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

Sector Showers

Note that the sector shower can not 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.

Sector showers are implemented as a non-default option and can be switched on using the flag

flag  Vincia:sectorShower   (default = off)

option off : No sector ordering is imposed. All antennae are allowed to contribute freely, independently of overlapping radiation.
option on : Only one antenna is allowed to contribute to each phase space point. Note: when this option is on, a set of additional terms will automatically be added to VINCIA's antenna functions, such that the entire collinear singularity of each gluon is present in each antenna.

When the sector option is on, the following switch determines whether the additional gluon-collinear terms are added to the global antenna functions in order to create proper sector antennae from them,

flag  Vincia:useSectorTerms   (default = on)
Switch for the additional gluon-collinear terms that should be added in the sector approach. Should be on for normal runs. Setting it to off will set the additional gluon-collinear sector antenna term coefficients to zero.

For the time being, only one option for how to distinguish between sectors has been implemented, as follows. A given trial emission will only be accepted if, after the branching, it has the lowest pT (as defined for Type 1 evolution above) of all possible colour-ordered 3→2 clusterings after the branching.

One-Loop Matching

Note that the one-loop matching can not 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.

mode  Vincia:matchingNLO   (default = 0; minimum = 0; maximum = 2)
Selects the order of one-loop matrix-element corrections, also called Next-to-Leading-Order matching. The value should interpreted as counting the total number of powers of alphaS beyond the Born that are matched to matrix elements. I.e., for the basic process X, setting this switch to 2 would invoke one-loop matching up to and including X+1 partons (one emission plus one loop gives two powers of alphaS), to the extent the relevant matrix elements are available in the code, see the list below. In other words, to include one-loop matching for X+n partons, this switch must be set to n+1. The value 0 is equivalent to switching matching off. Note: values larger than the leading-order value above will be ignored. Thus, to switch off all matrix-element corrections, it is sufficient to switch off the LO ones.

The choice of functional form of the renormalisation scale used for the alphaS power associated with the one-loop correction amounts to an NNLO effect and is hence formally beyond the explicit control of the NLO matching. It is controlled by the following parameter:

mode  Vincia:alphaSmodeNLO   (default = 0; minimum = 0; maximum = 1)

option 0 : The invariant mass of the parton system, m(ijk).
option 1 : Transverse momentum, defined as in ARIADNE, pT = m(ij)*m(jk)/m(ijk).
Note 1: for a generic multileg topology, the effective renormalisation scale is computed as the geometric mean of such scales, taken over all ordered three-parton clusterings in the event.
Note 2: the default value, 0, has been chosen to limit the absolute size of the NLO corrections, especially for soft branchings. For hard corrections, there should not be much difference between the two choices (though one can of course always argue about factors of 2). For soft corrections, differences appear starting from order aS2*Log(s/pT2). Thus, changing to 1 increases the absolute size of the NLO corrections for soft branchings. Ideally, the shower and hadronisation parameters should then be retuned.
Note 3: In the current formulation of the VINCIA NLO matching formalism, option 1 is intended mostly for theoretical reference. The matching expression used in the code is of the form (1 + V), which implicitly assumes that the correction, V, is small. For option 1, however, V becomes large for soft branchings. In this case, a resummed form of the matching expression would have to be used instead, but such an expression has so far not been derived.

mode  Vincia:matchingNLOnPointsMin   (default = 25; minimum = 10; maximum = 10000)
Minimal number of MC points used for the numerical evaluation of the real-emission correction integrals (deltaA) terms when computing NLO matching coefficients. Note that all possible forward branchings are cycled through, so e.g. for Z to 3 jets, four 4-parton matrix elements (2 qggq and 2 qqqq ones) are tested for each MC point.

mode  Vincia:matchingNLOnPointsMax   (default = 100000; minimum = 10; maximum = 1000000)
Max number of MC points used for the numerical evaluation of the real-emission correction integrals (deltaA) terms.


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.