Ho can I change the scale of alpha_s ?

Asked by Md.Raju on 2019-10-09

Respected Sir,

I am calculation double higgs production (g g -> h h)in madgraph so the strong coupling must be evaluated at energy scale 250 GeV
.So how can I set the scale in Madgraph ? The scale should be 250 GeV for on-shell di-Higgs production. I want to be sure how can use in the Madgraph ?

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
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Last query:
2019-10-09
Last reply:
2019-10-09

Hi,

The easiest is to type "set fixed_scale 250" when you have the following question:

Do you want to edit a card (press enter to bypass editing)?
/------------------------------------------------------------\
| 1. param : param_card.dat |
| 2. run : run_card.dat |
| 3. plot : plot_card.dat |
\------------------------------------------------------------/
 you can also
   - enter the path to a valid card or banner.
   - use the 'set' command to modify a parameter directly.
     The set option works only for param_card and run_card.
     Type 'help set' for more information on this command.
   - call an external program (ASperGE/MadWidth/...).
     Type 'help' for the list of available command
 [0, done, 1, param, 2, run, 3, plot, enter path][90s to answer]
>[timer stopped]
set fixed_scale 250

you will then see the following on screen:
INFO: modify parameter fixed_fac_scale of the run_card.dat to True
INFO: modify parameter fixed_ren_scale of the run_card.dat to True
INFO: modify parameter scale of the run_card.dat to 250.0
INFO: modify parameter dsqrt_q2fact1 of the run_card.dat to 250.0
INFO: modify parameter dsqrt_q2fact2 of the run_card.dat to 250.0

Equivalently you can edit your run_card.dat accordingly:
#*********************************************************************
# Renormalization and factorization scales *
#*********************************************************************
  True = fixed_ren_scale ! if .true. use fixed ren scale
  True = fixed_fac_scale ! if .true. use fixed fac scale
  250.0 = scale ! fixed ren scale
  250.0 = dsqrt_q2fact1 ! fixed fact scale for pdf1
  250.0 = dsqrt_q2fact2 ! fixed fact scale for pdf2
  -1 = dynamical_scale_choice ! Choose one of the preselected dynamical choices
  1.0 = scalefact ! scale factor for event-by-event scales

Cheers,

Olivier

Md.Raju (rahi199065) said : #2

Thanks that solve my question.

On Wed, Oct 9, 2019 at 3:53 PM Olivier Mattelaer <
<email address hidden>> wrote:

> Your question #684852 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/684852
>
> Status: Open => Answered
>
> Olivier Mattelaer proposed the following answer:
> Hi,
>
>
> The easiest is to type "set fixed_scale 250" when you have the following
> question:
>
> Do you want to edit a card (press enter to bypass editing)?
> /------------------------------------------------------------\
> | 1. param : param_card.dat |
> | 2. run : run_card.dat |
> | 3. plot : plot_card.dat |
> \------------------------------------------------------------/
> you can also
> - enter the path to a valid card or banner.
> - use the 'set' command to modify a parameter directly.
> The set option works only for param_card and run_card.
> Type 'help set' for more information on this command.
> - call an external program (ASperGE/MadWidth/...).
> Type 'help' for the list of available command
> [0, done, 1, param, 2, run, 3, plot, enter path][90s to answer]
> >[timer stopped]
> set fixed_scale 250
>
> you will then see the following on screen:
> INFO: modify parameter fixed_fac_scale of the run_card.dat to True
> INFO: modify parameter fixed_ren_scale of the run_card.dat to True
> INFO: modify parameter scale of the run_card.dat to 250.0
> INFO: modify parameter dsqrt_q2fact1 of the run_card.dat to 250.0
> INFO: modify parameter dsqrt_q2fact2 of the run_card.dat to 250.0
>
> Equivalently you can edit your run_card.dat accordingly:
> #*********************************************************************
> # Renormalization and factorization scales *
> #*********************************************************************
> True = fixed_ren_scale ! if .true. use fixed ren scale
> True = fixed_fac_scale ! if .true. use fixed fac scale
> 250.0 = scale ! fixed ren scale
> 250.0 = dsqrt_q2fact1 ! fixed fact scale for pdf1
> 250.0 = dsqrt_q2fact2 ! fixed fact scale for pdf2
> -1 = dynamical_scale_choice ! Choose one of the preselected dynamical
> choices
> 1.0 = scalefact ! scale factor for event-by-event scales
>
> 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/684852/+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/684852
>
> You received this question notification because you asked the question.
>

