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).

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.

Ordering Mode

Global Showers

For global showers, the user has to select what kind of ordering will be imposed, using the orderingMode parameter (note that this parameter will be used in the hard system only):

mode  Vincia:orderingMode   (default = 2; minimum = 0; maximum = 3)
This decides if and how ordering in the evolution variable is imposed on newly created dipole-antennae after a branching.
option 0 : No Ordering. Not recommended for physics runs. Newly created dipole-antennae are allowed to fill their full phase spaces, regardless of the ordering variable. Since energy and momentum are still conserved, a minimal amount of ordering will still occur, due to the post-branching dipole-antennae being smaller than the pre-branching one. This option could therefore also be called "phase-space ordering". The 2→4 approximation is given by products of nested 2→3 functions, without any further modification. This leads to a large amount of overcounting at the 2→4 level and should give answers similar to standard showers with virtuality-ordering with angular ordering switched off.
option 1 : Strong Ordering. This is identical to ordinary strongly ordered showers. Newly created antennae are restarted at the current evolution scale. Since the ordering condition acts like a step function in phase space, this choice generally implies that the shower may have some dead zones (points that are not reached by any strongly ordered path) starting from 2→4. For sensible evolution variables and maps (i.e., ones that have the appropriate LL singular limits), these dead zones only arise in non-LL-enhanced corners of the full 2→4 space, in which zero may not be such a terrible approximation, so they are not a priori problematic. However, their presence does preclude the use of strongly ordered showers as phase space generators for other purposes (e.g., for matching). The size of these zones depend on the evolution variable and kinematics maps and typically covers a few percent of phase space beyond 2→4 for the standard VINCIA variables (pT and mD).
option 2 : Smooth Ordering. This option smoothes out the ordinary strong ordering in QE by applying a smooth dampening instead of a sharp cutoff at the ordering scale. Nominally unordered branchings are thus allowed, but with a suppressed probability, defined as

where the hatted QE is the evolution scale evaluated on the pre-branching configuration (for branchings with more than one possible history, QEhat is the smallest of the corresponding evolution scales), and QE is the scale of the next branching. This removes most of the beyond-LL overcounting that would be obtained with option without any ordering at all (option 0) while simultaneously giving a better approximation to 2→4 over all of phase space, with no dead regions. Technically, this option is implemented in the following way: After each branching, all dipole-antennae are restarted at their full phase space, but subsequent branchings are subjected to a veto proportional to the Pimp factor above.
option 3 : Smooth Ordering up to Vincia:matchingLO, i.e. option 2 for the matrix-element corrected branchings and option 1 fo all further branchings.

A parameter with the same options controls the ordering of the shower in all other systems (MPI, decays that are not part of the hard system, ...):

mode  Vincia:orderingModeMPI   (default = 1; minimum = 0; maximum = 2)
The default value imposes strong ordering to ensure a faster evolution (compared to smooth ordering).

Hard Jets

To fill all of phase space the shower has to populate regions where the evolution variable is larger than the factorisation of the hard process. Based on the process, QCD (all hard processes with coloured objects in the final state, unless they arise from a decay, i.e. gg → H → gg is NOT a QCD process) or non-QCD, we allow the hard jets to be generated differently, with the following switches. Note that they only apply to the hard process.

QCD processes

For QCD processes VINCIA allows the first emission to be larger than the factorisation scale and treats it with smooth ordering (independent of what has be chosen for Vincia:orderingMode), where the factorisation scale acts as QEhat. By switching

flag  Vincia:hardJetsQCD   (default = on)

to off, the shower acts as an ordinary shower, starting at the factorisation scale.
Note 1: With smooth ordering and Vincia:hardJetsQCD switched off, the shower can still populate regions of phase space where the evolution variable is larger than the factorisation scale, starting from the second branching.
Note 2: Matrix element corrections are not applied if swiched off.

Non-QCD processes

For non-QCD processes VINCIA uses two event samples (not visible for the user): The first sample includes the inclusive Born events at the factorisation scale of the hard process. The second event sample generates Born + jets events, where the scale of the first jet is larger than the factorisation scale. This procedure slightly modifies the input cross section and comes with weighted events, mostly peaked around 2. By switching

flag  Vincia:hardJets   (default = on)

to off, the shower acts as an ordinary shower, starting at the factorisation scale, with one event sample.
Note 1: With smooth ordering the shower can still populate regions of phase space where the evolution variable is larger than the factorisation scale, starting from the second branching.
Note 2: Matrix element corrections cannot be applied beyond first order if Vincia:hardJets is switched off, as it would require clustering all the way to the Born kinematics. Note 3: The weights can of course be divided by 2, to be peaked around unity. If so the cross section needs to be multiplied by 2 to compensate for the change.

mode  Vincia:hardJetsAlphaS   (default = 0)
Controls the value of αs used for the first branching in the second event sample.
option 0 : Use the same scale and running of αs as the shower does.
option 1 : Use the same scale, but no CMW improvement.
option 2 : Use Q2fac+Q2branch as the scale.
option 3 : Use Q2fac+Q2branch as the scale and no CMW improvement.

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.