Applying Pythia merging setting doesnt work properly

Asked by Jay Sandesara on 2020-07-01


I am trying to generate g g -> 4 lep +0,1,2 jet events using MadGraph+Pythia. To do this, I have to generate each sample individually with MadGraph and run it through Pythia with CKKW-L merging settings applied. I then intend to use them together after proper normalization.

First I generate the lhe events with the following additional settings apart from the standard ones+ mmnl>180:

0 = ickkw
0.0 = drjj
30 = ktdurham
0.4 = parameter

After this, I setup the Pythia card. You can find this and the entire run directory here:

However as you may see In the log, even though it says that CKKW-L merging is on, I am not getting any vetoed events and the final sample contain any number of jets (3,4,..) even if I set I set nJetMax=1 or 2. I have also set TMS cut to 40.0 and it still doesn't reject any events and I end up getting the same cross section after merging and showering that I got for the initial MadGraph run.

Can you please take a look for what I am doing wrong?

Question information

English Edit question
MadGraph5_aMC@NLO Edit question
No assignee Edit question
Last query:
Last reply:
Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said :


If you have an issue with pythia8, you should contact Pythia8 author.

> he final sample contain any number of jets (3,4,..) even if I set I set nJetMax=1 or 2.

This is normal this parameter indicates to pythia your highest multiplicity sample generated with MG5aMC, such that such sample is handle in a different way (since you do not have double counting with higher sample).

> g g -> 4 lep +0,1,2 jet

Why gluon in the initial state? this is likely to create issue for the matching/merging since you do not include all flavor combination for the initial state



Revision history for this message
Jay Sandesara (jaysandesara) said :

Thanks, I will give it a try with full flavor initial states with filters on.

I have one question that might be related to how I am generating events in MadGraph. After the Pythia run I get the following warning:

Warning in Merging::mergeProcessCKKWL: No clusterings found. History incomplete.

Is this related to MadGraph generation. Should I be setting clusinfo=True for this to error to go away?

Can you help with this problem?

Provide an answer of your own, or ask Jay Sandesara for more information if necessary.

To post a message you must log in.