Offshell particle in event record

Asked by Junho Choi on 2018-06-27

Hello, I'm a cms guy doing analysis related to DrellYan process.

So, I generate events with command such as

generate p p > l+ l-

For onshell Z process, the event record contains Z boson and the record is like

q qbar > Z > l+ l-

.

But if Z OR gamma boson is offshell, event record doesn't store the Z*, so that the record is like

q qbar > l+ l-

which offshell Z is missing.

Actually I need intermediate boson even if the boson is offshell because I need to use PHOTOS++ simulator, one of the photon shower simulators.
PHOTOS++ needs intermediate Z boson because it finds a proper vertex such as Z > l+ l-. PHOTOS++ doesn't consider q qbar > l+ l- as a proper vertex. So, There's no photon radiation for offshell DY events when I plugin PHOTOS++ to MG5 sample.

I contacted PHOTOS++ authors and they said that user should add the missing intermediate boson, Z*.

Is there any way to include offshell particle in MG5"(without expanding decay width to force Z boson onshell. It may make the events unphysical.) As far as I know, other event generators contain intermediate particles even they are offshell.(MG4 as well)

Thank you for reading! If you have anything hard to understand, please let me know.

Best regards,

Junho Choi

Question information

Language:
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Status:
Answered
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MadGraph5_aMC@NLO Edit question
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Last query:
2018-06-28
Last reply:
2018-06-28

hi,

You can set bwcutoff (in the run_card) to very large number to always have the Z written in the lhe file.

> Is there any way to include offshell particle in MG5"(without expanding decay width to force Z boson onshell. It may make the events unphysical.) As far as I know, other event generators contain intermediate particles even they are offshell.(MG4 as well)

That's not true MG4 and MG5 have the exact same behaviour on that point.

Cheers,

Olivier

> On 27 Jun 2018, at 13:57, Junho Choi <email address hidden> wrote:
>
> New question #670506 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/670506
>
> Hello, I'm a cms guy doing analysis related to DrellYan process.
>
> So, I generate events with command such as
>
> generate p p > l+ l-
>
>
> For onshell Z process, the event record contains Z boson and the record is like
>
> q qbar > Z > l+ l-
>
> .
>
> But if Z OR gamma boson is offshell, event record doesn't store the Z*, so that the record is like
>
> q qbar > l+ l-
>
> which offshell Z is missing.
>
> Actually I need intermediate boson even if the boson is offshell because I need to use PHOTOS++ simulator, one of the photon shower simulators.
> PHOTOS++ needs intermediate Z boson because it finds a proper vertex such as Z > l+ l-. PHOTOS++ doesn't consider q qbar > l+ l- as a proper vertex. So, There's no photon radiation for offshell DY events when I plugin PHOTOS++ to MG5 sample.
>
> I contacted PHOTOS++ authors and they said that user should add the missing intermediate boson, Z*.
>
> Is there any way to include offshell particle in MG5"(without expanding decay width to force Z boson onshell. It may make the events unphysical.) As far as I know, other event generators contain intermediate particles even they are offshell.(MG4 as well)
>
>
> Thank you for reading! If you have anything hard to understand, please let me know.
>
>
> Best regards,
>
> Junho Choi
>
>
>
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Junho Choi (junhochoi) said : #2

From the explanation at the link below :

https://answers.launchpad.net/mg5amcnlo/+faq/2014

I can understand the role of the "bwcutoff" parameter.

So, DrellYan process which is generated by

 p p > l+ l-

has no change even if I set bwcutoff very large?

I mean, there's no difference between when bwcutoff is large and when bwcutoff is small?(because it's not a chain process)

I can check it one by one I want to make it sure.

Thank you for your answer!

Best,

Junho Choi

Junho Choi (junhochoi) said : #3

I checked the events with bwcutoff = 15(default), 13000, 99999999999999999.

(with
generate p p > e+ e-,
mmll = 200 which menas M(ll) > 200 GeV)

I fount Z boson with M(Z) > 200 GeV in event records with large values of bwcutoff(= 13000, 99999999999999999).

But I need all intermediate DrellYan(DY) boson, which means virtual gamma particles be included as well.

With large bwcutoff, there are 4538 events with Z boson. The other events(maybe q qbar > gamma* > e+ e- ?) doesn't have intermediate DY boson. So, the event records are like

q qbar > e+e-

NOT q qbar > gamma > e+ e-.

Is there other ways to store virtual gammas in MG5 events?

Thank you for reading .

Best regards,

Junho Choi

Hi,

Including systematically the photon is not supported since this is a massless particles.
For your case of interest, you will need to write a post-procesing script to add manually the detail of the photon for the events who do not have any s-channel explicitly.

If you like python, you can use our lhe_parser script for that:
madgraph/various/lhe_parser.py

Cheers,

Olivier

> On 28 Jun 2018, at 06:09, Junho Choi <email address hidden> wrote:
>
> Question #670506 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/670506
>
> Junho Choi gave more information on the question:
> I checked the events with bwcutoff = 15(default), 13000,
> 99999999999999999.
>
> (with
> generate p p > e+ e-,
> mmll = 200 which menas M(ll) > 200 GeV)
>
> I fount Z boson with M(Z) > 200 GeV in event records with large values
> of bwcutoff(= 13000, 99999999999999999).
>
> But I need all intermediate DrellYan(DY) boson, which means virtual
> gamma particles be included as well.
>
> With large bwcutoff, there are 4538 events with Z boson. The other
> events(maybe q qbar > gamma* > e+ e- ?) doesn't have intermediate DY
> boson. So, the event records are like
>
> q qbar > e+e-
>
> NOT q qbar > gamma > e+ e-.
>
>
> Is there other ways to store virtual gammas in MG5 events?
>
>
> Thank you for reading .
>
>
> Best regards,
>
> Junho Choi
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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