<chapter name="VINCIA User Reference"> 
 
<h2>The Antenna Shower Model (VINCIA)</h2> 
 
<h3>Overview</h3> 
 
<p/> 
The cornerstone of VINCIA is a pT-ordered model for QCD + QED/EW 
showers based on the antenna formalism.  Originally developed as a 
standalone plugin, it has been incorporated into the Pythia 8 source 
code since Pythia version 8.300. 
 
<p/> 
The main features are: 
<ul> 
<li><b>QCD:</b> VINCIA's QCD antenna functions incorporate both the 
collinear (DGLAP) and soft (eikonal) limits of QCD matrix elements at 
leading colour. They hence should exhibit improved colour-coherence 
effects relative to Pythia's original (DGLAP-based) simple shower 
model. </li> 
 
<li><b>QED:</b> 
VINCIA's default QED antenna-shower module is based on a fully coherent 
(multipole) treatment of photon radiation, which is 
interleaved with the QCD evolution. 
Photon splittings to lepton-antilepton and quark-antiquark pairs are 
also included. </li> 
 
<li><b>EW:</b> Optionally, the coherent QED shower can be replaced by a 
full-fledged electroweak (EW) shower module, which allows for 
branchings involving Z/W/h bosons as well, with antenna functions 
capturing the collinear limits of the relevant matrix elements, 
including ones involving gauge-boson self couplings. This can be used 
to include weak corrections in electroweak Sudakov factors and to 
resum multiple weak-boson emissions (and splittings). Note that 
the module operates on helicity states, and helicity dependence must 
therefore be enabled. Note also that, since 
this module is based on collinear approximations, it does not exhibit 
the full coherence of the pure-QED shower module. </li> 
 
<li><b>Mass Effects:</b> In all cases, the effects of final-state parton 
masses (e.g., bottom and top quark masses) are systematically 
included, and the massive antenna functions have the appropriate 
quasi-collinear limits. </li> 
 
<li><b>Efficient Multi-jet Merging:</b> see the section 
on <aloc href="VinciaMerging">Sector Merging (MESS)</aloc>.</li> 
 
<li><b>Interleaved Resonance Decays:</b> By default, resonance decays are 
interleaved with the rest of the perturbative evolution. This means 
that each resonance in the hard process (or emitted by the EW shower) 
is treated as stable until the evolution reaches a pT scale of order 
the offshellness of that resonance, at which point the resonance is 
replaced by its decay products and an internal "resonance shower" is 
performed in the decay system, starting from the invariant mass of the 
decaying resonance and ending at the offshellness scale. The 
decay+shower system is then merged back into the upstream system and 
the evolution of the event continues, starting from the offshellness 
scale. In case of sequential resonance decays, this procedure may be 
carried out either iteratively or in sequence, depending on the 
offshellnesses involved. 
</li> 
 
<li><b>Interleaved Evolution and MPI:</b> VINCIA's shower evolution is 
ordered in a measure of transverse momentum and is fully interleaved 
with Pythia's treatment of multiple parton interactions (MPI). When 
the Vincia shower model is selected, both the hard interaction and any 
MPI are showered by Vincia. 
</li> 
</ul> 
 
<p/> 
The name VINCIA stands for "VIrtual Numerical Collider with 
Interleaved Antennae". This reflects its main properties as a 
numerical model for collider-physics processes based on 
dipole-antennae and interleaved perturbative evolution.  It is also 
intended to allude to a progression from PYTHIA, a name originating 
in ancient Greece, to the renaissance era of Leonardo da Vinci. The 
logo of VINCIA is the "Vitruvian Man", by da Vinci, a choice which 
also reflects the combination of art and accuracy which is necessary 
to write a good event generator. Classical antiquity is still 
indirectly represented, via the namesake of the logo, Vitruvius, a 
first-century (BC) Roman author, architect, and engineer. 
 
