MadGraph5_aMC@NLO

Cross-section in Madgraph

Asked by Kanhaiya Gupta

How is the cross-section calculated in MadGraph?

Theoretically, expected cross-section = production cross-section * Branching_ratio

I generated the processes 1. p p > h (production by gluon fusion)

                                                  2. p p > h , h > b b~ ( Branching ratio for b b~ is 58%)

Theoretically, I should expect 0.58 of the production cross-section.

But using MadGraph, I am getting a nearly close cross-section.

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

Hi,

Theoretically, expected cross-section = production cross-section * Branching_ratio

Well this is an approximation but ok. Approximation that we DO NOT USE for our computation.

I generated the processes 1. p p > h (production by gluon fusion)

                                                 2. p p > h , h > b b~ ( Branching ratio for b b~ is 58%)

So in order to be fully LO accurate I have run the following command:
import model heft
generate p p > h, h > b b~
output
launch
set wh Auto
set cut_decays F

personally I get for the p p > h
# original cross-section: 17.6147161136
# PDF variation: +2.44% -2.44%

For the second
# original cross-section: 13.2230334866
# PDF variation: +2.46% -2.46%

If i compute the LO width in that model for the Higgs (as done above) I get:

DECAY 25 5.621956e-03
# BR NDA ID1 ID2 ...
   7.657121e-01 2 -5 5 # 0.00430479973487
   4.602490e-02 2 -15 15 # 0.000258749962704
   3.470500e-02 2 21 21 # 0.00019510998298
   2.306937e-02 3 -2 1 24 # 0.000129694983088
   2.306937e-02 3 -4 3 24 # 0.000129694983088
   2.283547e-02 3 -24 -1 2 # 0.000128380007579
   2.283547e-02 3 -24 -3 4 # 0.000128380007579
   7.638622e-03 3 -12 11 24 # 4.29439967846e-05
   7.638622e-03 3 -14 13 24 # 4.29439967846e-05
   7.589707e-03 3 -24 -11 12 # 4.26689988069e-05
   7.589707e-03 3 -24 -13 14 # 4.26689988069e-05
   7.549329e-03 3 -16 15 24 # 4.24419954675e-05
   7.536878e-03 3 -24 -15 16 # 4.23719964934e-05
   2.265759e-03 3 -1 1 23 # 1.27379974046e-05
   2.265759e-03 3 -3 3 23 # 1.27379974046e-05
   1.821430e-03 3 -5 5 23 # 1.02399993171e-05
   1.776873e-03 3 -2 2 23 # 9.98950182359e-06
   1.776873e-03 3 -4 4 23 # 9.98950182359e-06
   1.726125e-03 2 22 22 # 9.7041988005e-06
   1.019633e-03 3 -12 12 23 # 5.73233186215e-06
   1.019633e-03 3 -14 14 23 # 5.73233186215e-06
   1.019633e-03 3 -16 16 23 # 5.73233186215e-06
   5.073945e-04 3 -11 11 23 # 2.85254955364e-06
   5.073945e-04 3 -13 13 23 # 2.85254955364e-06
   4.989544e-04 3 -15 15 23 # 2.80509968281e-06

So since the LO width gives a 76% Branching ratio, the two results are coherent at LO accuracy (with LO width). Now if you do not want to use the LO width for the computation (which is tne default), then indeed in this case, you do not have the property of the BR. This is typically what people want since they have to re-scale the total cross-section anyway to N3LO (in this case)

Cheers,

Olivier

On 15 Mar 2019, at 06:28, Kanhaiya Gupta <<email address hidden><mailto:<email address hidden>>> wrote:

New question #679214 on MadGraph5_aMC@NLO:
https://answers.launchpad.net/mg5amcnlo/+question/679214

How is the cross-section calculated in MadGraph?

Theoretically, expected cross-section = production cross-section * Branching_ratio

I generated the processes 1. p p > h (production by gluon fusion)

                                                 2. p p > h , h > b b~ ( Branching ratio for b b~ is 58%)

Theoretically, I should expect 0.58 of the production cross-section.

But using MadGraph, I am getting a nearly close cross-section.

--
You received this question notification because you are an answer
contact for MadGraph5_aMC@NLO.

Revision history for this message
Kanhaiya Gupta (kanhaiya11) said : 		#2

Thank you so much for clarifying.

What is the significance of the two commands?
set wh Auto
set cut_decays F

Thanking you!

