# matching in decays with large pt cut

Asked by Tao Xu on 2017-08-13

Hi,

I’m interested in boosted fat jet, like in the p p > z z , z > vl vl~, z > j j process. I assume the two jets from Z boson could further be clustered by FastJet to form a fat jet. Since I want to pick out those events with boosted Z, I prefer to use a jet pt cut by setting a large value to htjmin in run_card.

The script I used is

generate p p > z z , z > vl vl~ , z > j j
add process p p > z z , z > vl vl~ , z > j j j
output jet/DJR
launch
shower=PYTHIA8
set ebeam1 7000
set ebeam2 7000
set ickkw 1
set auto_ptj_mjj True
set xqcut 20
set cut_decays True
set htjmin 400
set JetMatching:qCut 30
set JetMatching:nJetMax 3
set Merging:nJetMax 3

with other cuts at default values.

The cut_cecays=TRUE means htjmin cut is now on the jets from Z decay. However I don’t want to lose the phase space between the xqcut (and also Qcut) scale and the htjmin scale, so I added the process where the Z boson decays to three jets for a simple test.

After parton shower with pythia8 installed with Madgraph_v2_5_5, I checked the DJR plots but there’s no information about the contribution from 0-jet sample ,1-jet sample...

Now I’m not sure if this is the right way to generate events with large jet pt cut and proper matching. Can you help me with this problem?

1. Is it possible to do matching for the process I chose, as the jets are form an on-shell decay? According to https://answers.launchpad.net/mg5amcnlo/+question/407562, I know there could be some problem when doing this. Is it because of the difficulties in choosing xqcut and Qcut for matching/merging, but I could still trust the result?

2. What Qcut value should I choose if I already used some large ptj or htjmin cut, in order not to loose the phase space that should be filled by pythia? If the value should be choose to be 1/3~1/6 the hard scale, should I use htjmin*1/6 instead? But to my understanding, Qcut is the scale in parton shower, which is for low scale radiation, is it still valid when i choose some high scale value like 200 GeV?

Thanks a lot!
Tao Xu

## Question information

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Tao Xu
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2017-08-13
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2017-08-13
2017-08-13
 Olivier Mattelaer (olivier-mattelaer) said on 2017-08-13: #1

Hi,

MG5aMC is not able to perform matching/merging for particle coming from a on shell decay.

In this case where the on shell decay is the Z boson, I do not even think that using matching/merging is worth.
The parton-shower should generated the third-jet in a pretty good approximation. ( I would say that if you start to worry about this, then you should also worried about the NWA that you are using to separate those Feynman diagram from all the other Feynman diagram leading to the same final state.)

Cheers,

Olivier

> On 13 Aug 2017, at 05:44, Tao Xu <email address hidden> wrote:
>
> New question #655834 on MadGraph5_aMC@NLO:
>
> Hi,
>
> I’m interested in boosted fat jet, like in the p p > z z , z > vl vl~, z > j j process. I assume the two jets from Z boson could further be clustered by FastJet to form a fat jet. Since I want to pick out those events with boosted Z, I prefer to use a jet pt cut by setting a large value to htjmin in run_card.
>
> The script I used is
>
> generate p p > z z , z > vl vl~ , z > j j
> add process p p > z z , z > vl vl~ , z > j j j
> output jet/DJR
> launch
> shower=PYTHIA8
> set ebeam1 7000
> set ebeam2 7000
> set ickkw 1
> set auto_ptj_mjj True
> set xqcut 20
> set cut_decays True
> set htjmin 400
> set JetMatching:qCut 30
> set JetMatching:nJetMax 3
> set Merging:nJetMax 3
>
> with other cuts at default values.
>
> The cut_cecays=TRUE means htjmin cut is now on the jets from Z decay. However I don’t want to lose the phase space between the xqcut (and also Qcut) scale and the htjmin scale, so I added the process where the Z boson decays to three jets for a simple test.
>
> After parton shower with pythia8 installed with Madgraph_v2_5_5, I checked the DJR plots but there’s no information about the contribution from 0-jet sample ,1-jet sample...
>
> Now I’m not sure if this is the right way to generate events with large jet pt cut and proper matching. Can you help me with this problem?
>
> 1. Is it possible to do matching for the process I chose, as the jets are form an on-shell decay? According to https://answers.launchpad.net/mg5amcnlo/+question/407562, I know there could be some problem when doing this. Is it because of the difficulties in choosing xqcut and Qcut for matching/merging, but I could still trust the result?
>
> 2. What Qcut value should I choose if I already used some large ptj or htjmin cut, in order not to loose the phase space that should be filled by pythia? If the value should be choose to be 1/3~1/6 the hard scale, should I use htjmin*1/6 instead? But to my understanding, Qcut is the scale in parton shower, which is for low scale radiation, is it still valid when i choose some high scale value like 200 GeV?
>
> Thanks a lot!
> Tao Xu
>
> --

 Tao Xu (taoxu) said on 2017-08-13: #2

Hi Olivier,

1.）So for the hadronic Z boson decay, I would use pythia to generate the extra jets and do not have to set the xqcut and qcut values by myself.

2). I have another question about the choice of qcut value. If the process is

generate p p > j j
add process p p > j j j
add process p p > j j j j

and I still require the jets to have large pt by setting a large htjmin.
In this case, should I choose qcut according to htjmin( for example, from around 1/6*htjmin)?
Or should I just use the qcut value when there's no htjmin cut?

Best,
Tao Xu

 Olivier Mattelaer (olivier-mattelaer) said on 2017-08-13: #3

Hi,
> 1.）So for the hadronic Z boson decay, I would use pythia to generate the
> extra jets and do not have to set the xqcut and qcut values by myself.

Yes this is what I would do.
Now you can check this effect by comparing the spectrum of the third jet when you do that,
Compare to when you ask the Z to decay in 3 jet inside MG.

> 2). I have another question about the choice of qcut value. If the
> process is

I do not have experience in that process.
I would actually set a parameter at the Pythia level to indicate that the matching/merging should start at 2 jet here.

> In this case, should I choose qcut according to htjmin( for example, from around 1/6*htjmin)?
> Or should I just use the qcut value when there's no htjmin cut?

I would first try something like 1/6*htjmin. Then depending of the DJR, I would check for other value.

Cheers,

Olivier

> On 13 Aug 2017, at 16:58, Tao Xu <email address hidden> wrote:
>
> Question #655834 on MadGraph5_aMC@NLO changed:
>
>
> Tao Xu is still having a problem:
> Hi Olivier,
>
>
> 1.）So for the hadronic Z boson decay, I would use pythia to generate the
> extra jets and do not have to set the xqcut and qcut values by myself.
>
> 2). I have another question about the choice of qcut value. If the
> process is
>
> generate p p > j j
> add process p p > j j j
> add process p p > j j j j
>
> and I still require the jets to have large pt by setting a large htjmin.
> In this case, should I choose qcut according to htjmin( for example, from around 1/6*htjmin)?
> Or should I just use the qcut value when there's no htjmin cut?
>
> Best,
> Tao Xu
>
> --