Conflict between factorized decay and beam setup

Asked by Xunwu Zuo

Dear Whizard experts,

I am trying to generate some e+e- -> t tbar events, at sqrt=365 GeV, with specific top decay chains.
I am running Whizard 3.1.0 (/cvmfs/ as provided by the current standard FCC-ee configuration.
An example card of mine can be found at
in which the top decay chain (L40-42) is given by the factorized decay, as shown in Section 5.8.2 of the Whizard manual,
and the beam spread and ISR settings (L47-59) are the common settings used for FCC-ee.

If I run with both factorized decay and beam spread settings, I always get this error message in the integration step.
*** FATAL ERROR: Mismatch of process and beam setup (scattering/decay)

If I remove either of the factorized decay or the beam setup (as in the example card), the generation runs without any problem.

Could you let me know why there is such conflict and how I could fix the issue?


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Juergen Reuter Edit question
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Xunwu Zuo
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Juergen Reuter (j.r.reuter) said :

Dear Xunwu,
thanks for the question. Indeed, in your SINDARIN there is a
beams = e1, E1 => gaussian => isr
statement, that provides a global scope for beams. So Whizard applies a beams framework of electron on positron also to the two decays processes for t => Wp, b and Wp => E2, n2, and then complains about a mismatch of beam statements between scattering and decay processes.
There are two options to solve this issue:
Put the definition of unstable particles
unstable t (t_dec)
unstable Wp (W_dec)
_before_ the beams statement, such that Whizard integrate the decays without any beams statement, or
if you want to keep the unstable statements where they are, define a beams statement for them in a local scope:
unstable t (t_dec) { beams = t}
unstable Wp (W_dec) { beams = Wp}
Please do not hesitate open another ticket should you have further questions.
    JRR (Juergen)

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Xunwu Zuo (xunwu-zuo) said :

Dear Juergen,

Thanks a lot for the answer! It is very clear.
Just to make sure I got it right, are these two options equivalent?

My goal is to have the beam spread and ISR on electron and positron beams of course, but not on top and W mediators.
It seems like (2) would achieve the exact purpose.
But for (1), if the decay is already integrated (process generated if I understand correctly), does the beam statement still apply to the electron beams?


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Juergen Reuter (j.r.reuter) said :

Dear Xunwu,
yes, physicswise the two solutions are equivalent, in terms of the program structure there is a small difference. In the first case, the unstable statement is before any beam statement, so it gets the default beam statement, which is trivial beam contain of the "partonic" beam particle only (t and W+ in your case). Then there is the beam statement for the scattering process. In the second case,
there is the beam statement for the scattering process and explicit, but local beam statements for the decays. The local statements do not affect the global setting for the scattering process which still contains gaussian spread and ISR. Which of the two is more digestible depends probably on the user.

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Xunwu Zuo (xunwu-zuo) said :

Got it! Thanks a lot for the clear explanation and the quick response!