Hadronization

It is possible to pass the parton systems produced by VINCIA through PYTHIA's string hadronization model. Normally, this should happen automatically, according to the setting of the PYTHIA switch HadronLevel:all. The main parameter from the shower side is then the phase-space contour defined by the hadronization cutoff.

The hadronization cutoff, a.k.a. the infrared regularization scale, defines the resolution scale at which the perturbative shower evolution is stopped. Thus, perturbative emissions below this scale are treated as fundmanentally unresolvable and are inclusively summed over.

Important Note: when hadronization is switched on, there is a delicate interplay between the hadronization scale used in the shower and the parameters of the hadronization model. Ideally, the parameters of the hadronization model should scale as a function of the shower cutoff. This, however, is not the case for current hadronization models, such as the string model employed by PYTHIA and hence by VINCIA as well. Instead, the parameters of the hadronization model are tuned for one specific shower setting at a time. In order to be able to use PYTHIA's hadronization model together with VINCIA without major retuning efforts, it is therefore essential that VINCIA's cutoff be taken as close as possible to that used by PYTHIA in the PYTHIA tuning. What this means in practice is, firstly, that since PYTHIA's evolution variable is a pT-like variable, the default cutoff in VINCIA is likewise taken to be in a contour of pT. Secondly, there are a few different tunings of PYTHIA 8 to e+e- data available, each using a different numerical value of the cutoff. It is therefore important to match the value of the hadronization cutoff as well, depending on the PYTHIA tune.

Cutoff Variable

mode  Vincia:cutoffType   (default = 1; minimum = -1; maximum = 2)
This parameter selects the functional form of the infrared shower cutoff (a.k.a. hadronization cutoff), which defines the factorization border between the perturbative and non-perturbative regions. The possible options are:
option -1 : The cutoff is taken in PYTHIA 8's pTevol variable (times two, in order to use the same normalization as in the rest of Vincia). The cutoff scale is then set by the PYTHIA 8 parameter 2*TimeShower:pTmin. This is intended as a first crude way of using Vincia together with PYTHIA 8's hadronization model without having to retune the latter. Ultimately, dedicated tunes of the hadronization parameters using VINCIA should be used instead.
option 0 : Automatically set cutoffType equal to VinciaShower:evolutionType (only applies to infrared safe evolution variables)
option 1 : Cutoff in pT (defined as the type 1 evolution variable, see VinciaShower:evolutionType)
option 2 : Cutoff in dipole virtuality (defined as the type 2 evolution variable, see VinciaShower:evolutionType)

= -1 = 1 = 2
(cutoff at 2*TimeShower:pTmin) (cutoff at Vincia:cutoffScale) (cutoff at Vincia:cutoffScale)
Type-1 Type1 Type2

Cutoff Scale

parm  Vincia:cutoffScale   (default = 1.0)
When Vincia:cutoffType >= 0, this parameter sets the value (in GeV) of the shower cutoff, interpreted according to the functional form selected under Vincia:cutoffType. Note that all evolution/cutoff variables are normalized so that their maximum value is equal to the parent dipole mass. For instance, a type 1 cutoff at QI = 1.0 GeV therefore corresponds to a transverse-momentum cutoff at pT = 0.5 GeV. See VinciaShower:evolutionType for further information on the normalization.