WHIZARD

Vanishing cross section for specific parameter point in BSM model

Asked by Sheikh Farah Tabira

Hello,

I was checking the cross section for the process e+e- -> ZXX for a range of energies from 800-1500 gev. Only for 900, did whizard throw the warning: Intermediate decay of zero-width particle Ah2 may be possible, and no output was shown. I was wondering how to work around it. Thanks in advance!

 Time estimate for generating 10000 events: 0d:00h:00m:01s
| Restoring model 'br25'
sqrts = 9.000000000000E+02
| RNG: Initializing TAO random-number generator
| RNG: Setting seed for random-number generator to 17904
| Initializing integration for process eexx:
| Beam structure: e1, e1bar
| polarization (beam 1):
| @(-1: -1: ( 1.000000000000E+00, 0.000000000000E+00))
| polarization (beam 2):
| @(-1: -1: ( 1.000000000000E+00, 0.000000000000E+00))
| polarization degree = 0.8000000, 0.2000000
| Beam data (collision):
| e1 (mass = 5.1100000E-04 GeV) polarized
| e1bar (mass = 5.1100000E-04 GeV) polarized
| sqrts = 9.000000000000E+02 GeV
| Phase space: discarding old configuration (configuration parameters changed)
| Phase space: generating configuration ...
Warning: Intermediate decay of zero-width particle Ah2 may be possible.
| Phase space: ... success.
| Phase space: writing configuration file 'eexx.i1.phs'
| ------------------------------------------------------------------------
| Process [scattering]: 'eexx'
| Library name = 'default_lib'
| Process index = 1
| Process components:
| 1: 'eexx_i1': e1, e1bar => Z, a, a [omega]
| ------------------------------------------------------------------------
| Phase space: 5 channels, 5 dimensions
| Phase space: found 5 channels, collected in 3 groves.
| Phase space: Using 18 equivalences between channels.
| Phase space: wood
Warning: No cuts have been defined.
| Starting integration for process 'eexx'
| Integrate: iterations not specified, using default
| Integrate: iterations = 5:5000:"gw", 3:10000:""
| Integrator: 3 chains, 5 channels, 5 dimensions
| Integrator: Using VAMP channel equivalences
| Integrator: 5000 initial calls, 20 bins, stratified = T
| Integrator: VAMP
|=============================================================================|
| It Calls Integral[fb] Error[fb] Err[%] Acc Eff[%] Chi2 N[It] |
|=============================================================================|
| VAMP: parameter mismatch, discarding grid file 'eexx.m1.vg'
   1 0 0.0000000E+00 0.00E+00 0.00 0.00* 0.00
Warning: MC sampler initialization: sum of channel weights is zero
   2 0 0.0000000E+00 0.00E+00 0.00 0.00 0.00
Warning: MC sampler initialization: sum of channel weights is zero
   3 0 0.0000000E+00 0.00E+00 0.00 0.00 0.00
Warning: MC sampler initialization: sum of channel weights is zero
   4 0 0.0000000E+00 0.00E+00 0.00 0.00 0.00
Warning: MC sampler initialization: sum of channel weights is zero
   5 0 0.0000000E+00 0.00E+00 0.00 0.00 0.00
|-----------------------------------------------------------------------------|
   5 0 0.0000000E+00 0.00E+00 0.00 0.00 0.00 0.00 5
|-----------------------------------------------------------------------------|
   6 0 0.0000000E+00 0.00E+00 0.00 0.00 0.00
   7 0 0.0000000E+00 0.00E+00 0.00 0.00 0.00
   8 0 0.0000000E+00 0.00E+00 0.00 0.00 0.00
|-----------------------------------------------------------------------------|
   8 0 0.0000000E+00 0.00E+00 0.00 0.00 0.00 0.00 3
|=============================================================================|

Question information

Language:
English Edit question
Status:
Solved
For:
WHIZARD Edit question
Assignee:
Juergen Reuter Edit question
Solved by:
Juergen Reuter
Solved:
Last query:
Last reply:
Revision history for this message
Best Juergen Reuter (j.r.reuter) said : 		#1

Hi Tabira,
assuming that this is the same UFO model that you used for your previous question: the parameter point you were using is special as the pseudoscalar Higgs has a mass of 900 GeV, exactly the collider energy within your scan. I can integrate in the cross section without problem, so it might be numerical differences between Intel and M1/M2 chips. However, you have not set a width for the AH pseudoscalar Higgs (which is never a good idea). It looks like that there is a Feynman diagram with the electron Yukawa coupling of AH, so on-shell production e+e- -> AH with subsequent decay of AH into Z,a,a. Without a AH width this is numerically not defined, and could lead to a vanishing cross section. So the solution is to calculate the widths for the corresponding parameter points or to disallow the e+,e-,AH vertex.
Cheers,
    JRR

Revision history for this message
Sheikh Farah Tabira (farahtabira) said : 		#2

Thanks Juergen Reuter, that solved my question.