MadGraph5_aMC@NLO

Matching with VBF

Asked by Jamie Tattersall

Hi,

I was wondering what cuts should be set when you would like matching in a VBF process.

Specifically, I see that the 'auto_ptj_mjj' cut should be turned off to avoid cutting on the forward jets.

However, a cut is still required on any radiated jets. Is it possible to place a cut on only these jets in the run_card or does this need to be manually hacked into the code?

Many thanks,
Jamie

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
de Visscher Edit question
Solved by:
Olivier Mattelaer
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Last reply:
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Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi Jamie,

Please read this:
https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Matching

This indicates:
From MadGraph 5 v. 1.3.18 and Pythia/PGS package v. 2.1.10, non-radiation jets such as the scattered jets in VBF are not included in the matching (but final state radiation from such particles is matched consistently), which allows for variation of the matching scale (xqcut/QCUT) in a consistent way also for such processes.

Otherwise you have to specify the xqcut value since that's the value which control the matching.

Cheers,

Olivier

PS: You also have information on this link:
https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/IntroMatching

Revision history for this message
Jamie Tattersall (32-jamie-7j) said : 		#2

Thanks Olivier Mattelaer, that solved my question.

Revision history for this message
Jamie Tattersall (32-jamie-7j) said : 		#3

Hi Oliver,

I'm really sorry, somehow i read straight over that sentence.

In any thank you very much, that was what I was hoping for!

Cheers,
Jamie

Revision history for this message
Andrew Levin (amlevin-g) said : 		#4

I have another question about this.

How exactly did you implement in MadGraph 5 v. 1.3.18 and Pythia/PGS package v. 2.1.10 the feature that non-radiation jets are not matched?

Did you validate that it works?

Thanks.

Andrew

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#5

Hi Andrew,

This is not my work but the one of Johan. However Johan quits physics so I will refer your question to Simon who knows the best that part of the code.

Cheers,

Olivier

Revision history for this message
Kenneth Long (kdlong-e) said : 		#6

Hi Olivier,

Did you ever get more information on this? I am trying to understand the correct matching scheme for VBS of WZ. I have generated a sample with both the EWK only and QCD+EWK diagrams (specifically, it is pp > w+l+l-jj, w+ > l+ vl QED = 6). I am trying to understand how to consistently use a matching scheme on the QCD jets only. I need to be able to implement this outside of the MadGraph/Pythia/PGS interface in order to properly interface with the CMS Monte Carlo generation. I would greatly appreciate any advice.

Best,

Kenneth

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Olivier Mattelaer (olivier-mattelaer) said : 		#7

Dear Kenneth,

The first point that you have to do is to choose a Feynman Diagram for your event,
in madgraph this is done by the following the diagram enhancement method that we use for the phase-space integration.

Then when you have a single diagram, you have to define a rule of which jet are QCD and wich one are QED.
Most of the case (like a > j j, g > j j) are not ambiguous. Some other are ambiguous (like z > j j j), in that case, you basically have to apply a clustering algorithm to determine which one is the most likely not coming from the branching and tag that one as QED, while the other branch, you tag one as QED and the second one as QCD.

Cheers,

Olivier

PS: Note that Pythia8 contains very advanced matching/merging method, so try to not re-invent the wheel.