marked as solved

Md.Raju (rahi199065) said : #4

But if I apply 250 Gev for fixed scale its reduce the crossection by half
factor from when I am using the fixed scale 126 GeV. Why it is happening ?
I mean
cross section is 30 fb for fixed scale 126 Gev
cross section is 15 fb for fixed scale 250 Gev
which is correct ?

On Wed, Oct 9, 2019 at 4:18 PM Md.Raju <email address hidden>
wrote:

> Your question #684852 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/684852
>
> Status: Answered => Open
>
> You are still having a problem:
> Thanks that solve my question.
>
> On Wed, Oct 9, 2019 at 3:53 PM Olivier Mattelaer <
> <email address hidden>> wrote:
>
> > Your question #684852 on MadGraph5_aMC@NLO changed:
> > https://answers.launchpad.net/mg5amcnlo/+question/684852
> >
> > Status: Open => Answered
> >
> > Olivier Mattelaer proposed the following answer:
> > Hi,
> >
> >
> > The easiest is to type "set fixed_scale 250" when you have the following
> > question:
> >
> > Do you want to edit a card (press enter to bypass editing)?
> > /------------------------------------------------------------\
> > | 1. param : param_card.dat |
> > | 2. run : run_card.dat |
> > | 3. plot : plot_card.dat |
> > \------------------------------------------------------------/
> > you can also
> > - enter the path to a valid card or banner.
> > - use the 'set' command to modify a parameter directly.
> > The set option works only for param_card and run_card.
> > Type 'help set' for more information on this command.
> > - call an external program (ASperGE/MadWidth/...).
> > Type 'help' for the list of available command
> > [0, done, 1, param, 2, run, 3, plot, enter path][90s to answer]
> > >[timer stopped]
> > set fixed_scale 250
> >
> > you will then see the following on screen:
> > INFO: modify parameter fixed_fac_scale of the run_card.dat to True
> > INFO: modify parameter fixed_ren_scale of the run_card.dat to True
> > INFO: modify parameter scale of the run_card.dat to 250.0
> > INFO: modify parameter dsqrt_q2fact1 of the run_card.dat to 250.0
> > INFO: modify parameter dsqrt_q2fact2 of the run_card.dat to 250.0
> >
> > Equivalently you can edit your run_card.dat accordingly:
> > #*********************************************************************
> > # Renormalization and factorization scales *
> > #*********************************************************************
> > True = fixed_ren_scale ! if .true. use fixed ren scale
> > True = fixed_fac_scale ! if .true. use fixed fac scale
> > 250.0 = scale ! fixed ren scale
> > 250.0 = dsqrt_q2fact1 ! fixed fact scale for pdf1
> > 250.0 = dsqrt_q2fact2 ! fixed fact scale for pdf2
> > -1 = dynamical_scale_choice ! Choose one of the preselected
> dynamical
> > choices
> > 1.0 = scalefact ! scale factor for event-by-event scales
> >
> > 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/684852/+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/684852
> >
> > You received this question notification because you asked the question.
> >
>
> --
> You received this question notification because you asked the question.
>

Hi,

Yes you do expect a large dependence in the renormalization scale.
This is the issue with QCD, the cross-section (at leading order) computed has large dependence in this un-physical scale (which means we can not predict precisely the associated cross-section).

