MadGraph5_aMC@NLO

how to get a VBF result

Asked by Li

Hi,
  I have a problem that , I write a model add a particle sgoldstino, if you do not know this , you can just imagine this is a scalar like higgs but has a coupling to photon ,w boson , z boson while higgs dont couple to the photon .

  Then I want to generate the VBF process, using : generate u u~ > sphi u u~ /sigmas z g w+ w- $a QED=2 HIQ=0 QCD=0

  the result is that there are 2 diagrams , one is the VBF that u give a photon and ubar give a photon , the other one is that u u~ annihilate to a photon and this off shell photon decays to a sphi and a off shell photon and the second photon continue to decay to u ubar.

  I this that the second diagram contains a s channel photon so it should be disappear since I use $a but it didn't

  so how can I just produce a pure VBF process ?

  I didn't find a way to upload a diagram or something , if you need , I can upload the feynman diagram.

Best,
Li

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Olivier Mattelaer
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Revision history for this message
Li (huangli-itp) said : 		#1

Hi,
  I find that it is the same to the sm model , in that case I use : generate u u~ > u u~ h $z QCD=0 QED=4 and there are two diagrams one is VBF and another one is u ubar fusion . Maybe you can try this and tell me how to get rid of the u ubar fusion diagram.

Best,
Li

Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said : 		#2

Hi,

The $ syntax do not remove the diagram but apply on those a cut forcing the associate particles to be off-shell.
This is equivalent to an on-shell subtraction.

To remove the s-channel diagram the syntax is $$.
But this syntax has not guarantee to be meaningful (i.e. that syntax can break gauge and lorentz invariance.)
So please check carefully that you remove a gauge independent subset of Feynman diagram.

Cheers,

Olivier

> On Dec 31, 2015, at 07:13, Li <email address hidden> wrote:
>
> New question #280334 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/280334
>
> Hi,
> I have a problem that , I write a model add a particle sgoldstino, if you do not know this , you can just imagine this is a scalar like higgs but has a coupling to photon ,w boson , z boson while higgs dont couple to the photon .
>
> Then I want to generate the VBF process, using : generate u u~ > sphi u u~ /sigmas z g w+ w- $a QED=2 HIQ=0 QCD=0
>
> the result is that there are 2 diagrams , one is the VBF that u give a photon and ubar give a photon , the other one is that u u~ annihilate to a photon and this off shell photon decays to a sphi and a off shell photon and the second photon continue to decay to u ubar.
>
> I this that the second diagram contains a s channel photon so it should be disappear since I use $a but it didn't
>
> so how can I just produce a pure VBF process ?
>
> I didn't find a way to upload a diagram or something , if you need , I can upload the feynman diagram.
>
> Best,
> Li
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Li (huangli-itp) said : 		#3

Thanks Olivier Mattelaer, that solved my question.

Revision history for this message
Li (huangli-itp) said : 		#4

Hi,
  Sorry for I can not reply quick since there is a new year festival after a busy weak.

  I finally find a way that first use $$ then use / , and it works. When I first use / then $$ , for example, generate u u~ > sphi u u~ /sigmas z g w+ w- $$a QED=2 HIQ=0 QCD=0 , there is a bug said no particle $a , and the command generate u u~ > sphi u u~ $$a /sigmas z g w+ w- QED=2 HIQ=0 QCD=0 works.

  And, thanks for telling me the difference of these two command !

Best,
Li

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

Hi,

They are no bug in $a
Since the a has a zero mass and zero width this syntax do not mean anything (how can you have an on shell photon?)
Being meaningless this syntax should actually never be used.

Cheers,

Olivier

> On Jan 2, 2016, at 08:52, Li <email address hidden> wrote:
>
> Question #280334 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/280334
>
> Li posted a new comment:
> Hi,
> Sorry for I can not reply quick since there is a new year festival after a busy weak.
>
> I finally find a way that first use $$ then use / , and it works. When
> I first use / then $$ , for example, generate u u~ > sphi u u~ /sigmas z
> g w+ w- $$a QED=2 HIQ=0 QCD=0 , there is a bug said no particle $a ,
> and the command generate u u~ > sphi u u~ $$a /sigmas z g w+ w- QED=2
> HIQ=0 QCD=0 works.
>
> And, thanks for telling me the difference of these two command !
>
> Best,
> Li
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.