ISR Z jet

Asked by Amin Aboubrahim on 2018-05-30

Hi Olivier,

Is it possible to add an ISR Z jet to an incoming quark line?

Thank you,
Amin

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2018-06-06
Last query:
2018-06-06
Last reply:
2018-06-06

This question was reopened

Hi,

So you want to have the Z coming only for ISR? (and not from FSR)
Obviously this is not always gauge invariant but I assume that it is in your case.

We do not have any specific syntax for that, but this should be possible to do via the diagram filtering method.
( you basically have to look at the propagator associated to the Z and check that his ID is strictly lower than 3.)

Cheers,

Olivier

> On 30 May 2018, at 21:37, Amin Aboubrahim <email address hidden> wrote:
>
> New question #669789 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/669789
>
> Hi Olivier,
>
> Is it possible to add an ISR Z jet to an incoming quark line?
>
> Thank you,
> Amin
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Amin Aboubrahim (amin83) said : #2

Hi Olivier,

I am not familiar with this method. Is it the one explained here?

https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/FAQ-General-15

Thank you,
Amin

yes indeed

Olivier

> On 31 May 2018, at 01:42, Amin Aboubrahim <email address hidden> wrote:
>
> Question #669789 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/669789
>
> Status: Answered => Open
>
> Amin Aboubrahim is still having a problem:
> Hi Olivier,
>
> I am not familiar with this method. Is it the one explained here?
>
> https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/FAQ-General-15
>
> Thank you,
> Amin
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Amin Aboubrahim (amin83) said : #4

Thanks Olivier Mattelaer, that solved my question.

Amin Aboubrahim (amin83) said : #5

Hi Olivier,

Sorry for opening this again. I have been looking into this and I'm not sure that would be helpful in my case.
I am trying to consider the rare process of electroweak radiation, where a Z boson is radiated off an initial state quark. The filtering method assumes that this diagram exists and we are simply selecting it out and removing other diagrams.
When simulating a hard process, do you have electroweak radiation simulated as well, by default? For ISR and FSR, I have a switch which I use in Pythia8. Does a similar switch exist for electroweak radiation?

Thank you,
Amin

Hi,

I have (still now) no clue of what you try to achieve, what you describe does not really mean anything to me.
So my answer assume the following:
1) You want to do a LO computation
2) From this LO computation you want to have a Z coming from the initial state.

In that case, you have to explicitely have a Z in the final state definition of the process and then filtering this Z
to ensure that he is not radiate from the final state.

Otherwise our computation is inclusive and therefore include any additional radiation.
But such radiation will not be seen anywhere as long as you do not use a parton shower to pass from inclusive to exclusive computation.

If you want to forbid some radiation to be include in our framework you then need to include your Sudakov form-factor at some point of the computation. This is typically done via vetoing event within the parton-shower.
But can also be done via the bias method to multiply the matrix-element that MG5aMC integrates.

Cheers,

Olivier

> On 5 Jun 2018, at 07:02, Amin Aboubrahim <email address hidden> wrote:
>
> Question #669789 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/669789
>
> Status: Solved => Open
>
> Amin Aboubrahim is still having a problem:
> Hi Olivier,
>
> Sorry for opening this again. I have been looking into this and I'm not sure that would be helpful in my case.
> I am trying to consider the rare process of electroweak radiation, where a Z boson is radiated off an initial state quark. The filtering method assumes that this diagram exists and we are simply selecting it out and removing other diagrams.
> When simulating a hard process, do you have electroweak radiation simulated as well, by default? For ISR and FSR, I have a switch which I use in Pythia8. Does a similar switch exist for electroweak radiation?
>
> Thank you,
> Amin
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Amin Aboubrahim (amin83) said : #7

Hi Olivier,

Sorry for not explaining it properly but what you listed as your assumptions is what I am looking for.
Maybe if I tell you the process it will be clearer: I am looking at charged Higgs production is association with a top. The final states have a very similar topology to ttbar background and is very hard to discriminate the signal against background. What I want to do (an idea I got from a colleague at CERN which they use in their analyses) is have a Z radiated from the initial state which would then decay to a dilepton pair after which I can apply a cut on the dilepton mass to reduce ttbar background.
What I understood from your explanation is that I have to add a Z in the final state (generate p p > t~ H- Z) then filter it out since it may be coming from ISR or FSR. This leaves the Z coming from initial state. Then I pass the sample to Pythia for showering and use instructions as listed here:
http://home.thep.lu.se/Pythia/pythia82html/WeakShowers.html

I hope what I am trying to do is clear to you now.

Thanks a lot,
Amin

Hi,

Maybe I'm stupid but I'm still very confuse about what you try to do. I'll put here my concern.
But feel free to ignore them obviously if you know the answer to those (maybe stupid) question.
If you do and do not mind to teach me physics please go ahead.

 I am looking at charged Higgs production is association with a top.

