MadGraph5_aMC@NLO

Frame for the definition of helicity

Asked by Stephen Mrenna

I have a situation where someone provides Pythia(8) with an LHE file (generated by MG) containing undecayed W-bosons with specific polarizations. I then want to decay the W bosons with an angular distribution according to the helicity:
(1-cosT)^2, (1+cosT)^2, sinT^2. T is theta*, the azimuthal angle for the decay products.

I would like to clarify the frame of reference for polarization in MG. Particularly:
1. Is the helicity defined in the CMS frame of the partonic process?
2. Is the appropriate z-axis for the azimuthal angle the opposite to the direction of motion of the CMS frame in the rest frame of the W-boson?

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi Stephen,

> 1. Is the helicity defined in the CMS frame of the partonic process?

Yes it is.

> 2. Is the appropriate z-axis for the azimuthal angle the opposite to the direction of motion of the CMS frame in the rest frame of the W-boson?

Here I’m not 100% sure but it should be correct.

Cheers,

Olivier

On 11 May 2015, at 22:41, Stephen Mrenna <email address hidden> wrote:

> New question #266803 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/266803
>
> I have a situation where someone provides Pythia(8) with an LHE file (generated by MG) containing undecayed W-bosons with specific polarizations. I then want to decay the W bosons with an angular distribution according to the helicity:
> (1-cosT)^2, (1+cosT)^2, sinT^2. T is theta*, the azimuthal angle for the decay products.
>
> I would like to clarify the frame of reference for polarization in MG. Particularly:
> 1. Is the helicity defined in the CMS frame of the partonic process?
> 2. Is the appropriate z-axis for the azimuthal angle the opposite to the direction of motion of the CMS frame in the rest frame of the W-boson?
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Stephen Mrenna (stephen-mrenna) said : 		#2

For clarity, my original question should have included an example.

Assume the process is g g -> H g, and H -> W W.

Then, the helicity in the LHE file is defined in the (gg -> H g) rest-frame
and NOT the H rest frame, correct?

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

Yes it is correct.

Olivier
On 12 May 2015, at 15:51, Stephen Mrenna <email address hidden> wrote:

> Question #266803 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/266803
>
> Status: Answered => Open
>
> Stephen Mrenna is still having a problem:
> For clarity, my original question should have included an example.
>
> Assume the process is g g -> H g, and H -> W W.
>
> Then, the helicity in the LHE file is defined in the (gg -> H g) rest-frame
> and NOT the H rest frame, correct?
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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