MadGraph5_aMC@NLO

keeping 100MeV partial widths

Asked by John Doe

Hi MG5 developers, is it possible to keep partial widths that are less than 100 MeV? The default behavior is to
neglect them, which is fine. Except that in borderline cases, when the partial width can be slightly larger or smaller
than 100 MeV depending on a parameter, this default causes discontinuities. Is this default is hard-coded, could it
be edited by the user to some lower value? Thanks in advance!

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

Hi,

Is this the partial width for a coloured particle?
If yes, you can not really trust the computation for such small width since QCD is not perturbative in that case.
In that case, the 100MeV are indeed hardcoded since it correspond to roughly the mass of the pions.

In the case of not QCD particle, they are no hard cut-off, and you can simply play with the user parameter to improve the precision as much as needed.
(please look at 1402.1178 for details).

You can obviously change the value of the cut-off if you think that this is more physical.
This is in mg5decay/decay_objects.py
line 952

and line 6515 of madgraph/inteface/madgraph_interface.py

Cheers,

Olivier

On 27 Apr 2015, at 22:01, John Doe <email address hidden> wrote:

> New question #266010 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/266010
>
> Hi MG5 developers, is it possible to keep partial widths that are less than 100 MeV? The default behavior is to
> neglect them, which is fine. Except that in borderline cases, when the partial width can be slightly larger or smaller
> than 100 MeV depending on a parameter, this default causes discontinuities. Is this default is hard-coded, could it
> be edited by the user to some lower value? Thanks in advance!
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
John Doe (kwcpsn) said : 		#2

Thanks so much for your detailed reply. I agree with your statement, the perturbative
computation in that case is not meaningful. However, in order to verify that those
meaningless contributions are indeed negligible, I need the cross section to be analytic
in the couplings. Thanks again.

Revision history for this message
John Doe (kwcpsn) said : 		#3

Thanks Olivier Mattelaer, that solved my question.