3" which means HT/2
>
> Cheers,
>
> Olivier
>
> --
> If this answers your question, please go to the following page to let us
> know that it is solved:
>
> https://answers.launchpad.net/mg5amcnlo/+question/691949/+confirm?answer_id=4
>
> If you still need help, you can reply to this email or go to the
> following page to enter your feedback:
> https://answers.launchpad.net/mg5amcnlo/+question/691949
>
> You received this question notification because you asked the question.
>

--
Regards,
Disha Bhatia

Revision history for this message
Disha Bhatia (dishabhatia1989) said : 		#8

Thanks Olivier Mattelaer, that solved my question.