<h3>Matching and Merging with VINCIA</h3> 
 
<p/> 
VINCIA has its own dedicated approach to merging with 
higher-multiplicity tree-level matrix elements, called sector merging or MESS 
(for <b>M</b>atrix <b>E</b>lements + <b>S</b>ector <b>S</b>howers). 
It is similar in spirit to the CKKW(-L) approaches, but exploits the 
maximally-bijective nature of VINCIA's sector shower to vastly reduce 
the complexity of the merging procedure, so that merging especially 
at high multiplicities becomes more efficient. 
Settings for VINCIA's merging are described 
<aloc href="VinciaMerging">here</aloc>. 
 
<p/> 
VINCIA also provides dedicated hooks for vetoed showers in POWHEG NLO matching 
with input from the POWHEG-BOX program, attempting to correct for the mismatch 
between the ordering variables in POWHEG-BOX and VINCIA. They are 
contained in the same file 
<code>include/Pythia8Plugins/PowhegHooks.h</code> as the standard 
PYTHIA POWHEG hooks, and the usage is the same as for Pythia's 
<code>PowhegHooks</code>, described <aloc href="POWHEGMatching">here</aloc>. 
Any existing main program (e.g. <code>main152.cc</code>) that uses 
POWHEG-BOX+Pythia can be adapted to use VINCIA with POWHEG-BOX, simply 
by changing the shower model and setting 
<code>Vincia:pTmaxMatch = 2</code> and <code>Vincia:pTdampMatch = 3</code> 
as recommended <aloc href="POWHEGMatching">here</aloc>. 
 
<h3>Main Shower Switches and Starting Scales</h3> 
 
<modepick name="Vincia:nFlavZeroMass" default="4" min="2" max="6"> 
Controls the number of flavours that will be treated as massless by 
VINCIA, ie with massless kinematics and no mass corrections. The 
remaining flavours will be bookkept with massive kinematics and 
mass-corrected antenna functions.  Note that, even for flavours 
treated as massless, an elementary phase-space check is still made eg 
on all g&rarr;QQ branchings to ensure m(QQ) >= 2mQ.  Likewise, all 
heavy flavours in the initial state are forced to undergo a conversion 
into a gluon when the evolution variable reaches their mass threshold 
(with the threshold determined by the maximum of the PDF threshold and 
the relevant user-specifiable mass parameter given below). 
</modepick> 
 
<flag name="Vincia:helicityShower" default="off"> 
Switch to use helicity-dependent antenna functions (or not). Only 
meaningful when helicity information is present in the Born-level 
events. 
</flag> 
 
<flag name="Vincia:sectorShower" default="on"> 
Switch to (de-)activate the sector shower in VINCIA. In this 
version of VINCIA, the sector shower has become the default, 
due to its better scaling behaviour in multileg matching and merging, 
see below. 
</flag> 
 
<parm name="Vincia:sectorDamp" default="1.0" min="0.0" max="1.0"> 
In the symmetrisation over post-branching gluons that is done to 
derive the sector antenna functions from the global ones, the 
branching invariant with swapped gluons is nominally given by 
<ei>yijSym = yik = 1 - yij - yjk</ei>. If the swapped gluons are j and 
k (and straightforwardly generalised if they are i and j) then the 
collinear <ei>yjk&rarr;0</ei> limit does not change by adding or 
subtracting a term of order yjk. Therefore one could equally well use 
<ei>yijSym = 1 - yij</ei> (or something inbetween). This is still 
guaranteed to be positive definite and was indeed the choice in the 
original sector antenna shower papers. Since the latter definition 
produces a value for <ei>yijSym</ei> which is slightly larger than the 
former, the corresponding 1/yijSym singularities in the antenna 
function are damped slightly, so that larger values of the 
<code>sectorDamp</code> parameter produces sector antenna functions 
which have slightly smaller magnitudes outside the collinear 
limits. Strictly speaking this choice is an ambiguity that should be 
varied for uncertainty estimates, in which context we note that we 
expect it to be almost entirely degenerate with variations of 
nonsingular terms. 
</parm> 
 
<h4>Vincia Shower Starting Scales</h4> 
 
<p/> 
Similarly to PYTHIA, for processes that include at least one quark, 
gluon, or photon in the final state, the default choice in VINCIA is 
to start the shower from the factorisation scale used for the hard 
process (as given by PYTHIA for internal processes, or defined by the 
<code>scale</code> value for Les Houches input),while processes that 
do not include any such partons are allowed to populate the full phase 
space. This behaviour can be changed by the following option, which is 
anologous to the <code>SpaceShower:PTmaxMatch</code> option in PYTHIA. 
 
