MadGraph5_aMC@NLO

Recommended way to shower with Pythia8

Asked by Robin

Dear authors,

I am trying to do the common workflow of generating events in MG5_aMC@NLO (e.g. p p > b b~ or Z Z) then passing them to Pythia8 for showering. Having read through several other similar questions, as far as I can tell there are 2 approaches:

1) Generate events with MG5_aMC, then pass the LHE to standalone Pythia8 for showering.

2) Tell MG5_aMC to do the showering as part of event generation by setting parton_shower to PYTHIA8 in run_card.dat and setting the path to Pythia8 correctly

Is one of these more 'correct'? I guess the important part in option 1 is ensuring that the Pythia shower settings are correctly set (as per http://amcatnlo.web.cern.ch/amcatnlo/list_detailed2.htm#showersettings).

For option 2, if I want to make some changes to the pythia event loop (i.e. that can't be done in the generated Pythia8.cmd), which file(s) should I edit?

Apologies if this seems trivial or has been answered before, but I couldn't find a comprehensive answer comparing the two methods.

Many thanks!

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
Paolo Torrielli Edit question
Last query:
Last reply:
Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

Both method should be fine.

> For option 2, if I want to make some changes to the pythia event loop (i.e. that can't be done in the generated Pythia8.cmd), which file(s) should I edit?

This is a question for Paolo which is on holliday right now. So it might take some time to get an answer to the question.

Cheers,

Olivier

Revision history for this message
Robin (robinagg) said : 		#2

Many thanks for the prompt response, it cleared things up.

Revision history for this message
Paolo Torrielli (paolo-torrielli) said : 		#3

Dear Robin,
sorry for not being able to reply sooner.

As far as your first question is concerned, and as Olivier correctly says, the two things are equivalent.
There is just one point I would like to stress, that does not seem to emerge from your post: in both
cases (events showered within MG5_aMC or externally with standalone Pythia8) the parton_shower
parameter in run_card has to be set to PYTHIA8.
Indeed in the MC@NLO method the generated events do depend on the shower you plan to interface
to, and are not physical before showering (i.e. no physical plots can be obtained from the un-showered
event file), hence all steps in the procedure need to know what shower you plan to use.

In order to change settings in the Pythia8 event loop you can go in MCatNLO/srcPythia8 and edit the
files Pythia8*.cc, where '*' indicates one of the four possibilities you find in that directory (Pythia8.cc
if you use Pythia < 8.2 and internal analysis routines, Pythia82.cc for Pythia >= 8.2 and internal analysis
routines, Pythia8_hep.cc or Pythia82_hep.cc if you want to output HepMC n-tuples).
Pythia8 settings can be changed (respecting the recommendations on the webpage) towards the end
of MCatNLO/Scripts/MCatNLO_MadFKS_PYTHIA8.Script, that is the script that writes the Pythia8.cmd
card you find in the running folder at shower time.

Cheers.
Paolo

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