Vector Boson Fusion

Dear Matthew,

I think there is no exact way to do that with MG5 in the current version. What you can do is to use the $ syntax that
will take out resonance contribution during the running time (not diagrams). So this will effectively remove the case
where let's say pp > W+ W- j j, and the two jets come from a resonant W.

However, at the end the same can be achieved by asking the VBF cuts on the m(j,j)> 400 GeV or something like that
in a equivalent way. So there is no need to use the $ even in this case.

We don't have any other way just because it's not safe from the gauge invariance point of view.

Finally, please note that by asking W+, W- explicitly instead of l vl~ you will miss contributions where there are
no resonant W's, which are also possible.

Cheers

Fabio

Hi Fabio,

Thanks for the useful and timely response. Actually I had intended to use "generate p p > .... , w+ > l+ vl~" in any case, though I suppose that doing this will mean that using the $ syntax would also remove the resonance contribution from w+ > l+ vl~ also. I believe making the appropriate kinematic cuts as you suggested will do the trick for me.

Thanks,
- Matthew

Hello Matthew,

In MadGraph 5, the decay chain syntax means that whatever you specify for one part of the process has no impact on the other parts. So you can certainly do
generate p p > j j w+ w- $ w+ w- z, w+ > l+ vl, w- > l- vl~
and the "$" will affect only the core process, not the decays (it would have been quite useless if it did).

Cheers,
Johan

Hi,

Somewhat similar question to the VBF process, here i am concerning the perturbative unitarity of the vector boson fusions. Say for W+ W- -> W+ W- processes, there are contact interaction, the s/t channel Z-exchanging term, and the s-channel Higgs-exchanging term. May I know if there is any way to generate all these sub-processes together as a whole for the vector boson fusion process? thanks a lot.

ning

Hi Ning,

Do I understand correctly your question, if I rephrase it this way:
Is it possible from a VBF process:
(i.e. generate p p > j j w+ w- QCD=0)
to have access to the w+ w- > w+ w- sub-amplitude.
The first point that I don't know is how you define the sub-amplitude since the w from the T-channel are off-shell.

We don't have anything automatic for that. But this should be pretty easy (if you know the madgraph way to compute a matrix element) to do with a dedicated code hacking the code created by MG5.

So you need first to
1) generate all the diagrams:
2) create the output type that you want to modify
3) set to zero all the amplitude associate to diagrams which contains a sub-amplitude w+ w- > w+ w- but which are not a 4 point interaction. (AMP(i) = 0d0, instead of the call which computes it). Note that if you want to make the PS integration, you need to keep AMP2(i) different of zero
4) modify the helas routine(s) associate to the 4 point interaction and re-introduce the contribution of the three point interactions inside it
5) use that HELAS routine to define what you call sub-amplitude.

Cheers,

Olivier

Dear Olivier,

Sorry for some confusions in the beginning. Actually what I started was just typing "generate p p > w+ w- j j QCD=0", which leads to many diagrams including those I would expect for the vector boson fusion process. Then I typed "output vbf" to create a directory and typed "./bin/madevent" to launch a run to the parton level inside the "vbf" directory.

Afterwards, I was wondering where can I set the unwanted amplitudes to zero, i.e. to set the AMP(i)=0d0 as you mentioned before. Thank you very much for your help.

best,

ning

在 2013-11-20，下午10:36， Olivier Mattelaer 写道：

> Question #181720 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/181720
>
> Olivier Mattelaer proposed the following answer:
> Hi Ning,
>
> Do I understand correctly your question, if I rephrase it this way:
> Is it possible from a VBF process:
> (i.e. generate p p > j j w+ w- QCD=0)
> to have access to the w+ w- > w+ w- sub-amplitude.
> The first point that I don't know is how you define the sub-amplitude since the w from the T-channel are off-shell.
>
> We don't have anything automatic for that. But this should be pretty
> easy (if you know the madgraph way to compute a matrix element) to do
> with a dedicated code hacking the code created by MG5.
>
> So you need first to
> 1) generate all the diagrams:
> 2) create the output type that you want to modify
> 3) set to zero all the amplitude associate to diagrams which contains a sub-amplitude w+ w- > w+ w- but which are not a 4 point interaction. (AMP(i) = 0d0, instead of the call which computes it). Note that if you want to make the PS integration, you need to keep AMP2(i) different of zero
> 4) modify the helas routine(s) associate to the 4 point interaction and re-introduce the contribution of the three point interactions inside it
> 5) use that HELAS routine to define what you call sub-amplitude.
>
> Cheers,
>
> Olivier
>
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