How to apply center of mass energy cuts on a process

Asked by akash

Dear MadGraph team,

I am trying to reduce the SM background cross section for a particular BSM scecnario of interest. Since the particles in my BSM process are very massive, a partonic COM energy cut will reduce the background cross section greatly compared to the signal.

To do this, I uncomment lines 316-321 from cuts.f, within the SubProcesses folder for a particular run. However this leads to errors saying that the particular variables used in the cut (x1, x2, stot) are not defined.

I believe this is the old way of doing this. What would be the correct procedure to apply a partonic center of mass energy cut to any arbitrary process in the latest MG5 version? I would not like to rely on e_min_pdg since the background and signal have different particles.

Thank you.

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Olivier Mattelaer (olivier-mattelaer) said :

This is a cut that i would not implement in cuts.f but rather in setcuts.f
where you can add your threshold at this level:

c ensure that smin is bigger than mass threshold
      smin_p = 0d0
      do i=nincoming+1, nexternal
         smin_p = smin_p + pmass(i)
      smin = max(smin, smin_p**2)
      write(*,*) "Define smin to", smin

You should also add such type of cuts in cuts.f just in case one of the phase-space integrator does not respect that minimal value from scratch (which is not impossible).



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akash (ranade1) said :
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akash (ranade1) said :

Thanks Olivier Mattelaer, that solved my question.

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akash (ranade1) said :

Sorry for reopening this, but I am not that familiar with Fortran syntax. Is making this change as trivial as updating smin_p to whatever threshold I need? I tried this, but the cross section for the process I want to suppress (SM ttbar) does not decrease.
Could you please outline how one would apply such a cut?
Thank you for your time.

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Olivier Mattelaer (olivier-mattelaer) said :


Let me be clear that if you touch any Fortran file, you are responsible for doing the validation/... and that we do not provide support for any issue that you might face. If you are not confident in your computing (and/or Fortran) skill it is better to ask the help of someone in your lab with better knowledge.

So the variable which is important to update is smin, but since smin is set bigger than smin_p**2, setting smin_p to an hardcoded value might be enough.
You can look at the any of the channel log (accessible via the html link for example) if the print-statement was different before/after that setting.

Note that I still advise to set in top of the above a cut on S on the file dummy_fct.f where additional cuts can be designed for plugin (so this is also the place i would advise for adding additional cuts).



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