Shower Starting Scales

This page collects settings and parameters that control how the initial conditions for the shower evolutions are set, including also how the SCALUP parameter from external LHEF files is interpreted.

Factorisation Scale Interpretation (Wimpy vs Power Showers)

Similarly to PYTHIA, for processes that include at least one QCD parton in the final state, the default choice in VINCIA is to start the shower from the factorisation scale used for the hard process (as given by PYTHIA for internal processes, or defined by SCALUP for external ones), while processes that do not include any QCD partons are allowed to populate the full phase space. This behaviour can be changed by the following option, which is anologous to the SpaceShower:PTmaxMatch option in PYTHIA.

mode  Vincia:QmaxMatch   (default = 0; minimum = 0; maximum = 2)

option 0 : Showers off processes that include at least one final-state QCD parton are started at the factorisation scale, while processes that do not include any such partons are started at the phase-space maximum.
option 1 : Showers are always started at the factorisation scale.
option 2 : Showers are always started at the phase-space maximum. This option is not recommended for physics runs as it will lead to unphysical double counting in many cases.

When the first branching is limited by the factorisation scale for the hard process, a multiplicative factor can be applied to either increase or decrease the shower starting scale relative to the factorisation scale:

parm  Vincia:QmaxFudge   (default = 1.0; minimum = 0.1; maximum = 10.0)

parm  Vincia:QmaxFudgeMPI   (default = 1.0; minimum = 0.1; maximum = 10.0)
Same as above but for MPI systems, affecting the underlying event.

Note that for any (combination of) choices that result in ISR showers not using the factorisation scale as the starting scale, the generated Sudakov factor will produce leftover PDF ratios in the exclusive cross sections produced by the shower.

Use of External Scales (SCALUP)

VINCIA will ignore any externally provided scale information, such as SCALUP. For externally provided multi-parton systems, VINCIA will construct its own unique scales directly from the parton system.

The reasoning behind this is that any externally provided scales are not guaranteed to be compatible with those used by VINCIA, neither in functional form nor in normalisation. The exception to this is if the event was generated by VINCIA itself, in which case the correct scales will be reconstructed from the momenta.

Any more complete implementation of external scales should be implemented via PYTHIA's UserHooks interface, using the proper definition of the scale defined by the external program.