MadGraph5_aMC@NLO

External/internal parameters and new model

Asked by Francisco Casalinho

Hello MadGraph Team,

The only parameters I can change are the external parameters correct? I know that the internal parameters are already defined by the model and will be changed due to existing dependencies. Is the only way to change these internal parameters by creating a new model or is there another way?
How can I create a new model / impose new restrictions on existing models?

Thanks,

Francisco.

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

> The only parameters I can change are the external parameters correct?

Yes.

> Is the only way to change these internal parameters by creating a new model or is there another way?

in some cases, they might have other way but those will be very specific (like for the EW gauge sector)

> How can I create a new model / impose new restrictions on existing models?

For creating new model from scratch the best is to use FeynRules.
To apply additional (or change) restriction on a model you can check the FAQ:

https://answers.launchpad.net/mg5amcnlo/+faq/2312

Cheers,

Olivier

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Francisco Casalinho (francisco27) said : 		#2

Hi,

I changed the Z mass value in param.card, but the W mass value was not changed. Why?

Cheers,

Francisco

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Olivier Mattelaer (olivier-mattelaer) said : 		#3

What do you mean by not changed. MadGraph is certainly not using the value present in the param_card.dat if this is not an external parameter.

What can happen is that madgraph does not update the mass of the W in the used param_card. But the mass used will always be the updated. If you have any W in your lhe file, you should be able to check it. Otherwise, you can look at the mass information in the log file available from crossx.html. If you use too old version of MG5aMC in those the value within the param_card is never updated.

Note that for changing the EW parametrization of the model, we do have (in recent version of the code) the syntax:
set EWscheme MZ_MW_alpha
That put G_f as an internal parameter and put MW as an external parameter.

For older version of the code, the same is occurring if you using
set complex_mass_scheme True
But obviously in that case, not only the EW scheme is changed but we obviously used the complex mass scheme formalism

Cheers,

Olivier

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Francisco Casalinho (francisco27) said : 		#4

Thanks Olivier Mattelaer, that solved my question.

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cfgdfg (fhfh65) said : 		#5

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