VINCIA Shower Parameters
(nb: see the section on Initialization for how to set and change parameters.)

On/Off

flag  Vincia   (default = on)
Main switch for dipole-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).

flag  Vincia:uncertaintyBands   (default = off)
(Experimental flag. Not fully implemented yet. Leave in off position for physics studies.) Main switch for VINCIA's automatic evaluation of uncertainty bands.

flag  Vincia:nonRes   (default = off)
(Experimental flag. Not fully implemented yet. Leave in off position for physics studies.)

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.

Antenna Functions

AB->arb

The 2->3 (LL) VINCIA antennae have names such as

The generic name format is thus Vincia:AB:x, where A and B are the "mothers" and x is either emit or split, depending on whether the process is gluon emission or gluon splitting. The radiating (parent) antenna is interpreted as spanned between the Les Houches colour tag of A and the anti-colour tag of B, see illustration to the right.

Antenna Coefficients

The functional form of the antennae are specified by giving the coefficients of a double Laurent series in the two branching invariants (the third invariant, specifying a rotation around the dipole axis, is chosen uniformly). The Laurent expansion starts at power (-1,-1) corresponding to the double singularity. For so-called "global" antennae (the default in VINCIA), each antenna is fully specified by giving the coefficients LaurentC(i,j) of the following expansion:

A(y_ar,y_rb;s_AB) = 1/s_AB 4 pi alpha_s chargeFactor LaurentC(i,j) pow(y_ar,i) pow(y_rb,j);

where i and j are implicitly summed over, s_AB is the mass of the mother antenna, chargeFactor should be normalized to tend to NC raised to the number of new color lines created in the splitting in the large-NC limit (i.e., the limiting value should be 3 for gluon emission and 1 for gluon splitting), and y_ar,y_rb are the branching invariants scaled by the mass of the mother antenna, y_ij = s_ij/s_AB.

The antennae implemented in VINCIA are described in detail in a separate file:

The coefficients with negative indices are universal and should not be changed by the user. Finite terms, i.e., with both indices greater than or equal to zero, are arbitrary and in general the best choice to make will depend on the specific process considered. They are therefore not to be regarded as "tunable" parameters, but rather as an independent way of estimating the uncertainty due do uncalculated higher orders, an uncertainty which can be explicitly reduced by matrix-element matching. The default is therefore to allow these terms to be nonzero, but for special applications it may be convenient to have one global switch that switches them on and off:

flag  Vincia:useFiniteTerms   (default = on)
Global switch for all antenna function finite terms. Should be on for normal runs. Setting it to off will set all finite term coefficients to zero.

Using Non-Default Antennae

To use an alternative set of antennae (i.e., non-default), for instance to estimate uncertainties, a convenient way of specifying a collection of non-default parameters is to use

word  Vincia:antennaFile   (default = none)
Optional name of a file containing predefined settings for one or more non-default antenna functions. Note that this command file will be read in during construction of the VINCIA plug-in. Any desired further modifications of the antenna functions should therefore only be performed after construction of the plug-in. Some pre-defined antennae shipping with VINCIA are stored in a separate subdirectory called antennae/ (see below). More user-defined antennae can be added and stored in the same directory, if desired. To use them with a program that runs in your main VINCIA directory (default), include the following command in your main program or command file:

Vincia:antennaFile = antennae/antennae-GGG.cmnd
(Here we used the "GGG" antennae for illustration.) To use a user-defined antenna set with a program that does not run in your VINCIA main directory, or if the antennae you want to use are not stored in the antennae/ directory, give the full path- and filename instead, as in
Vincia:antennaFile = /Users/skands/vincia/antennae/antennae-GGG.cmnd
Note that the format of the antenna files is the same as that of ordinary PYTHIA 8 command files, hence any combination of VINCIA and PYTHIA 8 commands could in principle be included to define an antenna set, but in general we advise to keep only specific antenna-related parameters in the antenna definition files, to avoid confusions/conflicts between your main command file(s) and the antenna file.

