Main VINCIA Shower Settings

Master On/Off Switches

These switches allow to quickly switch major parts of VINCIA on and off.

VINCIA On/Off

flag  Vincia   (default = on)
Main switch for VINCIA antenna showers. When set to on and the VINCIA plug-in is linked correctly to PYTHIA 8 (see section on installation and linking and/or the example program vincia01.cc included with the VINCIA plug-in), PYTHIA will use the VINCIA showers instead of its internal ones. If set to off instead, PYTHIA will use its own internal showers, regardless of whether the VINCIA plug-in is linked (useful for quick comparisons).

Helicity Dependence

flag  Vincia:helicityShower   (default = off)
Switch to activate the helicity dependent showering (and matrix-element corrections) in VINCIA.

Matrix-Element Matching

Please see the section on Matrix-Element Corrections.

Uncertainty Bands

Please see the section on Uncertainty Bands.

Hadronisation

Hadronisation is handled by PYTHIA. Please see the section on Hadronisation.

Shower Starting Scales

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.