<modepick name="Vincia:pTmaxMatch" default="0" min="0" max="2"> 
<option value="0">Showers off hard processes that include at least one 
final-state quark, gluon, or photon, are started at the factorisation 
scale (wimpy showers), while processes that do not include any such 
partons are started at the phase-space maximum (power showers). Note 
that only partons that can be emitted by showers are included in this 
counting, hence top quarks are not included for 
<code>Vincia:nGluonToQuark &lt; = 5</code>.</option> 
<option value="1">Showers are always started at the factorisation 
scale (wimpy showers).</option> 
<option value="2">Showers off hard processes are always started at the 
phase-space maximum (power showers). This option is not recommended 
for physics runs as it will lead to unphysical double counting in many 
cases.</option> 
</modepick> 
 
<p/> 
When the first branching is limited by the factorisation scale for the 
hard process, a multiplicative factor can be applied to either 
increase or decrease the shower starting scale relative to the 
factorisation scale: 
<parm name="Vincia:pTmaxFudge" default="1.0" min="0.1" max="10.0"></parm> 
<parm name="Vincia:pTmaxFudgeMPI" default="1.0" min="0.1" max="10.0"> 
Same as above but for MPI systems, affecting the underlying event. 
</parm> 
 
<modepick name="Vincia:pTdampMatch" default="3" min="0" max="4"> 
For any systems that are treated with power showers, a dampening 
factor may be applied to dampen radiation above a given scale. 
<option value="0">emissions go up to the kinematical limit, 
with no special dampening.</option> 
<option value="1">emissions go up to the kinematical limit, 
but dampened by a factor <ei>k^2 Q^2_fac/(pT^2 + k^2 Q^2_fac)</ei>, 
where <ei>Q_fac</ei> is the factorization scale and <ei>k</ei> is a 
multiplicative fudge factor stored in <code>pTdampFudge</code> 
below.</option> 
<option value="2">emissions go up to the kinematical limit, 
but dampened by a factor <ei>k^2 Q^2_scale/(pT^2 + k^2 Q^2_scale)</ei>, 
where <ei>Q_scale</ei> is the geometric mean of the 
<code>scale</code> properties for the incoming partons for the given 
system. Note the interpretation of this option differs from that for 
the analogous value in the Simple Shower.</option> 
<option value="3">as option 1, but in addition to the standard requirements 
for dampening it is further necessary to have at least two top or 
beyond-the-Standard-Model coloured particles in the final state. 
Examples include <ei>t tbar</ei> and <ei>squark gluino</ei> 
production.</option> 
<option value="4">as option 2, but in addition to the standard requirements 
for dampening it is further necessary to have at least two top or 
beyond-the-Standard-Model coloured particles in the final state. 
Examples include <ei>t tbar</ei> and <ei>squark gluino</ei> 
production.</option> 
</modepick> 
 
<parm name="Vincia:pTdampFudge" default="1.0" min="0.25" max="4.0"> 
In cases above where a dampening is imposed at around a specific Q 
scale, this parameter allows the <ei>pT</ei> scale of dampening of 
radiation by a half to be shifted by this factor relative to the 
default scale. This number ought to be in the neighbourhood of unity, 
but variations away from this value could do better in some processes. 
</parm> 
 
<p/> 
Note that for any (combination of) choices that result in ISR showers 
not using the factorisation scale as the starting scale, the generated 
Sudakov factor will effectively produce leftover PDF ratios in the 
exclusive cross sections produced by the shower. 
 
<h3>Kinematics and Recoils</h3> 
 
<p/> 
While the CM momenta of a <ei>2&rarr;3</ei> 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 be significant in regions where the leading logs are 
suppressed or absent. 
 
<p/> 
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 <ei>1&rarr;2</ei> 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). 
 