The default antennae have been chosen to be close to the GGG ones and have been verified to give a good average agreement with Z->n matrix elements. The following two examples, included with the VINCIA package in the antennae/ subdirectory, have been defined so as to try to span a reasonable min-max uncertainty range:

Other predefined antenna functions included in the same directory are:

Gluon Splitting

The number of quark flavours allowed in gluon splittings, phase space permitting, is given by

mode  Vincia:nGluonToQuark   (default = 3; minimum = 0; maximum = 5)
Number of allowed quark flavours in gluon splittings, g -> q qbar, during the shower evolution. E.g., a change to 4 would exclude g -> b bbar but would include the lighter quarks, etc. Note that quark mass effects are currently not taken into account. Note also that this parameter does not directly affect the running coupling (see the section on alphaS).

Ordering

These choices govern how the shower fills phase space, and hence how the logarithms generated by it are ordered. This does not affect the LL behaviour, but does affect the tower of higher (subleading) logs generated by the shower and can therefore be signficiant in regions where the leading logs are suppressed or absent. Note that, by construction, the dipole formalism automatically ensures an exact treatment of coherence effects to leading logarithmic order, and hence additional constraints, such as angular ordering, are not required.

Evolution Variable

mode  Vincia:evolutionType   (default = 1)
Choice of functional form of the shower ordering variable (a.k.a. evolution variab le), for AB -> a r b branchings (see, e.g., illustration of dipole-antenna branching above and/or further illustrations below):
option 1 : Transverse Momentum. This evolution variable is roughly equal to the inverse of the antenna function for gluon emission, and hence is in some sense the most natural evolution variable. We define it as in Ariadne, but with a normalization that makes it equal to S_AB at the upper edge of phase space,

Q_E^2 = 4pT^2 = 4 s_ar*s_rb/s_AB

option 2 : Daughter Antenna Mass. This mass-like variable represents a fairly moderate variation on the transverse momentum. It will give slightly more priority to soft branchings over collinear branchings, as compared to transverse momentum. We define it as
Q_E^2 = 2m_D^2 = 2*min(s_ar,s_rb)

option 3 : Energy (of emitted parton, in dipole-antenna CM). This option gives the highest possible prioritization of collinear branchings over soft ones, and is in that sense the asymptotic extreme of type 1 above. We define it as
Q_E^2 = (s_ar + s_rb)^2/s_AB
Note that this evolution variable does not go to zero on the collinear boundaries of phase space. We therefore include it mostly for theoretical reference and highly discourage using it for physics studies.
option 4 : V, an artificial measure constructed so as to give the highest possible prioritization to soft branchings over collinear ones, and is in that sense an more extreme variant of type 2 above. It is defined as
Q_E^2 = s_AB ( (s_ar + s_rb)^(1/2) - |s_ar - s_rb|^(1/2) )
Due to its somewhat contrived nature and more extreme form, this variable is also mostly included for theoretical reference. For normal physics studies, considering types 1 and 2 alone should give a reasonable idea of the uncertainty due to the choice of evolution variable.
The normalizations are chosen such that the maximum value of the evolution variable is equal to the mass of the parent dipole-antenna, Q_max = m_AB = m_arb.

The contours below illustrate the progression of each evolution variable over the dipole-antenna phase space for three fixed values of y_E = Q_E^2/s_AB:

Types 1 and 2: moderate variation
Type1 Type2
Types 3 and 4: extreme variation
Type3 Type3

Note that energy-ordering (type 3) is not infrared safe, since contours of finite value of that evolution variable intersect the collinear region along the axes. This would nominally lead to infinitely many collinear branchings being generated during a finite evolution interval, rendering our shower formalism inapplicable. It is therefore not possible to choose energy ordering with a hadronization cutoff in the evolution variable. Instead, energy ordering must be used with a cutoff either in pT or in mass, which is sufficient to regulate the divergence. Note that even with this regularization this ordering should still result in a logarithmically enhanced preponderance of near-collinear branchings.

Ordering Mode

mode  Vincia:orderingMode   (default = 3; minimum = 0)
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 with QE-dampening. 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,

Pimp = 1 / (1 + QE'^2/QE^2) ,
where QE' is the evolution scale evaluated on the pre-branching configuration (for branchings with more than one possible history, QE' 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 with pT-dampening. As for option =2 but with pT scales used instead of QE for computing the suppression factor Pimp, regardless of which evolution variable is used. For QE = pT (see evolutionType) this option is obviously identical to =2, but for other evolution types this choice makes the effective antenna functions independent of the evolution variable and gives an extremely good approximation all the way through 2->6, which is the highest order we have checked explicitly (comparing tree-level expansions of VINCIA with Leading-Color matrix elements from MadGraph).