How do you preserve gauge invariance when you force the Z to come from ISR?
You should have a quark line connecting initial state and final state right?
In that case separating ISR and FSR are not gauge invariant ( which in our context will also lead to non lorentz invariant result, i.e. bullshit result)

What I understood from your explanation is that I have to add a Z in the final state (generate p p > t~ H- Z) then filter it out since it may be coming from ISR or FSR. This leaves the Z coming from initial state. Then I pass the sample to Pythia for showering and use instructions as listed here:
http://home.thep.lu.se/Pythia/pythia82html/WeakShowers.html

1) I would keep all Z (both from initial and final state) since
   a) how do you distinguish them experimentally?
   b) ignoring FSR will break gauge invariance (but if I'm wrong)
2) Why do you need weak shower? You already have your Z in the final state.

Cheers,

Olivier

On 5 Jun 2018, at 16:27, Amin Aboubrahim <<email address hidden><mailto:<email address hidden>>> wrote:

Question #669789 on MadGraph5_aMC@NLO changed:
https://answers.launchpad.net/mg5amcnlo/+question/669789

   Status: Answered => Open

Amin Aboubrahim is still having a problem:
Hi Olivier,

Sorry for not explaining it properly but what you listed as your assumptions is what I am looking for.
Maybe if I tell you the process it will be clearer: I am looking at charged Higgs production is association with a top. The final states have a very similar topology to ttbar background and is very hard to discriminate the signal against background. What I want to do (an idea I got from a colleague at CERN which they use in their analyses) is have a Z radiated from the initial state which would then decay to a dilepton pair after which I can apply a cut on the dilepton mass to reduce ttbar background.
What I understood from your explanation is that I have to add a Z in the final state (generate p p > t~ H- Z) then filter it out since it may be coming from ISR or FSR. This leaves the Z coming from initial state. Then I pass the sample to Pythia for showering and use instructions as listed here:
http://home.thep.lu.se/Pythia/pythia82html/WeakShowers.html

I hope what I am trying to do is clear to you now.

Thanks a lot,
Amin

--
You received this question notification because you are an answer
contact for MadGraph5_aMC@NLO.

Amin Aboubrahim (amin83) said : #9

Hi Olivier,

There must be a misunderstanding here. Also, I cannot teach you physics, not my intention. You definitely know more than I do.
Here's my understanding of electroweak radiation:
For high enough pT of the incoming quark, the electroweak Sudakov factor should be high enough to cause electroweak radiation with a W or Z emitted from a light quark just like EM and gluon emissions. Am I right?
So I don't understand why having a Z boson emitted in such a process would break gauge invariance. A qqZ vertex is perfectly allowed. Unless you mean that this is part of the parton shower and not the hard process, i.e. not a Madgraph thing?

Thanks again,
Amin

Hi,

> Also, I cannot teach you physics, not my intention. You definitely know more than I do.

Maybe I do (or not) does not really matter.

> For high enough pT of the incoming quark, the electroweak Sudakov factor should be high enough to cause electroweak radiation with a W or Z emitted from a light quark just like EM and gluon emissions. Am I right?

The Sudakov factor being the probability of NO emission, What you say does not make sense.
(if that probablity is high, then you should not have radiation)

> So I don't understand why having a Z boson emitted in such a process would break gauge invariance. A qqZ vertex is perfectly allowed.

A qqZ is perfectly allowed both in final state and in final state. The problem is that your quark line should connect your initial state from your final state and therefore the choice of gauge can change the
relative importance of the ISR and FSR. (i.e. this is not a physical quantity in this case).

I'm not fully sure how Parton-Shower handles this. But they for sure use a lot of approximation.
Like the collinear approximation which allow them to assume that each radiation factorizes.
Already for QCD, Parton-Shower have to include angular ordering to restore some interference.
I'm not an expert in QED shower to know what they exactly include (if how when they do).

Cheers,

Olivier

> Unless you mean that this is part of the parton shower and not the hard process, i.e. not a Madgraph thing?

> On 6 Jun 2018, at 00:13, Amin Aboubrahim <email address hidden> wrote:
>
> Question #669789 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/669789
>
> Status: Answered => Open
>
> Amin Aboubrahim is still having a problem:
> Hi Olivier,
>
> There must be a misunderstanding here. Also, I cannot teach you physics, not my intention. You definitely know more than I do.
> Here's my understanding of electroweak radiation:
> For high enough pT of the incoming quark, the electroweak Sudakov factor should be high enough to cause electroweak radiation with a W or Z emitted from a light quark just like EM and gluon emissions. Am I right?
> So I don't understand why having a Z boson emitted in such a process would break gauge invariance. A qqZ vertex is perfectly allowed. Unless you mean that this is part of the parton shower and not the hard process, i.e. not a Madgraph thing?
>
> Thanks again,
> Amin
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Amin Aboubrahim (amin83) said : #11

Hi Olivier,

> The Sudakov factor being the probability of NO emission, What you say does not make sense.
(if that probablity is high, then you should not have radiation)

Sorry, I misspoke here. I meant the opposite.

I understand what you mean now. Thanks for the clarification and patience.

Best wishes,
Amin

Amin Aboubrahim (amin83) said : #12

Thanks Olivier Mattelaer, that solved my question.