VINCIA will generate weighted events for all non-QCD LHC processes, i.e. for all processes with incoming partons and no outgoing QCD jets. For those processes, VINCIA uses two event samples (not visible to the user), see the section about hard jets. This procedure modifies the normal PYTHIA 8 weight.
Another example is when VINCIA is used to shower a set of externally generated weighted events, in which case VINCIA normally just passes the weights through, without modification.
This initial event weight that VINCIA uses to start off the showering, can
be accessed by weightInit()
method in the VINCIA plugin. For
instance, if the VinciaPlugin
object is called vincia
the initial event weight can be accessed by
vincia.weightInit();
examples/vincia24.cc
on how to
take this into account properly.
The method
bool vincia.weightingOccurred();
In a few cases VINCIA may actively modify the initial event weight, as, e.g., in the automatic evaluation of uncertainty bands.
In some cases, VINCIA's shower or matching scheme may cause violations of the Monte Carlo overestimates used to drive the evolution. In such cases, warnings are normally printed to the output, giving the magnitude of the violation as well as some minimal information about what caused the violation. VINCIA automatically adjust the overestimates on the fly, i.e. in case of a violation the overestimate will be increased slightly. The overestimates will also be optimised (i.e. if the overestimate is a too large VINCIA will slowly decrease it), depending on the following flag:
flag
Vincia:MCoptimization
(default = on
)option
on : The overestimates will be decreased slowly if
the physical branching probabilies are much smaller than the
overestimate.
option
off : No optimisation. The overestimates will only be increased,
never decreased.
Note that switching Vincia:MCoptimization
off generally leads to
fewer violations; however, it also decreases the speed a bit.
flag
Vincia:allowReweighting
(default = off
)If changes to the initial event weight are made, these are bookkept and the final weight is accessible through
vincia.weight();
A convenient yes/no shorthand for checking whether any initial event weights were modified during the current run is provided by the method
bool vincia.reweightingOccurred();
PYTHIA's own weight has not been changed and is still accessible with
it's own info.weight()
method
(note that for non-QCD processes at the LHC this does not
include VINCIA's weighting factor to generate two event samples).