External Scales

flag  Vincia:useCreationScales   (default = on)
Sets whether to use "creation scale" information from the matrix element as an upper bound on the evolution variable for showering. Not required for VINCIA's internal matching, but can be useful when using VINCIA's showers with alternative matching schemes relying on externally generated matrix-element-level events.

parm  Vincia:pTmaxFudge   (default = 1.0)
Copy of the PYTHIA 8 parameter TimeShower:pTmaxFudge allowing the one used for VINCIA showers to be changed independently of the PYTHIA 8 one. See the PYTHIA 8 documentation for more info.

Sector Ordering (optional)

Important note: sector ordering should only be used together with dedicated antenna functions whose collinear singularities have been tailored explicitly to work in this mode. If used with the default kind of antenna functions, such as the GGG or ARIADNE ones, correct DGLAP evolution will not result.

flag  Vincia:sectorOrdering   (default = off)
This decides whether to allow radiation from different dipole-antennae to overlap in each phase space point, such that the total result for n partons is obtained as a sum over all clusterings to (n-1) partons, or whether to only allow one dipole-antenna to contribute in each phase space point.
option off : No sector ordering is imposed. All antennae are allowed to contribute freely, independently of overlapping radiation. Warning: this option should only be used with so-called "global" antenna functions (the default), whose singular terms are such that the collinear singularity of one gluon is obtained by summing two neighbouring antennae over all of phase space.
option on : Only one antenna is allowed to contribute to each phase space point. Warning: this option should only be used with so-called "sector" antenna functions, whose singular terms are such that the collinear singularity of one gluon is represented entirely by the antenna which has the smallest value of pT for the given phase space point.

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 color-ordered 3->2 clusterings after the branching.

In the case of sector-ordered antennae (as opposed to the "global" ones described above), explicit singularities at y_ij->1 may also be present. In addition to the C_ij coefficients defined above, we therefore also allow for the following additional terms in the antenna function parametrization when sector ordering is switched on:

... + LaurentD(i) pow(y_ar,i) / (1-y_rb) + LaurentE(i) pow(y_rb,i) / (1-y_ar) + K / (1-y_ar) / (1-y_rb);

Recoil Strategy

While the CM momenta of a 2->3 branching are fixed by the generated invariants (and hence by the antenna function), the global orientation of the produced 3-parton system with respect to the rest of the event (or, equivalently, with respect to the original dipole-antenna axis) suffers from an ambiguity outside the LL limits, which can affect the tower of subleading logs generated and can be significant in regions where the leading logs are suppressed or absent.

To illustrate this ambiguity, consider the emissision of a gluon from a qqbar antenna with some finite amount of transverse momentum (meaning transverse to the original dipole-antenna axis, in the CM of the dipole-antenna). The transverse momenta of the qqbar pair after the branching must now add up to an equal, opposite amount, so that total momentum is conserved, i.e., the emission generates a recoil. By an overall rotation of the post-branching 3-parton system, it is possible to align either the q or the qbar with the original axis, such that it becomes the other one that absorbs the entire recoil (the default in showers based on 1->2 branchings such as old-fashioned parton showers and Catani-Seymour showers), or to align both of them slightly off-axis, so that they share the recoil (the default in VINCIA, see illustration below).

Kinematics Map

2->3
			      kinematics

mode  Vincia:kineMapType   (default = 1; minimum = 1; maximum = 3)
Selects which method to use for choosing the Euler angle for the global orientation of the post-branching kinematics construction. The default option is illustrated in the figure (right). This setting applies to all antennae, irrespective of the branching type.
option 1 : The ARIADNE angle (see illustration). The recoiling mothers share the recoil in proportion to their energy fractions in the CM of the dipole-antenna. Tree-level expansions of the VINCIA shower compared to tree-level matrix elements through third order in alphaS have shown this strategy to give the best overall approximation, followed closely by the KOSOWER map below.
option 2 : LONGITUDINAL. The parton which has the smallest invariant mass together with the radiated parton is taken to be the "radiator". The remaining parton is taken to be the "recoiler". The recoiler remains oriented along the dipole axis in the branching rest frame and recoils longitudinally against the radiator + radiated partons which have equal and opposite transverse momenta (transverse to the original dipole-antenna axis in the dipole-antenna CM). Comparisons to higher-order QCD matrix elements show this to be by far the worst option of the ones so far implemented, hence it could be useful as an extreme case for uncertainty estimates, but should probably not be considered for central tunes. (Note: exploratory attempts at improving the behaviour of this map, e.g., by selecting probabilistically between the radiator and the recoiler according to approximate collinear splitting kernels, only resulted in marginal improvements. Since such variations would introduce additional complications in the VINCIA matching formalism, they have not been retained in the distributed version.)
option 3 : The KOSOWER map. Comparisons to higher-order QCD matrix elements show only very small differences between this and the ARIADNE map above, but since the KOSOWER map is sometimes used in fixed-order contexts, we deem it interesting to include it as a complementary possibility. (Note: the KOSOWER maps in fact represent a whole family of kinematics maps. For experts, the specific choice made here corresponds to using r=sij/(sij+sjk) in the definition of the map.)