Cheers,

Olivier

> On 9 Oct 2019, at 13:33, Md.Raju <email address hidden> wrote:
>
> Question #684852 on MadGraph5_aMC@NLO changed:
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684852&amp;data=02%7C01%7C%7C0fc5aff59b6f415bbbcb08d74cac8647%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637062176181029280&amp;sdata=qGGy2Blr89RbRqn5YI8SHPIIazwYIj2u6z2xAKAvLOY%3D&amp;reserved=0
>
> Md.Raju posted a new comment:
> But if I apply 250 Gev for fixed scale its reduce the crossection by half
> factor from when I am using the fixed scale 126 GeV. Why it is happening ?
> I mean
> cross section is 30 fb for fixed scale 126 Gev
> cross section is 15 fb for fixed scale 250 Gev
> which is correct ?
>
> On Wed, Oct 9, 2019 at 4:18 PM Md.Raju <email address hidden>
> wrote:
>
>> Your question #684852 on MadGraph5_aMC@NLO changed:
>> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684852&amp;data=02%7C01%7C%7C0fc5aff59b6f415bbbcb08d74cac8647%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637062176181029280&amp;sdata=qGGy2Blr89RbRqn5YI8SHPIIazwYIj2u6z2xAKAvLOY%3D&amp;reserved=0
>>
>> Status: Answered => Open
>>
>> You are still having a problem:
>> Thanks that solve my question.
>>
>> On Wed, Oct 9, 2019 at 3:53 PM Olivier Mattelaer <
>> <email address hidden>> wrote:
>>
>>> Your question #684852 on MadGraph5_aMC@NLO changed:
>>> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684852&amp;data=02%7C01%7C%7C0fc5aff59b6f415bbbcb08d74cac8647%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637062176181029280&amp;sdata=qGGy2Blr89RbRqn5YI8SHPIIazwYIj2u6z2xAKAvLOY%3D&amp;reserved=0
>>>
>>> Status: Open => Answered
>>>
>>> Olivier Mattelaer proposed the following answer:
>>> Hi,
>>>
>>>
>>> The easiest is to type "set fixed_scale 250" when you have the following
>>> question:
>>>
>>> Do you want to edit a card (press enter to bypass editing)?
>>> /------------------------------------------------------------\
>>> | 1. param : param_card.dat |
>>> | 2. run : run_card.dat |
>>> | 3. plot : plot_card.dat |
>>> \------------------------------------------------------------/
>>> you can also
>>> - enter the path to a valid card or banner.
>>> - use the 'set' command to modify a parameter directly.
>>> The set option works only for param_card and run_card.
>>> Type 'help set' for more information on this command.
>>> - call an external program (ASperGE/MadWidth/...).
>>> Type 'help' for the list of available command
>>> [0, done, 1, param, 2, run, 3, plot, enter path][90s to answer]
>>>> [timer stopped]
>>> set fixed_scale 250
>>>
>>> you will then see the following on screen:
>>> INFO: modify parameter fixed_fac_scale of the run_card.dat to True
>>> INFO: modify parameter fixed_ren_scale of the run_card.dat to True
>>> INFO: modify parameter scale of the run_card.dat to 250.0
>>> INFO: modify parameter dsqrt_q2fact1 of the run_card.dat to 250.0
>>> INFO: modify parameter dsqrt_q2fact2 of the run_card.dat to 250.0
>>>
>>> Equivalently you can edit your run_card.dat accordingly:
>>> #*********************************************************************
>>> # Renormalization and factorization scales *
>>> #*********************************************************************
>>> True = fixed_ren_scale ! if .true. use fixed ren scale
>>> True = fixed_fac_scale ! if .true. use fixed fac scale
>>> 250.0 = scale ! fixed ren scale
>>> 250.0 = dsqrt_q2fact1 ! fixed fact scale for pdf1
>>> 250.0 = dsqrt_q2fact2 ! fixed fact scale for pdf2
>>> -1 = dynamical_scale_choice ! Choose one of the preselected
>> dynamical
>>> choices
>>> 1.0 = scalefact ! scale factor for event-by-event scales
>>>
>>> Cheers,
>>>
>>> Olivier
>>>
>>> --
>>> If this answers your question, please go to the following page to let us
>>> know that it is solved:
>>>
>>>
>> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684852%2F%2Bconfirm%3Fanswer_id%3D0&amp;data=02%7C01%7C%7C0fc5aff59b6f415bbbcb08d74cac8647%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637062176181029280&amp;sdata=Svnlt3FwFljCxgmHaKPDsR81wQgjE3PVYbVcbbWIcFg%3D&amp;reserved=0
>>>
>>> If you still need help, you can reply to this email or go to the
>>> following page to enter your feedback:
>>> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684852&amp;data=02%7C01%7C%7C0fc5aff59b6f415bbbcb08d74cac8647%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637062176181029280&amp;sdata=qGGy2Blr89RbRqn5YI8SHPIIazwYIj2u6z2xAKAvLOY%3D&amp;reserved=0
>>>
>>> You received this question notification because you asked the question.
>>>
>>
>> --
>> You received this question notification because you asked the question.
>>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Md.Raju (rahi199065) said : #6

