# Calculation of multiprocesses matrix element

Good day,

Right now I'm trying to calculate the matrix element of VBF Higgs production and I don't quite understand how to combine all the channels together.

When I use the following commands
import model HC_UFO
generate p p > j j x0 \$\$ w+ w- z a QCD=0
output standalone_cpp VBFME
I got 77 different matrix elements in SubProcesses folder.
Is there some reasonable way to calculate the "combined" matrix element?
I wouldn't actually like to calculate them separately and then weight them by PDF's values myself...

Nikita

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Olivier Mattelaer
Solved:
2018-06-08
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2018-06-08
2018-06-08
 Olivier Mattelaer (olivier-mattelaer) said on 2018-06-05: #1

Hi Nikita,

You need to define what you call "combined" matrix element before I can comment.
But i guess that in any case you will need to define it yourself.

Olivier

> On 5 Jun 2018, at 12:43, Nikita <email address hidden> wrote:
>
> New question #669983 on MadGraph5_aMC@NLO:
>
> Good day,
>
> Right now I'm trying to calculate the matrix element of VBF Higgs production and I don't quite understand how to combine all the channels together.
>
> When I use the following commands
> import model HC_UFO
> generate p p > j j x0 \$\$ w+ w- z a QCD=0
> output standalone_cpp VBFME
> I got 77 different matrix elements in SubProcesses folder.
> Is there some reasonable way to calculate the "combined" matrix element?
> I wouldn't actually like to calculate them separately and then weight them by PDF's values myself...
>
> Nikita
>
> --

 Nikita (zerousik) said on 2018-06-05: #2

Hi Oliver,

By "combined" matrix element I meant the matrix element that takes into account all the VBF Higgs production diagrams, i.e. the matrix element of the p p > j j x0 process, where p and j can be any allowed quarks or gluons.

Best Regards,
Nikita

 Olivier Mattelaer (olivier-mattelaer) said on 2018-06-05: #3

Hi,

You mean just do the sum of them?

If you want to change the output to have a fully optimal way to compute such sum.
You will need to create a quite complex plugin to do that.

Cheers,

Olivier

> On 5 Jun 2018, at 17:42, Nikita <email address hidden> wrote:
>
> Question #669983 on MadGraph5_aMC@NLO changed:
>
>
> Nikita is still having a problem:
> Hi Oliver,
>
> By "combined" matrix element I meant the matrix element that takes into account all the VBF Higgs production diagrams, i.e. the matrix element of the p p > j j x0 process, where p and j can be any allowed quarks or gluons.
>
> Best Regards,
> Nikita
>
> --

 Nikita (zerousik) said on 2018-06-06: #4

Hi Oliver,

> You mean just do the sum of them?
I'm afraid it's not simply the sum, because if we take into account different subprocesses we will face with the interference effects between different diagrams while calculating the amplitude, so the resulting matrix element will have additional terms.
So, if I understood correctly, there is no "out-of-the-box" way to do that?

Best Regards,
Nikita

 Olivier Mattelaer (olivier-mattelaer) said on 2018-06-08: #5

Hi,

Which kind of interference are you talking about?
The different processes have different final state and therefore are distinguishable (at least in the S-matrix formalism)
So the interference term is zero. I obviously know some situation where this is not the case but this is typically the case when perturbation theory are not valid anymore.

> So, if I understood correctly, there is no "out-of-the-box" way to do that?

Yeah that's for sure (especially since I still do not know how you want to compute such interference)

Cheers,

Olivier

 Nikita (zerousik) said on 2018-06-08: #6

Thanks Olivier Mattelaer, that solved my question.