VINCIA Internal Parameters

Checks and Warnings

Check of antenna functions

By default, Vincia checks antenna functions for positivity and absence of dead zones.

flag  Vincia:CheckAntennae   (default = on)
Switch to control whether to perform antenna self-consistency checks or not.

mode  Vincia:nPointsCheck   (default = 1000; minimum = 0; maximum = 1e6)
Number of random points to check each antenna functions for positivity.

Numerical Parameters

Warning: do not change these parameters unless you have a very good reason to do so.

Paccept > 1 Verbosity

parm  Vincia:PacceptFuss   (default = 1.05; minimum = 1.0; maximum = 2.0)
Warning messages about P>1 violations will be suppressed if lower than this level.

parm  Vincia:PacceptPT0   (default = 1.5; minimum = 0.5; maximum = 10.0)
Warning messages about P>1 violations will be suppressed if the pT scale of the offending branching is lower than this scale.

flag  Vincia:PrintPacceptVariations   (default = off)
Switch to control whether to print a warning when violations of the accept probability in the shower variations occur.

flag  Vincia:PrintReWeightVariations   (default = off)
Switch to control whether to print a warning when the reweight factor in the shower variations is reset (reject weight).

Dead Zone Avoidance Factor

parm  Vincia:deadZoneAvoidance   (default = 0.0001; minimum = 0.0; maximum = 1.0)
During initialisation, warnings are issued if any antenna functions (in dimensionless form, with the Eikonal proportional to 2/y1/y2) become smaller than this number, anywhere in the resolved part of phase space (away from phase-space boundaries). This is to warn against spurious radiation zeroes or large negative finite terms creating "dead zones", or near-dead zones, in the shower. For LL showering and matching up to NLO, there is in principle no problem in taking this parameter to zero if so desired. However, for the NLL and higher-order matching corrections, very small values of this parameter may result in weights greater than unity being generated, since the corrections are multiplicative and large reweighting factors may be needed to "make up" for any near-dead zones at the previous branching step.

Experimental / Development / Advanced Parameters

Singular Coefficients

Within the antenna formalism, the collinear singularity of two gluons j and k is distributed between two neighboring antennae. One contains the singularity for j becoming soft, one the singularity for k becoming soft. In showers based on so-called global antenna functions (as opposed to sector functions, which are no longer implemented in VINCIA), the two antennae share the collinear singularity, j||k, point by point in phase space, and only after summing over both is the full collinear AP splitting kernel recovered. The parameter below controls the repartition ambiguity and gives the value of "half" the gluon splitting function on its finite end.

parm  Vincia:octetPartitioning   (default = 0.0; minimum = 0.0; maximum = 1.0)
Gluon-collinear α parameter. Only used for final-final global antennae. Note: only the default value (0) is consistent with the initial-final (and initial-initial) antenna functions in VINCIA. Special values of interest are: α=0, which corresponds to the Gehrmann-Gehrmann-de Ridder-Glover (GGG) partitioning, and α=1, which corresponds to the Gustafson (ARIADNE) partitioning.

Sector Showers

Note that the sector shower cannot be used in this version of VINCIA. In order to use the sector shower shower for the final state in electron-positron annihilation we refer to Vincia version 1.X.

One-Loop Matching

Note that the one-loop matching cannot be used in this version of Vincia. In order to use it for Z decay electron-positron annihilation we refer to Vincia version 1.X.


flag  Vincia:hyperjet   (default = off)
(Experimental flag. Not fully implemented yet. Leave in off position for physics studies.) When this flag is switched on, VINCIA will automatically use modified gluon emission antenna functions, in which the Eikonal (soft) terms have been subtracted out, for the showers off the hardest interaction. The gluon splitting antennae and the gluon emission antennae for radiation inside resonance decays (and for showers off MPI) are left unmodified. Since the modified gluon emission antennae no longer contain a soft singularity, this will NOT generate the correct DGLAP evolution if used together with normal matrix elements for the hard process. This option is therefore only intended for use together with matrix element events in which the soft Eikonal has already been resummed, such as when VINCIA is used together with J. Andersen's high-energy-limit matrix elements. Since these matrix element expressions are correct in the high-y-per-jet limit, we have dubbed this running mode HyperJet.

flag  Vincia:postponeMarkov   (default = off)
This flag controls whether generated trial branchings are considered by the accept/reject veto algorithm at their nominal branching scale or whether they are "postponed" until the evolution scale reaches the Markov scale of the would-be post-branching event.