MadGraph5_aMC@NLO

importing scale into model

Asked by Arian Abrahantes

dear experts:

this may sound as a naive question. It refers to scales used in calculations. In my case renormalization scale is needed somewhere within my model files and I would like to have it from MG commons. I have checked that scale-related variables reside in

common/to_scale/

included into run.inc file

I could hardwire those lines into my model files but common name can be changed from version to version or variable population of the common, their ordering etc. Is it there a MG-preferred way to import commons like that into user model files?

Actually, I believe my question can be extended to the point of what would be a good policy to include MG variables (variable names, commons, parameters) into user model files? The program already calculates and storage many things events per event and user model may profit from it.

I know it is kind of simple doubt but a standard/efficient way to solve this may deccrease considerably cmputation time in user models approach.

thanks n advance,

arian

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Olivier Mattelaer
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Arian Abrahantes (arian-abrahantes) said : 		#1

what if I'd like to evaluate my couplings in a decay on event-by-event basis in a dynamical/event-by-event renormalization scale. the simplest example in the default renoramlization scale of the event calculated by MG.

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Olivier Mattelaer (olivier-mattelaer) said : 		#2

Hi Arian,

You are talking about MG4 model (usermod?) model right?

In principle, you don't have to link to the scale.
If you choose a dynamical scale, the value of G will be updated according to your scale definition (on an event by event basis)
After this update on G we call the routine
setpara(param_name,.False.)
The second parameter prevent to read the param_card and prevent to update the coupling which are not scale dependent.

So you need to be sure that your couplings is indeed not in the
if (readlha) then

endif
part of couplings.f

and provide an expression of this coupling in term of G

Cheers,

Olivier

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Arian Abrahantes (arian-abrahantes) said : 		#3

dear oliver thanks for your reply.

> You are talking about MG4 model (usermod?) model right?
>
>
R: Yes, MG4 user model.

>
> So you need to be sure that your couplings is indeed not in the
> if (readlha) then
>
> endif
> part of couplings.f
>

R: Ok, I understood this. Yet what if I have another quantity but G in my
couplings depending on that dynamic scale. As I said scale I may have said
any dynamic parameter calculated by MG on event-by-event basis.

best regards and thanks in advence

arian

>
> and provide an expression of this coupling in term of G
>
> Cheers,
>
> Olivier
>
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Best Olivier Mattelaer (olivier-mattelaer) said : 		#4

Hi Arian,

Let me rewrite what you ask, just in order to be on the same page.
So you want to have a second parameter (let's call it alpha_s2) which is running like alpha_s (but with a different running function).
And you want to use the same scale for both.

In that case, you need to define the second running function inside your model and have access to the scale which indeed request to have access to the common to_scale.

Our Philosophy of the model is to be 100% independent of the generator. This is a library which could in principle be passed to another code than MG. And such model clearly breaks such idea.

>Actually, I believe my question can be extended to the point of what would be a good policy to include MG variables (variable >names, commons, parameters) into user model files? The program already calculates and storage many things events per event >and user model may profit from it.

As you point the problem with that is that you can't ensure that your model will work with all future version of MG5. I have simply no real answer to this problem.

My advice in that case would be to put on your website both your model and the source code of MG5 that you use (such that you know that this version is working).
Even better would be a script would check if the latest version Mg5 is still compatible with your model.

Cheers,

Olivier

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Arian Abrahantes (arian-abrahantes) said : 		#5

dear oliver:

 that was a perfect reformulation of my question. so, there is no standard way to do it and I'll have to use as many includes (of MG variables) as needed. It will be up-to-me to check which would be the common with the parameters I'll request in my model file and verify the MG version it is working with. The script seems a pretty good idea for the latter. Ok, first I'll try to do the physics then try to publish it ;-). deeply thankful for your concern,

arian

Revision history for this message
Arian Abrahantes (arian-abrahantes) said : 		#6

Thanks Olivier Mattelaer, that solved my question.