MadGraph5_aMC@NLO

Question about single top with jet matching

Asked by Kelci Mohrman

Hi MG Experts,

I have a question about jet matching. We are trying to make a leading order t-channel single top sample; we are using the 4-flavor scheme and we want to include an extra parton. In our process card, we have:

generate p p > t b~ j l+ l- $$ w+ w-
add process p p > t~ b j l+ l- $$ w+ w-
add process p p > t b~ j j l+ l- $$ w+ w-
add process p p > t~ b j j l+ l- $$ w+ w-

Based on the MG wiki (specifically in [1], the paragraph towards the end that begins "Note that there are special processes, such as p p > t b~ j + p p > t b~ j j ..."), we initially thought that we should set the Pythia parameter nJetMax to 1. However, when we did this, we found that the events from the 3rd and 4th processes in our process card were being 100% vetoed. We changed nJetMax to be 2, and there was no longer 100% vetoing for any process. This makes sense to us since there are two light flavor jets in the process, but we're still a bit confused because only one of them is a radiated jet. We have three main questions about this situation:

1) For our case, is it correct to set nJetMax=2?

2) Does it matter if we leave nJet at its default value of -1, or should we set it to 1, as seemed to be suggested in a different question on this forum [2]?

3) As far as we can tell, it seems that pythia is treating the "j" in pp > tbjZ as an extra parton and applying the matching to it. We are concerned about this because the jet comes from a QED vertex and so the parton shower should not be able to fill in this extra jet. Is our understanding of the situation correct? Is there an issue with the situation? If there is a problem, is there a way of specifying that only the second jet should be matched?

Thanks very much for the help! - Kelci

[1] https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Matching#no1
[2] https://answers.launchpad.net/mg5amcnlo/+question/672638

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Maybe you should ask a pythia8 expert here.

What is clear is that:
generate p p > t b~ j l+ l- $$ w+ w-
add process p p > t~ b j l+ l- $$ w+ w-

need to be treated in exclusive mode while

add process p p > t b~ j j l+ l- $$ w+ w-
add process p p > t~ b j j l+ l- $$ w+ w-

should be treated in inclusive mode.

The pythia8 parameter for such treatment is the following: (http://home.thep.lu.se/~torbjorn/pythia81html/JetMatching.html)

mode JetMatching:exclusive (default = 2; minimum = 0; maximum = 2)
Exclusive or inclusive merging.
option 0 : The merging is run in inclusive mode. All partons must match jets, but additional jets are allowed, provided they are not harder than the matched jets.
option 1 : The merging is run in exclusive mode. All partons must match jets, and no additional jets are allowed.
option 2 : If nJet < nJetMax, then the merging is run in exclusive mode, otherwise it is run in inclusive mode. For Madgraph-style matching, this is checked on an event-by-event basis, which is useful when an LHEF contains a "soup" of partonic multiplicities. If nJetMax < 0 or nJet < 0, then the algorithm defaults to exclusive mode.

mode JetMatching:nJet (default = -1; minimum = -1)
When JetMatching:exclusive = 2, nJet indicates the minimum number of additional light jets in the incoming process. This value may be set automatically.

mode JetMatching:nJetMax (default = -1; minimum = -1)
When JetMatching:exclusive = 2, nJetMax is used to indicate the maximum number of jets that will be matched.

So I will keep njet to default value (-1) since this need to be re-evaluated event by event.
And would put JetMatching:nJetMax=1.

The fact that you do not have any jet from the 1j might means that your merging scale is not appropriate.

Cheers,

Olivier

On 28 Aug 2019, at 16:13, Kelci Mohrman <<email address hidden><mailto:<email address hidden>>> wrote:

generate p p > t b~ j l+ l- $$ w+ w-

Can you help with this problem?

Provide an answer of your own, or ask Kelci Mohrman for more information if necessary.

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