On Fri, Mar 15, 2019 at 1:38 PM Olivier Mattelaer <
<email address hidden>> wrote:

> Your question #679214 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/679214
>
> Status: Open => Answered
>
> Olivier Mattelaer proposed the following answer:
> Hi,
>
> Theoretically, expected cross-section = production cross-section *
> Branching_ratio
>
> Well this is an approximation but ok. Approximation that we DO NOT USE
> for our computation.
>
> I generated the processes 1. p p > h (production by gluon fusion)
>
> 2. p p > h , h > b b~
> ( Branching ratio for b b~ is 58%)
>
> So in order to be fully LO accurate I have run the following command:
> import model heft
> generate p p > h, h > b b~
> output
> launch
> set wh Auto
> set cut_decays F
>
>
> personally I get for the p p > h
> # original cross-section: 17.6147161136
> # PDF variation: +2.44% -2.44%
>
> For the second
> # original cross-section: 13.2230334866
> # PDF variation: +2.46% -2.46%
>
> If i compute the LO width in that model for the Higgs (as done above) I
> get:
>
> DECAY 25 5.621956e-03
> # BR NDA ID1 ID2 ...
> 7.657121e-01 2 -5 5 # 0.00430479973487
> 4.602490e-02 2 -15 15 # 0.000258749962704
> 3.470500e-02 2 21 21 # 0.00019510998298
> 2.306937e-02 3 -2 1 24 # 0.000129694983088
> 2.306937e-02 3 -4 3 24 # 0.000129694983088
> 2.283547e-02 3 -24 -1 2 # 0.000128380007579
> 2.283547e-02 3 -24 -3 4 # 0.000128380007579
> 7.638622e-03 3 -12 11 24 # 4.29439967846e-05
> 7.638622e-03 3 -14 13 24 # 4.29439967846e-05
> 7.589707e-03 3 -24 -11 12 # 4.26689988069e-05
> 7.589707e-03 3 -24 -13 14 # 4.26689988069e-05
> 7.549329e-03 3 -16 15 24 # 4.24419954675e-05
> 7.536878e-03 3 -24 -15 16 # 4.23719964934e-05
> 2.265759e-03 3 -1 1 23 # 1.27379974046e-05
> 2.265759e-03 3 -3 3 23 # 1.27379974046e-05
> 1.821430e-03 3 -5 5 23 # 1.02399993171e-05
> 1.776873e-03 3 -2 2 23 # 9.98950182359e-06
> 1.776873e-03 3 -4 4 23 # 9.98950182359e-06
> 1.726125e-03 2 22 22 # 9.7041988005e-06
> 1.019633e-03 3 -12 12 23 # 5.73233186215e-06
> 1.019633e-03 3 -14 14 23 # 5.73233186215e-06
> 1.019633e-03 3 -16 16 23 # 5.73233186215e-06
> 5.073945e-04 3 -11 11 23 # 2.85254955364e-06
> 5.073945e-04 3 -13 13 23 # 2.85254955364e-06
> 4.989544e-04 3 -15 15 23 # 2.80509968281e-06
>
>
> So since the LO width gives a 76% Branching ratio, the two results are
> coherent at LO accuracy (with LO width). Now if you do not want to use the
> LO width for the computation (which is tne default), then indeed in this
> case, you do not have the property of the BR. This is typically what people
> want since they have to re-scale the total cross-section anyway to N3LO (in
> this case)
>
>
> Cheers,
>
> Olivier
>
>
> On 15 Mar 2019, at 06:28, Kanhaiya Gupta <
> <email address hidden><mailto:
> <email address hidden>>> wrote:
>
> New question #679214 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/679214
>
> How is the cross-section calculated in MadGraph?
>
> Theoretically, expected cross-section = production cross-section *
> Branching_ratio
>
> I generated the processes 1. p p > h (production by gluon fusion)
>
> 2. p p > h , h > b b~
> ( Branching ratio for b b~ is 58%)
>
> Theoretically, I should expect 0.58 of the production cross-section.
>
> But using MadGraph, I am getting a nearly close cross-section.
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.
>
> --
> 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/679214/+confirm?answer_id=0
>
> 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/679214
>
> You received this question notification because you asked the question.
>

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

Hi,

Those two commands, are editting the cards, to change the value from their default

set wh Auto

Change the "wh" parameter of the param_card (this is the width of the higgs)
and set it to Auto (arXiv:1402.1178<http://arXiv.org/abs/arXiv:1402.1178>) which recomputes the width at LO.
(In general this is bad for the Higgs due to the lack of loop-induced process, but this is ok in the heft obviously)

set cut_decays F

So this change that parameter of the run_card. I believe that the default is actually already False.
But I wanted to be sure.
This forbids to have any cut on the particle coming from an onshell decay.

