Lepton/Anti-lepton energy asymmetry

Asked by David Vannerom on 2021-02-04

I am producing events using the model given in this paper (https://arxiv.org/abs/2007.03701, see "Ancillary files"). I generate sterile neutrino (n4) decays as:

define v = v1 v2 v3
generate n4 > v e+ e- QED=2 / w+ w- h

In the parameter card, I turn off the couplings of the sterile to the electron and muon flavours (thetae and thetamu = 0) for reasons that make sense in my analysis.

My problem/question concerns the energy distribution of the electron and the positron: they are slightly different, the spectrum of the electron is harder. The leptons are produced in pair in the decay of the Z boson. I know about the forward-backward asymmetry in the way the Z boson treats leptons and anti-leptons, but I have never heard of something similar in the energy distribution. I want to check with you that this asymmetry does not come from a wrong configuration or used of the tool.

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Last query:
2021-02-04
Last reply:
2021-02-04

Hi,

> I am producing events using the model given in this paper (https://arxiv.org/abs/2007.03701, see "Ancillary files").

This does not include any UFO model. I do not have mathematica so I can not generate such model.
In any case, I doubt that I can safely reproduce your issue with that few information.

> My problem/question concerns the energy distribution of the electron and the positron: they are slightly different, the spectrum of the electron is harder. The leptons are produced in pair in the decay of the Z boson. I know about the forward-backward asymmetry in the way the Z boson treats leptons and anti-leptons, but I have never heard of something similar in the energy distribution.

Difficult to comment without saying the distribution that you are comparing.
The first question is how many event did you generate? (i.e. are you sure that this is significant)
Second question: Are you not breaking lorentz/gauge invariance by removing particles (if you do then you can obviously have a lot of funny behavior)
Third question, can this be a spin-correlation effect? (I guess n4 is a scalar and v are all vector, in that case it could be).
Fourth question, Is foward-backward assymetry an observable?

Cheers,

Olivier

> On 4 Feb 2021, at 12:50, David Vannerom <email address hidden> wrote:
>
> New question #695368 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/695368
>
> I am producing events using the model given in this paper (https://arxiv.org/abs/2007.03701, see "Ancillary files"). I generate sterile neutrino (n4) decays as:
>
> define v = v1 v2 v3
> generate n4 > v e+ e- QED=2 / w+ w- h
>
> In the parameter card, I turn off the couplings of the sterile to the electron and muon flavours (thetae and thetamu = 0) for reasons that make sense in my analysis.
>
> My problem/question concerns the energy distribution of the electron and the positron: they are slightly different, the spectrum of the electron is harder. The leptons are produced in pair in the decay of the Z boson. I know about the forward-backward asymmetry in the way the Z boson treats leptons and anti-leptons, but I have never heard of something similar in the energy distribution. I want to check with you that this asymmetry does not come from a wrong configuration or used of the tool.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

David Vannerom (dvannerom) said : #2

Hello Olivier,

Is there a way for me to share the UFO model with you?

In any case, let me answer your questions:

- I generate 1 million events, the effect is clear.
- Removing the diagrams involving W bosons is actually redundant with requiring that the sterile only couple to nutaus, and the Higgs contribution is negligible. I tried withall bosons and got the same result. I also tried leaving the couplings to default (equal coupling to all flavours) and I actually see a stronger effect! I may have to bring this to the authors actually, they may be best placed to answer this question.
- The spin-correlation lead is interesting, I'll try to look into that!
- I believe you can indeed observe the AFB.

Cheers,
David

You can create a bug report and attach file for that.

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