Is the parameter scalefact in the LO run_card equivalent to shower_scale_factor in the NLO card?

Hi,

The shower_scale_factor does not have a corresponding factor at LO (at least within the run_card).
This parameter control at which scale the shower is starting.
in general, one can use mu_R for that but some people also use \sqrt(s) (which is called power shower if I remember correctly).

The equivalent for scalefact at NLO is:
muR_over_ref
muF_over_reg

Therefore for getting HT/4 at LO, I would do
dynamical_scale_choice = 3
scalefact=0.5
use_syst=False
or
dynamical_scale_choice = 2
scalefact=0.25
use_syst=False

To be sure that you do not have to use sqrt(2), I have done the following runs

1) dynamical_scale_choice=2
returns 387.46 and via reweighting returns the value for Ht/2 as 494 pb
2) dynamical_scale_choice=2, scalefact=0.5, usesyst=False
returns 504 pb
3) dynamical_scale_choice=2, scalefact= 0.3535534 , usesyst=False
returns 580 pb
4) dynamical_scale_choice=2, scalefact= 0.25 , usesyst=False
returns 661pb
5) dynamical_scale_choice=3, scalefact= 0.5 , usesyst=False
return 667 pb
6) dynamical_scale_choice=3, scalefact= 1 , usesyst=True
returns 503 pb
via reweighting returns HT/4 as 656 pb

and you can see that this sounds consistent.

Cheers,

Olivier

> On 7 Sep 2021, at 20:45, Khawla Jaffel <email address hidden> wrote:
>
> New question #698655 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/698655
>
> Dear expert,
>
> I am wondering if `shower_scale_factor` and `scalefact` are equivalent consecutively in the NLO, LO run_card ?
>
>
> In the NLO run_card we have such block :
> #***********************************************************************
> # Include the NLO Monte Carlo subtr. terms for the following parton *
> # shower (HERWIG6 | HERWIGPP | PYTHIA6Q | PYTHIA6PT | PYTHIA8) *
> # WARNING: PYTHIA6PT works only for processes without FSR!!!! *
> #***********************************************************************
> PYTHIA8 = parton_shower
> 0.3535534 = shower_scale_factor ! multiply default shower starting
> ! scale by this factor
> #***********************************************************************
> # Renormalization and factorization scales *
> # (Default functional form for the non-fixed scales is the sum of *
> # the transverse masses divided by two of all final state particles *
> # and partons. This can be changed in SubProcesses/set_scales.f or via *
> # dynamical_scale_choice option) *
> #***********************************************************************
> False = fixed_ren_scale ! if .true. use fixed ren scale
> False = fixed_fac_scale ! if .true. use fixed fac scale
> 91.118 = mur_ref_fixed ! fixed ren reference scale
> 91.188 = muf1_ref_fixed ! fixed fact reference scale for pdf1
> 91.188 = muf2_ref_fixed ! fixed fact reference scale for pdf2
> -1 = dynamical_scale_choice ! Choose one (or more) of the predefined
> ! dynamical choices. Can be a list; scale choices beyond the
> ! first are included via reweighting
>
>
> However in the LO card :
>
> # Renormalization and factorization scales *
> #*********************************************************************
> False = fixed_ren_scale ! if .true. use fixed ren scale
> False = fixed_fac_scale ! if .true. use fixed fac scale
> 91.188 = scale ! fixed ren scale
> 91.188 = dsqrt_q2fact1 ! fixed fact scale for pdf1
> 91.188 = 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
>
>
> Why I am asking ? Because I want to set the `dynamical_scale_choice ` to `HT/4` in my LO run_card.
> So I am wondering if I can use ` scalefact` you see above to change the default value of the dynamical_scale_choice ? .
>
> I know the deafult for LO( HT) and for NLO (HT/2).
> does it make sense to set: `scalefact = 2/sqrt(2)` and
> `dynamical_scale_choice = 3 ` (==> which give HT/2 )
> to achieve this purpose ?
>
> Many thanks in advance.
> Cheers,
> Khawla
>
>
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