Cheers,

Olivier

On 15 Mar 2019, at 10:02, Kanhaiya Gupta <<email address hidden><mailto:<email address hidden>>> wrote:

Question #679214 on MadGraph5_aMC@NLO changed:
https://answers.launchpad.net/mg5amcnlo/+question/679214

   Status: Answered => Open

Kanhaiya Gupta is still having a problem:
Thank you so much for clarifying.

What is the significance of the two commands?
set wh Auto
set cut_decays F

Thanking you!

On Fri, Mar 15, 2019 at 1:38 PM Olivier Mattelaer <
<email address hidden><mailto:<email address hidden>>> wrote:

Your question #679214 on MadGraph5_aMC@NLO changed:
https://answers.launchpad.net/mg5amcnlo/+question/679214

   Status: Open => Answered

Olivier Mattelaer proposed the following answer:
Hi,

Theoretically, expected cross-section = production cross-section *
Branching_ratio

Well this is an approximation but ok. Approximation that we DO NOT USE
for our computation.

I generated the processes 1. p p > h (production by gluon fusion)

                                                2. p p > h , h > b b~
( Branching ratio for b b~ is 58%)

So in order to be fully LO accurate I have run the following command:
import model heft
generate p p > h, h > b b~
output
launch
set wh Auto
set cut_decays F

personally I get for the p p > h
# original cross-section: 17.6147161136
# PDF variation: +2.44% -2.44%

For the second
# original cross-section: 13.2230334866
# PDF variation: +2.46% -2.46%

If i compute the LO width in that model for the Higgs (as done above) I
get:

DECAY 25 5.621956e-03
# BR NDA ID1 ID2 ...
  7.657121e-01 2 -5 5 # 0.00430479973487
  4.602490e-02 2 -15 15 # 0.000258749962704
  3.470500e-02 2 21 21 # 0.00019510998298
  2.306937e-02 3 -2 1 24 # 0.000129694983088
  2.306937e-02 3 -4 3 24 # 0.000129694983088
  2.283547e-02 3 -24 -1 2 # 0.000128380007579
  2.283547e-02 3 -24 -3 4 # 0.000128380007579
  7.638622e-03 3 -12 11 24 # 4.29439967846e-05
  7.638622e-03 3 -14 13 24 # 4.29439967846e-05
  7.589707e-03 3 -24 -11 12 # 4.26689988069e-05
  7.589707e-03 3 -24 -13 14 # 4.26689988069e-05
  7.549329e-03 3 -16 15 24 # 4.24419954675e-05
  7.536878e-03 3 -24 -15 16 # 4.23719964934e-05
  2.265759e-03 3 -1 1 23 # 1.27379974046e-05
  2.265759e-03 3 -3 3 23 # 1.27379974046e-05
  1.821430e-03 3 -5 5 23 # 1.02399993171e-05
  1.776873e-03 3 -2 2 23 # 9.98950182359e-06
  1.776873e-03 3 -4 4 23 # 9.98950182359e-06
  1.726125e-03 2 22 22 # 9.7041988005e-06
  1.019633e-03 3 -12 12 23 # 5.73233186215e-06
  1.019633e-03 3 -14 14 23 # 5.73233186215e-06
  1.019633e-03 3 -16 16 23 # 5.73233186215e-06
  5.073945e-04 3 -11 11 23 # 2.85254955364e-06
  5.073945e-04 3 -13 13 23 # 2.85254955364e-06
  4.989544e-04 3 -15 15 23 # 2.80509968281e-06

So since the LO width gives a 76% Branching ratio, the two results are
coherent at LO accuracy (with LO width). Now if you do not want to use the
LO width for the computation (which is tne default), then indeed in this
case, you do not have the property of the BR. This is typically what people
want since they have to re-scale the total cross-section anyway to N3LO (in
this case)

Cheers,

Olivier

On 15 Mar 2019, at 06:28, Kanhaiya Gupta <
<email address hidden><mailto:<email address hidden>><mailto:
<email address hidden><mailto:<email address hidden>>>> wrote:

New question #679214 on MadGraph5_aMC@NLO:
https://answers.launchpad.net/mg5amcnlo/+question/679214

How is the cross-section calculated in MadGraph?

Theoretically, expected cross-section = production cross-section *
Branching_ratio

I generated the processes 1. p p > h (production by gluon fusion)

                                                2. p p > h , h > b b~
( Branching ratio for b b~ is 58%)

Theoretically, I should expect 0.58 of the production cross-section.

But using MadGraph, I am getting a nearly close cross-section.

--
You received this question notification because you are an answer
contact for MadGraph5_aMC@NLO.

--
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/679214/+confirm?answer_id=0

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/679214

You received this question notification because you asked the question.

--
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Revision history for this message
Kanhaiya Gupta (kanhaiya11) said : 		#4

Thanks Olivier Mattelaer, that solved my question.