variation in parton level processes with fixed scales and no pdf's

[Maikel de Vries]

Hi,

I have been generating events with a fixed scale:

 True = fixed_ren_scale ! if .true. use fixed ren scale
 True = fixed_fac_scale ! if .true. use fixed fac scale

and without pdfs:

     0 = lpp1 ! beam 1 type
     0 = lpp2 ! beam 2 type

using the commands:

import model mssm
generate go go > u u~ / ul~ ur~

and then comparing to an analytic formula. There seems to be quite some variation in this process if generated for multiple mass points. One does not obtain a smooth line, and there seems to be very bad convergence of the error (fluctuations of a few percent with 10k events per mass point). This problem does not appear fro the process:

generate go go > g g / ul~ ur~

where the cross section is spot on (per-mille level errors).

The same happens for the model I am working on myself.

[Olivier Mattelaer (olivier-mattelaer)]

Hi,

Something realy funny seems to appear here.
Some of the helicity are sometimes extremelly suppressed while contributing for 50% of the cross-sections.
Such behavior make the helicity selector to sometimes discard those helicities and this creates some fake determination of the cross-sections.

The solution is to be more conservative on that selection of helicity.
This can be done by modifying Template/LO/Source/genps.inc

you can modify the lines:
c Parameters for helicity sums in matrixN.f
c*************************************************************************
      REAL*8 LIMHEL
      PARAMETER(LIMHEL=1e-6)
      INTEGER MAXTRIES
      PARAMETER(MAXTRIES=10)

I have set LIMHEL to 1e-9 and then I do not observe such behavior anymore (in 65 different consecutive run).
increasing MAXTRIES should help as well.

I will do a combination of both for the next release.

Cheers and thanks for the information,

Olivier

PS: I still have 35 run schedule, I will keep you posted if any or those have the buggy behavior.