How to make a UFO model calculates the width of particle without making it decay??

Asked by Igal on 2019-09-29


This question is linked to question #682829 (

I would like the UFO model to calculate the particle width of a customized model without making this particle decay. Is there a way to do that?

Also with the following commands

process DECAY = "zp" => "mu+", "mu-"

error_threshold = 1E-8
auto_decays_multiplicity = 2
?auto_decays_radiative = false
unstable "zp" (DECAY) { ?auto_decays = true }

I have this error: FATAL ERROR: Mismatch of process and beam setup (scattering/decay)

but if unstable "zp" (DEACY) { ?auto_decays = true } is replaced by unstable "zp" (DECAY) it is working

Also to which value should be set the error_threshold?

Thanks, Igal.

Question information

English Edit question
WHIZARD Edit question
Wolfgang Kilian Edit question
Last query:
Last reply:
Juergen Reuter (j.r.reuter) said : #1

The following works for me in the Standard Model:

process foo = Z => "mu+", "mu-"

error_threshold = 1E-8
auto_decays_multiplicity = 2
?auto_decays_radiative = false
unstable Z (foo) { ?auto_decays = true }

So please check your UFO model, or try out the new version 2.8.2 of WHIZARD.
The variable error_threshold just considers all absolute values of
errors below the value given as exactly zero, so partial widths in GeV. 10^-8 GeV
tells you that errors smaller 10 eV are considered to be zero.