WHIZARD 2 fails to integrate VBF channels correctly

[Juergen Reuter]

The setup for VBF does not integrate with the same result as in WHIZARD 1, Helac or Sherpa. The process is e1, E1 => e1, E1, u, U, d, D, the result from WHIZARD 1 is 0.6521(55) fb. The WHIZARD 2 result however is 0.866(35) fb where the integration is stuck and does not improve at all:

| Initializating integration for process xxxxx:
| Phase space file 'xxxxx.phs' not found.
| Generating phase space configuration ...
| ... done.
| ... found 795 phase space channels, collected in 55 groves.
| Phase space: found 1051 equivalences between channels.
| Wrote phase-space configuration file 'xxxxx.phs'.
| iterations = 5:100000, 25:200000
| Applying user-defined cuts.
| Creating VAMP integration grids:
| Using phase-space channel equivalences.
| 100000 calls, 795 channels, 14 dimensions, 12 bins, stratified = T
| Integrating process 'arsch':
|=============================================================================|
| It      Calls  Integral[fb]  Error[fb]   Err[%]    Acc  Eff[%]   Chi2 N[It] |
|=============================================================================|
   1     100000  5.7038854E-01  9.58E-02   16.80   53.13*   1.21
   2     100000  5.4088309E-01  5.46E-02   10.09   31.90*   0.78
   3     100000  6.9953138E-01  6.56E-02    9.38   29.66*   0.24
   4     100000  9.0424999E-01  1.75E-01   19.33   61.12    0.17
^[[1;5C   5     100000  8.3137084E-01  1.26E-01   15.17   47.97*   0.05
|-----------------------------------------------------------------------------|
   5     500000  6.3176528E-01  3.60E-02    5.69   40.26    0.05    2.30   5
|-----------------------------------------------------------------------------|
   6     200000  8.3751280E-01  1.54E-01   18.34   82.00    0.02
   7     200000  8.7231558E-01  1.45E-01   16.60   74.25*   0.02
   8     200000  8.2172036E-01  4.00E-02    4.87   21.78*   0.01
   9     200000  8.4897070E-01  3.68E-02    4.33   19.36*   0.01
  10     200000  8.6302096E-01  3.54E-02    4.10   18.32*   0.01
  11     200000  8.6603706E-01  3.53E-02    4.08   18.23*   0.02
  12     200000  8.6603852E-01  3.53E-02    4.08   18.23*   0.03
  13     200000  8.6604215E-01  3.53E-02    4.08   18.23*   0.02
  14     200000  8.6608253E-01  3.53E-02    4.08   18.23*   0.00
  15     200000  8.6612555E-01  3.53E-02    4.08   18.23*   0.01
  16     200000  8.6612643E-01  3.53E-02    4.08   18.23*   0.04
  17     200000  8.6612643E-01  3.53E-02    4.08   18.23*   0.02
  18     200000  8.6612645E-01  3.53E-02    4.08   18.23*    NaN
  19     200000  8.6612647E-01  3.53E-02    4.08   18.23*    NaN

I attach the corresponding SINDARIN input file as well as the WHIZARD 1 input and output files.

[Juergen Reuter]

W1 output

[Juergen Reuter]

Process definition

[Juergen Reuter]

WHIZARD 1 cut file

[Juergen Reuter]

WHIZARD 1 input file

[Juergen Reuter]

WHIZARD 2 SINDARIN file

[Juergen Reuter]

It looks like that this is indeed a wrong cut set-up, I did an angle cut between jet-jet which should not be there, and forgot the angle cut between lepton-lepton. Now it looks much better.

[Juergen Reuter]

There are loads of numbers completely off. Either the phase space setup of WHIZARD 2 is BY FAR too sensitive to small fluctuations from different channels or there are some problems with the cuts not properly transported from one process to the other or there are still some problems lingering around with collinear singularities. It seems that problems are inherent always then if there are electrons in the final state for electron-positron initial state. I'm stymied.

[Juergen Reuter]

The very problem seems to be the population of channels. It seems that according whether WHIZARD hits or misses a channel the outcome of the result is more or less arbitrary. So in principle for a 2->6 process we should always at least demand 15 iterations or even more (???).

[Juergen Reuter]

Started test runs for the same process ten times. It really looks like the result is more or less arbitrary. Waiting for the final results.