MadGraph5_aMC@NLO

Polarised WW scattering at NLO

Asked by Ibano Heyth

Dear MG authors,
I'm trying to study polarised WW scattering in SM with/out complex-mass scheme. I succeeded to do it at LO and now trying at NLO.
I've tried:
                 import model loop_qcd_qed_sm
                generate w+{0} w-{0} > w+{0} w-{0} [virt=QCD QED]
and got the error:
              InvalidCmd : Polarization restriction can not be used for NLO processes
Is it possible in the current version to perform such processes ? any hints ?

Thanks,
Ibano

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Olivier Mattelaer
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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

What you ask is not NLO since you only ask for the loop.
Standalone mode for loop might actually be working but I have not checked it.
If you are brave enough you can comment the line responsible for the crash and check if the output makes sense or not.

Cheers,

Olivier

> On 1 Oct 2020, at 14:30, Ibano Heyth <email address hidden> wrote:
>
> New question #693193 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/693193
>
> Dear MG authors,
> I'm trying to study polarised WW scattering in SM with/out complex-mass scheme. I succeeded to do it at LO and now trying at NLO.
> I've tried:
> import model loop_qcd_qed_sm
> generate w+{0} w-{0} > w+{0} w-{0} [virt=QCD QED]
> and got the error:
> InvalidCmd : Polarization restriction can not be used for NLO processes
> Is it possible in the current version to perform such processes ? any hints ?
>
> Thanks,
> Ibano
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Ibano Heyth (heyth) said : 		#2

Thanks Olivier for the quick reply.

Just few things for more clarity.

When you say "standalone mode for loop" you mean "MadLoop", right? By standalone, do you mean I should try it outside the MG5 environment ?

What I'm actually looking for is the full NLO result for the process mentioned above. Perhaps I used the wrong syntax. Should it be like that
           import model loop_qcd_qed_sm
          generate w+{0} w-{0} > w+{0} w-{0} [QCD]
instead ? Would this include both qcd and qed loop diagrams ?

Thanks

Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said : 		#3

Yes I mean MadLoop.
This is a code that can evaluate the matrix-element for one phase-point and that's it.
The syntax that you use [virt=QCD QED] is a madloop syntax in the sense that it will produce such code.

So the syntax that you are technically looking for is
generate w+{0} w-{0} > w+{0} w-{0} [QCD QED]

Now this syntax is not going to work for many reason:
1) polarization are not supported at NLO in QCD (this can be possible in the future but for the moment we failed the implementation of such feature)
2) polarization are not supported at NLO EW ( this I do not even dream to support it --if this is only possible--)
3) for NLO-EW computation, I do not think that you can assign a "W" in the initial state (like it does not make sense to ask u u~ > t t~ at NLO in QCD)

So such computation has to stay at LO (and I doubt that they are ANY code able to do such computation at NLO-EW in a consistent way)

Cheers,

Olivier

Revision history for this message
Ibano Heyth (heyth) said : 		#5

Thanks Olivier Mattelaer, that solved my question.