The cross-section changes strangely when width surpasses 5%

Asked by Manuel Szewc on 2018-05-21

Hi,

I am studying a new particle production cross section and it has an odd behavior in the region when the width of said particle surpasses the 5% of the mass. The cross-section, which I expected to fall as I change the parameter, suddenly grows then starts to fall again.
I was wondering if there exist a discontinuity in the matrix element definition around said width value. Looking at http://pdg.lbl.gov/2017/reviews/rpp2017-rev-resonances.pdf equation 49.22, it mentions that the definition can change between narrow and broad resonances ( "For a narrow resonance it is common to replace √s by MBW"). I would like to know, if possible, if there exists a change in definition and if it could affect the behavior of the cross-section in an unphysical way.

Thanks in advance,
Manuel

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Last query:
2018-05-24
Last reply:
2018-05-24

Hi,

The formula for the matrix element never use the narrow width approximation.
Now we have a threshold on the width of the particles above which a particle is never consider onshell.
Above that threshold, those particles are not written in the lhe file anymore and this is going to change the behaviour of the parton-shower program.
This can also change the behaviour of some cuts if you play with onshell/offshell definition/cuts.

What is your model/process? I would be interested to run those myself to check if I can spot something weird.

Cheers,

Olivier

> On 21 May 2018, at 06:03, Manuel Szewc <email address hidden> wrote:
>
> New question #669477 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/669477
>
> Hi,
>
> I am studying a new particle production cross section and it has an odd behavior in the region when the width of said particle surpasses the 5% of the mass. The cross-section, which I expected to fall as I change the parameter, suddenly grows then starts to fall again.
> I was wondering if there exist a discontinuity in the matrix element definition around said width value. Looking at http://pdg.lbl.gov/2017/reviews/rpp2017-rev-resonances.pdf equation 49.22, it mentions that the definition can change between narrow and broad resonances ( "For a narrow resonance it is common to replace √s by MBW"). I would like to know, if possible, if there exists a change in definition and if it could affect the behavior of the cross-section in an unphysical way.
>
> Thanks in advance,
> Manuel
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Manuel Szewc (mszewc) said : #2

Dear Olivier
Thank you for answering! This odd behavior is already found at the parton level, without showering. The behavior persists if I change the cuts_decay command.
I am trying to reproduce the problem in a model I can post here (my current model has too many parameters). If I am able to do it, I will post it here. The process is single production associated with two other particles. I was wondering if maybe this was because of my syntax (I used the dreaded / command in order to ban an antiparticle appearing in the single production diagram of the particle).
Thank you again for your answers
Manuel

As you want, but if you do not find an easy example. Do not hesitate to send me your current setup.
I should be able to check the issue (and potentially find a simpler example)

You can send your model/additional information either to my email
<email address hidden>
or you can promote your question to a bug (bug report are allowed to have attachment.

Cheers,

Olivier

> On 24 May 2018, at 20:52, Manuel Szewc <email address hidden> wrote:
>
> Question #669477 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/669477
>
> Status: Answered => Open
>
> Manuel Szewc is still having a problem:
> Dear Olivier
> Thank you for answering! This odd behavior is already found at the parton level, without showering. The behavior persists if I change the cuts_decay command.
> I am trying to reproduce the problem in a model I can post here (my current model has too many parameters). If I am able to do it, I will post it here. The process is single production associated with two other particles. I was wondering if maybe this was because of my syntax (I used the dreaded / command in order to ban an antiparticle appearing in the single production diagram of the particle).
> Thank you again for your answers
> Manuel
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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