Thank you sir for your patient response
But which fixed scale should be used for double higgs production in this
case ?

On Wed, Oct 9, 2019 at 5:13 PM Olivier Mattelaer <
<email address hidden>> wrote:

> Your question #684852 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/684852
>
> Olivier Mattelaer posted a new comment:
> Hi,
>
> Yes you do expect a large dependence in the renormalization scale.
> This is the issue with QCD, the cross-section (at leading order) computed
> has large dependence in this un-physical scale (which means we can not
> predict precisely the associated cross-section).
>
> Cheers,
>
> Olivier
>
>
>
> > On 9 Oct 2019, at 13:33, Md.Raju <email address hidden>
> wrote:
> >
> > Question #684852 on MadGraph5_aMC@NLO changed:
> >
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684852&amp;data=02%7C01%7C%7C0fc5aff59b6f415bbbcb08d74cac8647%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637062176181029280&amp;sdata=qGGy2Blr89RbRqn5YI8SHPIIazwYIj2u6z2xAKAvLOY%3D&amp;reserved=0
> >
> > Md.Raju posted a new comment:
> > But if I apply 250 Gev for fixed scale its reduce the crossection by half
> > factor from when I am using the fixed scale 126 GeV. Why it is happening
> ?
> > I mean
> > cross section is 30 fb for fixed scale 126 Gev
> > cross section is 15 fb for fixed scale 250 Gev
> > which is correct ?
> >
> > On Wed, Oct 9, 2019 at 4:18 PM Md.Raju <
> <email address hidden>>
> > wrote:
> >
> >> Your question #684852 on MadGraph5_aMC@NLO changed:
> >>
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684852&amp;data=02%7C01%7C%7C0fc5aff59b6f415bbbcb08d74cac8647%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637062176181029280&amp;sdata=qGGy2Blr89RbRqn5YI8SHPIIazwYIj2u6z2xAKAvLOY%3D&amp;reserved=0
> >>
> >> Status: Answered => Open
> >>
> >> You are still having a problem:
> >> Thanks that solve my question.
> >>
> >> On Wed, Oct 9, 2019 at 3:53 PM Olivier Mattelaer <
> >> <email address hidden>> wrote:
> >>
> >>> Your question #684852 on MadGraph5_aMC@NLO changed:
> >>>
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684852&amp;data=02%7C01%7C%7C0fc5aff59b6f415bbbcb08d74cac8647%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637062176181029280&amp;sdata=qGGy2Blr89RbRqn5YI8SHPIIazwYIj2u6z2xAKAvLOY%3D&amp;reserved=0
> >>>
> >>> Status: Open => Answered
> >>>
> >>> Olivier Mattelaer proposed the following answer:
> >>> Hi,
> >>>
> >>>
> >>> The easiest is to type "set fixed_scale 250" when you have the
> following
> >>> question:
> >>>
> >>> Do you want to edit a card (press enter to bypass editing)?
> >>> /------------------------------------------------------------\
> >>> | 1. param : param_card.dat |
> >>> | 2. run : run_card.dat |
> >>> | 3. plot : plot_card.dat |
> >>> \------------------------------------------------------------/
> >>> you can also
> >>> - enter the path to a valid card or banner.
> >>> - use the 'set' command to modify a parameter directly.
> >>> The set option works only for param_card and run_card.
> >>> Type 'help set' for more information on this command.
> >>> - call an external program (ASperGE/MadWidth/...).
> >>> Type 'help' for the list of available command
> >>> [0, done, 1, param, 2, run, 3, plot, enter path][90s to answer]
> >>>> [timer stopped]
> >>> set fixed_scale 250
> >>>
> >>> you will then see the following on screen:
> >>> INFO: modify parameter fixed_fac_scale of the run_card.dat to True
> >>> INFO: modify parameter fixed_ren_scale of the run_card.dat to True
> >>> INFO: modify parameter scale of the run_card.dat to 250.0
> >>> INFO: modify parameter dsqrt_q2fact1 of the run_card.dat to 250.0
> >>> INFO: modify parameter dsqrt_q2fact2 of the run_card.dat to 250.0
> >>>
> >>> Equivalently you can edit your run_card.dat accordingly:
> >>> #*********************************************************************
> >>> # Renormalization and factorization scales *
> >>> #*********************************************************************
> >>> True = fixed_ren_scale ! if .true. use fixed ren scale
> >>> True = fixed_fac_scale ! if .true. use fixed fac scale
> >>> 250.0 = scale ! fixed ren scale
> >>> 250.0 = dsqrt_q2fact1 ! fixed fact scale for pdf1
> >>> 250.0 = dsqrt_q2fact2 ! fixed fact scale for pdf2
> >>> -1 = dynamical_scale_choice ! Choose one of the preselected
> >> dynamical
> >>> choices
> >>> 1.0 = scalefact ! scale factor for event-by-event scales
> >>>
> >>> Cheers,
> >>>
> >>> Olivier
> >>>
> >>> --
> >>> If this answers your question, please go to the following page to let
> us
> >>> know that it is solved:
> >>>
> >>>
> >>
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684852%2F%2Bconfirm%3Fanswer_id%3D0&amp;data=02%7C01%7C%7C0fc5aff59b6f415bbbcb08d74cac8647%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637062176181029280&amp;sdata=Svnlt3FwFljCxgmHaKPDsR81wQgjE3PVYbVcbbWIcFg%3D&amp;reserved=0
> >>>
> >>> If you still need help, you can reply to this email or go to the
> >>> following page to enter your feedback:
> >>>
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684852&amp;data=02%7C01%7C%7C0fc5aff59b6f415bbbcb08d74cac8647%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637062176181029280&amp;sdata=qGGy2Blr89RbRqn5YI8SHPIIazwYIj2u6z2xAKAvLOY%3D&amp;reserved=0
> >>>
> >>> You received this question notification because you asked the question.
> >>>
> >>
> >> --
> >> You received this question notification because you asked the question.
> >>
> >
> > --
> > You received this question notification because you are an answer
> > contact for MadGraph5_aMC@NLO.
>
> --
> You received this question notification because you asked the question.
>

