MadGraph5_aMC@NLO

beam polarization

Asked by Ezequiel Alvarez

Hello,

the beam polarization variables in run_card.dat are useful only for e+e- collisions? Or does it also mean a polarized p in pp collision?

Thank you very much, best, Ezequiel.

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

Hi,

In the case of proton beam, the effect will apply on the Parton level (on the quark and gluon).
Indeed, I doubt that this fully make sense for the proton collider case.

Cheers,

Olivier

> On 20 Jan 2017, at 07:58, Ezequiel Alvarez <email address hidden> wrote:
>
> New question #444786 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/444786
>
> Hello,
>
> the beam polarization variables in run_card.dat are useful only for e+e- collisions? Or does it also mean a polarized p in pp collision?
>
> Thank you very much, best, Ezequiel.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Ezequiel Alvarez (sequi76) said : 		#2

Hi Olivier

thanks!

So you mean that if I generate

1) u u~ > t t~
or
2) g g > t t~

it will have some effect, but it will not if I generate

3) p p > t t~ ?

In 1 I see it reasonably to talk about L and R polarization. But in 2 would it end up meaning helicity = +/-1 ?

Or may be it does mean nothing in both 1) and 2)?

Than you very much, cheers, Ezequiel.

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

Hi,

Obviously we did not implement such option for hadron collider, so
A) we do not have validation about the physics
B) we do not have any check if such effect are meaningful.
Please do not consider this discussion as any approval of computation.
I’m just explaining how the code reacts if you want to activate such mode.
I’m not saying that this is the correct way to do (I really doubt that such behaviour can be useful for proton collider)

> 3) p p > t t~ ?

Using “p p” is just a keyword for
generate g g > t t~
add process u u~ > t t~
# plus the others.
Using p p is actually more efficient since this allows to have a better code structure of the generated code.
But in the case of the helicity the effect of switching ON those polarisation would be the same as if you were generated those one by one (either in the same or in different output directory)

> But in 2
> would it end up meaning helicity = +/-1 ?

For photon/gluon in the initial state this means circular polarisation.
(You can either look at the helas manual and/or to this recent paper on related topic: https://arxiv.org/pdf/1701.02754.pdf)

To come back to the physics, let me stress that the PDF do not include any information related to the polarisation of the beam and/or Parton.
This in itself is likely to kill completely the physical meaning of using such switch for hadron collider.

Cheers,

Olivier

> On 20 Jan 2017, at 11:22, Ezequiel Alvarez <email address hidden> wrote:
>
> Question #444786 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/444786
>
> Status: Answered => Open
>
> Ezequiel Alvarez is still having a problem:
> Hi Olivier
>
> thanks!
>
> So you mean that if I generate
>
> 1) u u~ > t t~
> or
> 2) g g > t t~
>
> it will have some effect, but it will not if I generate
>
> 3) p p > t t~ ?
>
> In 1 I see it reasonably to talk about L and R polarization. But in 2
> would it end up meaning helicity = +/-1 ?
>
> Or may be it does mean nothing in both 1) and 2)?
>
> Than you very much, cheers, Ezequiel.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Ezequiel Alvarez (sequi76) said : 		#4

Thanks Olivier Mattelaer, that solved my question.