Incorrect polarization vector for primary leptons

[simonthor]

The polarization vector of the primary lepton seems to be set to -1 (antiparallel to the momentum vector), regardless of the energy of the lepton (based on this code). This approximation is correct for high energies but not for lower energies, e.g., when 5 GeV tau neutrinos produce tau leptons.

It seems like there is a suggestion in a comment in the code to implement a more sophisticated calculation of the polarization. This feature would be helpful, especially as lower energy neutrinos and tau production become more important.

Are there any current plans for when this polarization is planned to be implemented?

In the mean time, it would be useful if a warning is raised or it is written explicitly in the documentation that:

• the polarization vector is always antiparallel to the momentum
• the polarization vector is in the lab frame (not in the lepton rest frame, as one might assume)
• the current approximation is correct for high energies ($E >> m_l$) but not otherwise.

Thank you for your help and consideration of this request!

[idkakorin]

Dear Simon,
Yes, we have a plan to implement polarization, but, you know, the common problem of lack of time. But since the request has been received, work in this direction will be accelerated.

[simonthor]

Thank you for the update! That is of course understandable that other features are sometimes prioritized.

[ghkareem]

Hello - just to follow along from this; can I ask if there has been any effort towards this? I notice that indeed stdheppolz outputs polarizations which seem to give a tailed distribution to the tau polarization centred at -1

Thanks for any update!

[idkakorin]

Dear Karim,

Thank you for reaching out and for your interest in our work. Since the polarization calculation feature is currently under development, and we are working with a limited amount of manpower, it has not yet been included in the official release of GENIE. However, you can expect this option to be incorporated in one of the upcoming official releases of the GENIE code.

Thanks again for your understanding and support. If you have any further questions or need additional information, please don’t hesitate to let me know.

[ghkareem]

Dear Igor, thanks for this information.
Does this mean the stdheppolz is not usable at this current stage for instance?

At least it appears to be retrievable, though I'm not certain. (I assume it to take the place of the polarisation vector in future releases, but already seems to output something that looks physical above say 10x lepton masses?)

Many thanks and best wishes, Kareem

[idkakorin]

Dear Karim,

Does this mean the stdheppolz is not usable at this current stage for instance?

In the current implementation of GENIE, all charged leptons are assumed to be massless. Negative leptons have polarization -1, and positive ones have polarization +1, relative to the direction of the lepton's motion. The stdheppolz is filled according to the above information.

Best regards,
Igor

[ghkareem]

Dear Igor thanks for your help, I greatly appreciate it,
According to my distribution I shared, there appears to be a tail - is this simply then numerical, even though the tau energy is ~100 GeV?
Many thanks and best wishes, Kareem

[idkakorin]

Dear Karim,

is this simply then numerical, even though the tau energy is ~100 GeV?

Yes, I think this is somehow related to a numerical issue. To verify this, could you plot similar distributions for an energy of, say, 5 GeV, please?

Best regards,
Igor