aMCatNLOError : Poles do not cancel, run cannot continue

Created by marco zaro on on 2015-06-16
Keywords:
pole check

For some processes it can happen that the error
"aMCatNLOError : Poles do not cancel, run cannot continue"
is raised after compilation.
This should happen only if at the process generation time the warning
"WARNING: Some loop diagrams contributing to this process are discarded because they are not pure (QCD)-perturbation.
Make sure you did not want to include them."
is printed on screen.

What happens is that for these kind of processes the loops with non-QCD particles (W/Z/photons/higgs/...) in the loops are discarded.

One example is the case of pentagons (gluon exchange between the two quark lines) in vector-boson fusion.

These diagrams are often 0 for color conservation. In the case of VBF-like processes the non-vanishing diagrams are known to give a very little (<1%) contribution to the cross-section.

In some cases, however, they do not vanish.

With the current version of MG5_aMC@NLO one can continue the run disabling the poles check by editing the
Cards/FKS_params.dat
file and setting
#IRPoleCheckThreshold
and
#PrecisionVirtualAtRunTime
to -1d0

The next version capable of handling mixed QCD-EW expansion will allow the user to include also such diagrams.