Matching/Merging with UNLOPS

Asked by Soumyadip Barman on 2021-05-11

Hello,

I am trying to run a process with UNLOPS (ickkw = 4) merging. For that, I have an LHE file containing both LO and NLO processes. Now I want to shower the LHE file with Pythia8. I have two questions regarding this.

1. What are the parameters that must be synced between MG5@NLO and Pythia8, so that the merging process takes place properly?

2. I know that in UNLOPS the cross-section doesn't change after the merging, so does the number of events after the merging remains also same, or does it change after merging just like FxFx merging?

Please let me know if there are any dedicated wiki about UNLOPS.

Thanks,
Soumyadip

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
Rikkert Frederix Edit question
Last query:
Last reply:

This question was reopened

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Rikkert Frederix (frederix) said :
#1

Dear Soumyadip,

First of all, let me remind you that one cannot use the MG5_aMC interface to pythia8 to "automatically" shower the events. You have write your own pythia code that showers LHE events. There are 2 examples in the pythia8 example folder that deal with UNLOPS events: main88 and main89. Starting from those, and the information at http://home.thep.lu.se/Pythia/pythia82html/MatchingAndMerging.html it should be straight-forward.

Best regards,
Rikkert

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Soumyadip Barman (soumyadipbarman) said :
#2

Dear Rikkert,

Thanks, Rikkert for your reply.
So, please let me try to elaborate a bit on this. I am trying to set up the UNLOPS interface within CMS. In CMS it is two different steps to create the final output. One LHE step and then Showering step.

I have successfully completed the LHE step. But we don't have a default UNLOPS showering setup from Pythia8. I am trying to create that one. I am aware of the standalone Pythia8 setup. But I want to create a setup within CMS. So to create that setup I am trying to find the answers to the above-mentioned questions.

So, if you could give some info about that, it will be very helpful for me to work on it.

Thanks,
Soumyadip

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Rikkert Frederix (frederix) said :
#3

Dear Soumyadip,

I'm not sure I understand what would be the difference between a standalone Pythia8 setup and a setup within CMS. I can only recommend about the standalone Pythia8 setup, since that's only what I know. And that part is rather well-explained on the webpage I linked as well as in the pythia8 main88 and main89 example files. I don't think I can help much more than that since I'm not at all familiar with the CMS framework.

best,
Rikkert

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Soumyadip Barman (soumyadipbarman) said :
#4

Dear Rikkert,

Ok, Thanks Rikkert for your suggestions. I will look into it and try to set it up within CMS.

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Soumyadip Barman (soumyadipbarman) said :
#5

Thanks Rikkert Frederix, that solved my question.

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Soumyadip Barman (soumyadipbarman) said :
#6

Hi Rikkert,

Sorry to open up the conversation again. I need some more information.
I want to produce a gridpack for the Drell-yan process for UNLOPS with icckw = 4 in the run card.
I am a bit confused with the process card settings.

[1]
generate p p > e+ e- [QCD] @0
add process p p > e+ e- j [QCD] @1
add process p p > e+ e- j j [QCD] @2

add process p p > e+ e- j [LOonly=QCD] @3
add process p p > e+ e- j j [LOonly=QCD] @4
add process p p > e+ e- j j j [LOonly=QCD] @5
*****************************************************

[2]
generate p p > e+ e- [QCD] @0
add process p p > e+ e- j [QCD] @1
add process p p > e+ e- j j [QCD] @2

add process p p > e+ e- j j j [LOonly=QCD] @3
******************************************************

Which setting is the correct one for producing the gridpack for the Drell-yan process.

Thanks,
Soumyadip

Revision history for this message
Rikkert Frederix (frederix) said :
#7

Dear Soumyadip,

As can be seen from the main89 in the examples directory from pythia8, it's [1].

Best,
Rikkert

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Soumyadip Barman (soumyadipbarman) said :
#8

Hi,

In main89 in the examples directory of pythia8, it expects 2 LHE files : one with NLO and one with LO.
But if I want to create a mixed gridpack of NLO and LO then it is also good to go with [1]?
Because in this case isn't there some double-counting with process ids 1 ,2 and 3 ,4 ?

Thanks,
Soumyadip

Revision history for this message
Rikkert Frederix (frederix) said :
#9

Dear Soumyadip,

I should work either way.

best,
Rikkert

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