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)

Numerical Parameters

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

Dead Zone Avoidance Factor

parm  Vincia:deadZoneAvoidance   (default = 0.001; minimum = 0.0; maximum = 1.0)
Antenna functions (in dimensionless form, with the Eikonal proportional to 2/y1/y2) are not allowed to become smaller than this number. During initialization of each antenna function, its finite coefficients will be adjusted, if necessary, until this condition is satisfied everywhere in phase space. This is to avoid 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 will be needed to "make up" for any near-dead zones at the previous branching step.

Shower/Matching Headroom Factor

parm  Vincia:headRoomFactor   (default = 1.5; minimum = 1.0; maximum = 10.0)
A multiplicative factor applied to the trial generator to prevent P > 1 from occurring during trial accept/reject stage. (When matching is switched off, this factor is set to unity.) Note that, the better the shower approximation, the smaller this factor can be chosen. The default value is chosen to work optimally with the ARIADNE kinematics map. For other kinematics maps, higher settings may be needed to avoid P > 1 warnings. Note, however, that the code automatically increases this number (in 10% steps) when P > 1 warnings are encountered, so such warnings should gradually disappear for long runs, at the expense of slower running.

Experimental / Development / Advanced Parameters

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.