beam polarization of electrons and positrons

Asked by Nej on 2021-05-25

Hello,

I have a question please.
When i generate the e-e+ -> ttbar process, by default the polarization of the beam 1 (e-) and beam 2 (e+) are set to zero in the run card, it means that 50% of the electrons (positrons) are Left handed, and 50% are right-handed, which means that the beams are not polarized.
If it is correct, the problem is that when i plot the helicity histogram, i find out that the beams are polarized ! i don't understand why.. for the electron beam the number of right handed particle are more than the left handed particle, and vise versa for the positron beam.
Can you please explain to me why is that ?

Thank you,
Best regards.

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Olivier Mattelaer (olivier-mattelaer) said :
#1

Hi,

> If it is correct,

Yes.

> the problem is that when i plot the helicity histogram, i find out that the beams are polarized

Your interpretation is not correct. The cross-section is not the same for the various helicity and therefore you have an assymetry in the event file. That does not mean that your beam is polarized.

To show this I have run the following script:

> generate e+{L} e- > t t~
> output
> launch -f
> generate e+{R} e- > t t~
> output
> launch -f
> generate e+{L} e- > Z > t t~
> output
> launch -f
> generate e+{R} e- > Z > t t~
> output
> launch -f
> generate e+{L} e- > t t~ / Z
> output
> launch -f
> generate e+{R} e- > t t~ / Z
> output
> launch -f

and I get the following cross-section:
> e+{L} e- > t t~: 0.1033
> e+{R} e- > t t~ : 0.2293
> e+{L} e- > Z > t t~. : 0.029

> e+{R} e- > Z > t t~ : 0.045

> e+{L} e- > t t~ / Z : 0.1231
> e+{R} e- > t t~ / Z : 0.1231

Where you can see that the assymetry in the cross-section is related to the Z boson.

Cheers,

Olivier

> On 25 May 2021, at 09:15, Nej <email address hidden> wrote:
>
> New question #697233 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/697233
>
> Hello,
>
> I have a question please.
> When i generate the e-e+ -> ttbar process, by default the polarization of the beam 1 (e-) and beam 2 (e+) are set to zero in the run card, it means that 50% of the electrons (positrons) are Left handed, and 50% are right-handed, which means that the beams are not polarized.
> If it is correct, the problem is that when i plot the helicity histogram, i find out that the beams are polarized ! i don't understand why.. for the electron beam the number of right handed particle are more than the left handed particle, and vise versa for the positron beam.
> Can you please explain to me why is that ?
>
> Thank you,
> Best regards.
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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