Hi,

Technically you can use the one that you want.
I would argue that using 125 GeV is likely too small for this process and I would advise to use something in the range of 250 to 400 GeV. (I would personally use 300).

Now this large dependencies shows that you expect large contribution from NLO computation. And that you should rescale such generation to the most precise theoretical computation for this process (and check the associated theoretical uncertainty)

Cheers,

Olivier

Tamasi (tamasi-kar) said : #8

Hi Oliver,

I have a question related to factorization(μF)and renormalization(μR) scales. In some phenomenological paper that I went through, I read μF = μR = HT/2. Also, on page 123 of https://arxiv.org/pdf/1405.0301.pdf I read that by default, the dynamical scales are set equal to HT/2. I had a look at the various options available for the dynamical_scale_choice in the file setscales.f in the sub-folder SubProcesses. I noticed that options -1 and 3 for dynamical_scale_choice do represent μF = μR = HT/2 for NLO processes. However, for LO processes if I look at the same file the default option -1 reads something like :

      if (dynamical_scale_choice.eq.-1) then
c Cluster external states until reducing the system to a 2->2 topology whose transverse mass is used for setting the scale.
c This is not done in this file due to the clustering.
           rscale=0d0

So I guess, I will have to modify the dynamical_scale_choice in the runcard to 3 for LO processes?

Also, what are the advantages or disadvantages of using a fixed scale over dynamical_scale?