Small decay width treatment

Hi,

What happens if you keep the "small_width_treatment" parameter to the default value?
You should then have a warning stating the following:

Particle 9000006 will use a fake width ( XXX instead of YYY ).
Cross-section will be rescaled according to NWA if needed.
To force exact treatment reduce the value of 'small_width_treatment' parameter of the run_card

This should allows to have a nice handling of the resonances by avoiding any numerical issues.
As stated the code will correct the cross-section according to the NWA.

In the unlikely case where you have other non-resonant diagram that contribute significantly to the cross-section, the scaling does not occur on such contribution of the cross-section.
Now this assumes that interference term between the onshell and off-shell diagram are negligeable (since otherwise NWA does not make sense).

So expect for some of the problematic case. This mode is equivalent to
1) make the computation with a "large" width (large means here 1e-8 times the mass)
2) correct the cross-section by the ratio of the fake width by the real width
Obviously the shape of the Breit-wigner will be much too large (in your case by 10 order of magnitude) but this should not be a problem for any experience.

This was actually the recomendation before (asking the user to do that manually), the small_width_treatment is simply an automation of this procedure.

Cheers,

Olivier

Dear Oliver,

Thanks for your reply. Without adding small_width_treatment I get both the warning and the error:

WARNING: Particle 9000006 will use a fake width ( 0.000324 instead of 3.67072e-18 ).
Cross-section will be rescaled according to NWA if needed.To force exact treatment reduce the value of 'small_width_treatment' parameter of the run_card
INFO: Update the dependent parameter of the param_card.dat
ERROR: The width of particle 9000006 is too small for an s-channel resonance (3.67072e-18). If you have this particle in an s-channel, this is likely to create numerical instabilities .

I indeed think that at least in my particular case there is no other non-resonant diagram that interferes with the present one.

Do I understand correctly that you suggest to remove small_width_treatment parameter and that in this case I can trust the computations of the cross section and its error bars?

Hi,

> Thanks for your reply. Without adding small_width_treatment I get both
> the warning and the error:
>
> WARNING: Particle 9000006 will use a fake width ( 0.000324 instead of 3.67072e-18 ).
> Cross-section will be rescaled according to NWA if needed.To force exact treatment reduce the value of 'small_width_treatment' parameter of the run_card
> INFO: Update the dependent parameter of the param_card.dat
> ERROR: The width of particle 9000006 is too small for an s-channel resonance (3.67072e-18). If you have this particle in an s-channel, this is likely to create numerical instabilities .

Ok this should be fine, can you send me your model/ process/...
such that I can check why you have both message and see if I can remove the printing of the error in that case (that should be the case). You can send that to <email address hidden>

> Do I understand correctly that you suggest to remove
> small_width_treatment parameter and that in this case I can trust the
> computations of the cross section and its error bars?

Cross-section and error-bars should be fine.
The invariant mass distribution will be too large (with a width of 0.000324 instead of 3.67072e-18)
and therefore all observables correlated with that mass might be technically impacted but the impact should be very small.

Cheers,

Olivier

> On 2 May 2019, at 14:47, Anastasia <email address hidden> wrote:
>
> Question #680594 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/680594
>
> Status: Answered => Open
>
> Anastasia is still having a problem:
> Dear Oliver,
>
> Thanks for your reply. Without adding small_width_treatment I get both
> the warning and the error:
>
> WARNING: Particle 9000006 will use a fake width ( 0.000324 instead of 3.67072e-18 ).
> Cross-section will be rescaled according to NWA if needed.To force exact treatment reduce the value of 'small_width_treatment' parameter of the run_card
> INFO: Update the dependent parameter of the param_card.dat
> ERROR: The width of particle 9000006 is too small for an s-channel resonance (3.67072e-18). If you have this particle in an s-channel, this is likely to create numerical instabilities .
>
> I indeed think that at least in my particular case there is no other
> non-resonant diagram that interferes with the present one.
>
> Do I understand correctly that you suggest to remove
> small_width_treatment parameter and that in this case I can trust the
> computations of the cross section and its error bars?
>
> --
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