Photon asymmetry in SM W+gamma FxFx

Hi Benedikt,

This indeed looks very strange and cannot be correct.

A very naive question. I assume that the cards are used to generate a 'grid-pack'. And from those you submit many jobs to generate the actual events. If this is the case, might it be that something went wrong in the setting up of the grids that went unnoticed? Did you retry?

In any case, I'll try to reproduce your results locally.

Best,
Rikkert

Hi Rikkert,

That's right, events have been generated from gridpacks. However, we were carefully looking at the logs of their production and couldnt find anything strange. All seemed normal, for instance here:

https://www.dropbox.com/s/9vao8uhmwyp4rs5/WAToLNuA01j_5f_pta130_NLO_FXFX.txt?dl=0

Thanks a lot for looking into it!

Best,
Benedikt

(and we generated the gridpacks with different cuts on the photon, like 130 or 500 GeV, and also generated the 1j component alone. In all cases everything looked good, and all show this asymmetry)

Hi Benedikt,

I've been able to reproduce your results. Indeed, this very much looks like a (serious?) bug to me. I'll keep investigating.

Best,
Rikkert

Hi Benedikt,

I finally found the problem. It's there only for processes that have diagrams with 1->3 decays (i.e., in your case where the electron radiates the photon).
We have just frozen the a version to be released in the next few days (which will be 2.5.5). I don't think the fix for this bug can still be part of it, and therefore I think it will only be part of the 2.5.6. Let me know if you want me to send you a patch by e-mail.

Best,
Rikkert

Hi Rikkert,

That's very nice to hear!! Yes if you could send me the patch (<email address hidden>), we could implement it in the MG version in our CMS repository.

Best,
Benedikt

Hi Rikkert,

I work with Benedikt and asked him to ask you about the problem. Thank you very much for looking into it!
I have a quick follow-up question to understand which data sets we have generated should be invalidated: When you say 1->3, that is then not limited to W->l nu gamma but can also be Z->l l gamma with an ~on-shell Z, correct? Also, is it any 1->3 decay, or does the 1 have to be a W or Z?

Thank you,
Yutaro

Hello Yutaro,

In principle, any 1->3 decay could be affected. In practice, the problem should be most severe for the W boson, since the possible interactions it can have with quarks are more restricted than the Z boson, since it's flavour changing. I think it would be better to also re-generate the l+l-+gamma sample, since the problem might also be there.

B.t.w., the release of 2.5.5 has been delayed by a couple of days, which means that the fix will still be part of that release. It should become available in the next couple of days if all the final tests go smooth.

Best,
Rikkert

Hi Rikkert,

Thank you for the clarification. And that's good (?) news about 2.5.5.

Best,
Yutaro