The Strong Coupling

Reference Value

The amount of QCD radiation in the shower is determined by

parm  Vincia:alphaSvalue   (default = 0.138)
The alpha_strong value at scale mZ. The default is chosen to agree with LEP results for default shower settings, and is appropriate for a leading-order / leading-log shower (as compared, e.g., to leading-order extractions of alpha_strong at LEP).

Order

mode  Vincia:alphaSorder   (default = 1; minimum = 0; maximum = 1)
Order at which alpha_strong runs,
option 0 : zeroth order, i.e. alpha_strong is kept fixed.
option 1 : first order. This option is recommended for LO matrix elements and LL showers.

Argument of Running Coupling

When Vincia:alphaSorder is non-zero, the actual value is then regulated by running to the scale K*muR, at which the shower evaluates alpha_strong. The functional form of muR is given by Vincia:alphaSmode and the scale factor K is given by Vincia:alphaSscaleFactor.

mode  Vincia:alphaSmode   (default = 3; minimum = 1; maximum = 3)
The functional form of muR is given by
option 1 : The evolution variable (specified by Vincia:evolutionType) evaluated at the current branching.
option 2 : The invariant mass of the mother antenna.
option 3 : Transverse momentum, specifically the Type 1 Evolution variable, regardless of what ordering variable is being used in Vincia:evolutionType. Note that, since the VINCIA normalization of the transverse-momentum variable corresponds to 2pT, this should normally be used with Vincia:alphaSscaleFactor = 0.5, see below.

parm  Vincia:alphaSscaleFactor   (default = 0.5; minimum = 0.0)
If different from unity, alpha_strong is evaluated at the scale defined by Vincia:alphaSmode times this scale factor, i.e., it gives the value of Kmu in the argument to alphaS(Kmu*muR). Thus, e.g., for transverse momentum, this scale factor should be 0.5, since VINCIA uses 2pT as evolution variable.

Infrared Freezeout Scale

parm  Vincia:alphaSmuMin   (default = 0.5; minimum = 0.0)
Smallest Kmu*muR scale at which alphaS will be evaluated. I.e., the strong coupling is treated as frozen below this scale.

Max Coupling

parm  Vincia:alphaSMax   (default = 1.0; minimum = 0.0)
Largest allowed numerical value for alphaS.

Beyond-LL Matching

Matching to beyond-LL splitting kernels is still at a development stage. However, a matching to the parts generated purely by the running of alphaStrong is already accessible, which can be used to gauge the improvement in the scale dependence that a full matching to second-order antenna functions would give. The stabilizing corrections are currently themselves multiplied by alphaS evaluated at the invariant mass of the splitting dipole-antenna. This ensures that the effect of the stabilization is as small as possible, and hence conservative.

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

option -1 : Off. Scale variations in alphaS are not compensated at all in the shower evolution.
option 0 : Scale Cancellation Only. Scale variations in alphaS are compensated up to 1st order by matching to the explicit b0-dependent terms in the second-order (NLL) antenna functions.
option 1 : (Reserved for future use.) Full matching to second-order antenna functions.

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 Vincia:evolutionType (only applies to infrared safe evolution variables)
option 1 : Cutoff in pT (defined as the type 1 evolution variable, see Vincia:evolutionType)
option 2 : Cutoff in daughter antenna mass (defined as the type 2 evolution variable, see Vincia: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 Q_I = 1.0 GeV therefore corresponds to a transverse-momentum cutoff at pT = 0.5 GeV. See Vincia:evolutionType for further information on the normalization.

