UFO error with Majorana fermions

[Wojciech Kotlarski]

Dear Whizard authors,

There seems to be a problem with UFO interface in the presence of Majorana fermions. This affects Whizard version >= 2.8. To trigger it, it's enough to have an add-on model on top of the SM in FeynRules which contains only a Majorana fermion, without any interactions for this fermion. This then gives

*** ERROR: Simulate: no process has a valid matrix element.

for e+ e- > e+ e-. It looks like the sole existence of a Majorana fermions messes up all vertices.

Regards,
Wojciech

[Thorsten Ohl (thomega)]

Dear Wojciech,

I'm still debugging the support for UFO models with Majorana fermions. (I'm a bit behind our internal deadline for this, but such is life, sorry.)

In the meantime: are the vertices in your model such, that you could treat it as a Dirac fermion by adding an unphysical charge?

Cheers,
-Thorsten

[Juergen Reuter (j.r.reuter)]

Dear Wojciech,
we are still working on the Majorana support for UFO models. The complete UFO support is scheduled for v2.8.3 to be expected in a single-digit number of weeks. I agree that the error symptoms are not very clear, for the moment we should just veto this. Please give us a little more time.
Cheers,
   JRR

[Juergen Reuter (j.r.reuter)]

This bug is gone, WHIZARD with version 2.8.3 will now generate matrix elements for models with Majorana fermions. There are still some (known) bugs in the signs for Fermi statistics with quite
complicated matrix elements, but these would deserve their own dedicated bug reports, so I closed that one.

[Wojciech Kotlarski (wojciech-kotlarski)]

Dear Juergen,

Thanks a lot! Unfortunately, version 2.8.3 does not solve our problem. Also, to be clear, the problem we were facing was that the presence of Majorana fermions in the UFO model caused no matrix elements to be generated (even for processes which don't involved Majorana fermions). Maybe I could send you our UFO model for debugging?

cheers,
Wojciech

[Juergen Reuter (j.r.reuter)]

Dear Wojciech,
Daniel Jeans from KEK was faster to notice that there was some problem
that we overlooked.
The O'Mega matrix element generator for models with Majorana fermions is
working well,
but it is incorrectly steered in 2.8.3. There was too much haste for the
release because of the
pressure from the Linear Collinear Generator Group as they want to start
the full simulation
complete SM 2 ab^-1 MC production. There will be a bug fix release 2.8.4
tomorrow morning
that takes this into account.
Cheers,
     JRR

Am 07.07.20 um 23:27 schrieb Wojciech Kotlarski:
> Dear Juergen,
>
> Thanks a lot! Unfortunately, version 2.8.3 does not solve our problem.
> Also, to be clear, the problem we were facing was that the presence of
> Majorana fermions in the UFO model caused no matrix elements to be
> generated (even for processes which don't involved Majorana fermions).
> Maybe I could send you our UFO model for debugging?
>
> cheers,
> Wojciech
>

--
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---------------------------------
Juergen Reuter
DESY Theory Group, Bldg. 2a
Notkestr. 85
22607 Hamburg, GERMANY
Tel [phone]
Fax [phone]
skype: jr_reuter
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