MadGraph5_aMC@NLO

warning: small width

Asked by Sarah

Hello
I got this warning( Particle 36 with small width detected)
I saw the answer https://answers.launchpad.net/mg5amcnlo/+faq/3053, but I don't understand it.

do I have to write this line 1e-12 = small_width_treatment in run card after the running of the process is done?

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

Depend on how you want to handle that width.
In general, not doing anything is the best thing to do.

Cheers,

Olivier

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Sarah (sarahwarad) said : 		#2

Thank you for your answer

Is it something related to narrow width approximation?

I am trying to reproduce data of a published paper and I got slightly different results.
Maybe the story be behind this

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Best Olivier Mattelaer (olivier-mattelaer) said : 		#3

Yes and No,

If the width is too small, then the numerical evaluation of the matrix-element and the associated phase-space integration can be numerically unstable.
This mode triggers special mode of computation to avoid such numerical issue.
To avoid such issue we indeed assume narrow-width-approximation (which should be a very good approximation when this mode triggers) to keep the cross-section correct, however offshell effect will not be handle in an exact fashion (which given the small width should be reasonable once again). Indeed, the full matrix-element will use a fake width (artificially large to avoid numerical issue) and this will impact offshell effect.

the parameter 'small_width_treatment' can change the threshold where this mode activates and how much artificially large the width is going to be in this mode.

Cheers,

Olivier

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Sarah (sarahwarad) said : 		#4

Thank you Olivier for the clarification.
I appreciate you

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Sarah (sarahwarad) said : 		#5

Thanks Olivier Mattelaer, that solved my question.