Tunes

VINCIA accepts input of tune presets in the form of a standard PYTHIA 8 command file whose name and location can be specified by the user. In this way, a particular set of user-defined parameters can easily be made into a tune set by simply copying the relevant parts of the user's normal command file (i.e., omitting the process-specific and program control parameters) into a new file that can then be shared and/or submitted to the VINCIA authors for possible inclusion in future distributions.

Although there are obviously parameters that it makes more sense to tune than others, there is no explicit restriction imposed on what parameters are allowed to be present in this file. This implies some responsibility on the part of the user. As a guideline, the main parameters that need to be properly tuned are the non-perturbative hadronization parameters used in PYTHIA's string fragmentation model. Since PYTHIA and VINCIA treat soft radiation somewhat differently, there can be important differences between the two in the soft region that the hadronization model will not re-absorb automatically and which therefore only a retuning can address. Apart from this, the value of the strong coupling used in the shower is also normally considered a tuning parameter.

In versions before 1.022, no systematic attempt had been made to tune VINCIA to describe, e.g., LEP data, beyond setting the default shower alphaS and cutoff parameters to be similar to those used by PYTHIA 8 and then relying on universality of the corrections below the cutoff. Starting from VINCIA version 1.022, however, dedicated tunes of VINCIA itself have been included with the standard distribution.

word  Vincia:tuneFile   (default = tunes/jeppsson2.cmnd)
Name of a command file containing tune presets for VINCIA. Note: the requested file will only be read in when VINCIA is switched on, in order not to interfere with the PYTHIA settings when VINCIA is switched off. Note: a special value for this parameter is "none", in which case no tune file will be used (i.e., PYTHIA's parameters will be used as they are). Note: the entries in the tune file will be superseded by any user modifications made in the main command file given to the VINCIA constructor. This should allow sufficient flexibility to explore user variations away from the tuned values. The list of currently available tunes is as follows:
option none : No tune file will be read.
option tunes/jeppsson.cmnd : A first tune of VINCIA+PYTHIA to LEP data, by M. H. Jeppsson, April 2010.
option tunes/jeppsson2.cmnd : A slight modification of the original Jeppssson tune of VINCIA+PYTHIA to LEP data, by M. H. Jeppsson and P. Skands, April 2010.

Note on user tunes: in order to make the tunings more stable against possible changes in the program defaults (be it PYTHIA or VINCIA), it is advisable to include all relevant parameter values explicitly in the tune file, rather than letting parameters that retain their (version-specific) default values be defined implicitly.

VINCIA Internal Parameters

Output and Debug Information

mode  Vincia:verbose   (default = 1; minimum = 0; maximum = 9)
Level of detail of information written to standard output on what goes on inside VINCIA. Settings different from zero and one are intended for debugging purposes and hence should not be used for normal runs.
option 0 : No runtime output.
option 1 : Normal runtime output.
option 2 : Enhanced runtime output. Also, internal VINCIA diagnostics histograms are booked and filled, especially for matching. These can be printed by the user at any time (e.g., after a run) using the printHistos() method.
option 3 : As for =2. And: a consistency check is added to each branching by reclustering the resulting momenta back using the corresponding inverse kinematics map and checking that the original momenta are recovered within the desired numerical precision.
option 4 : As for =3. And: each prepare() and pTnext() call is explicitly announced, with system number and restart scale printed out, respectively. .
option 5 : As for =4. And: momentum listings are printed for each configuration that violates P < 1.
option 6 : As for =5. And: each main function call is explicitly announced with begin and end printed to output.
option 7 : As for =6. And: all function calls are explicitly announced with begin and end printed to output.
option 8 : As for =7. And: last semi-sensible level of output.
option 9 : As for =8. And: all possible output.

Numerical Parameters

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

parm  Vincia:TINY   (default = 1e-5; minimum = 1e-12; maximum = 1e-2)
Target for numerical precision. For reasonable precision do not increase above 1e-4. For reasonable speed, do not decrease below 1e-6 (though in principle the Sudakov integrals have been tested Numerical = Analytical all the way down to 1e-12, beyond which the internal double precision handling would no longer suffice).

parm  Vincia:deadZoneAvoidanceFactor   (default = 0.10; minimum = 0.0; maximum = 1.0)
The finite terms of each antenna function are not allowed to cause the full antenna function to become smaller than this factor times the singular terms. During initialization of each antenna function, its C00 coefficient 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.

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