<p/> 
<a href="http://home.fnal.gov/~skands/vincia/img/vincia-kin.png"> 
  <img src="vincia-kins.png" align="right" border="0" hspace="20" 
       vspace="20" width="300" height="254" alt="2to3-kinematics"/> </a> 
 
<h4>Kinematics and Recoils for II Antennae </h4> 
 
<p/> 
The post-branching momenta are fixed by the following requirements: 
<br/> 
1) The direction of the initial state partons is aligned with the beam axis 
(z-axis). 
<br/> 
2) The invariant mass and the rapidity of the final state recoiler are not 
changed by the branching. This allows a direct construction of the 
post-branching momenta in the lab frame. 
 
<a name="kinematicsIF"></a> 
<h4>Kinematics and Recoils for IF Antennae</h4> 
<p/> 
For IF branchings, Vincia contains implementations of two different 
kinematics maps, called "local" and "global" in what follows. 
 
<p/> 
In the 
"local map", the initial-state parton recoils 
longitudinally, and there is no recoil imparted to any partons 
that do not participate directly in the branching. (I.e., 
recoil effects are absorbed locally within the branching antenna, and 
no partons outside of it are affected.) This is equivalent to saying 
that any transverse momentum associated with the emitted parton (j) is 
absorbed by the other final-state parton (k). This 
allows a simple construction  of the post-branching momenta in the 
centre-of-mass frame of the initial-final antenna. 
 
<p/> 
The "global map" allows for an overall transverse recoil associated with the 
initial-state leg to be imparted to the system of final-state partons 
other than those participating directly in the branchings. This is 
equivalent to saying that any transverse momentum associated with the 
emitted parton (j) is absorbed by the initial-stage leg (a), after 
which a Lorentz transformation brings it (plus the final-state system) 
back to having beam-collinear kinematics. The recoil vanishes For 
final-state collinear kinematicsbut is in general nonzero outside that 
limit. 
 
<p/> 
Intuitively, the local map should be appropriate for final-state 
splittings, while the global one would be appropriate for initial-state 
ones. The full story is more complicated, partly since soft wide-angle 
radiation intrinsically represents interference between the two cases, 
and partly because the phase-space limits for the two maps (outside of 
the strict soft and collinear limits) are different. (The x &lt; 1 
constraint translates to slightly different constraints on the 
branching invariants for the two maps, as does positivity of the Gram 
determinant.) A probabilistic selection is therefore made between 
the local and 
global maps, using a form obtained by R. Verheyen based on comparisons 
to DIS matrix elements, <ei>P(global) = (sAK - saj)^2/[ (sAK + sjk)^2 + 
(sAK - saj)^2 ] * Theta( sAK - saj )</ei>, with Theta the unit step 
function (since the momenta in the global map always become unphysical 
for saj > sAK). 
 
<p/> 
<modepick name="Vincia:kineMapIF" default="1" min="1" max="3"> 
<option value="1">Local recoil map.</option> 
<option value="2">Gluon emissions use a probabilistic selection 
between the global and local maps. Antennae that only contain 
initial-state singularities always use the global one. Antennae that 
only contain final-state singularities always use the local one.</option> 
<option value="3">Probabilistic selection between the global and 
and local maps, for all IF branchings irrespective of their 
singularity structure.</option> 
</modepick> 
 
<p/> 
When using the probabilistic selection, it is possible (in 
phase-space regions well away from the strict soft and collinear 
limits) that the selected kinematics map produces unphysical momenta 
(with x &gt; 1 or negative energies) for the given branching 
invariants, while the other map would give physical momenta. In such 
cases, one has to choose whether the given phase-space point should be 
vetoed, or whether the other map should be allowed to be used instead 
to construct the kinematics. 
<flag name="Vincia:kineMapIFretry" default="off"> 
<option value="off">If the map selected according to the probabilistic 
choice above returns unphysical momenta, the trial branching is 
vetoed.</option> 
<option value="on">If the map selected according to the probabilistic 
choice above returns unphysical momenta, the other map is tried. Only 
if both maps fail to produce physical momenta is the trial branching 
vetoed.</option> 
</flag> 
 
<h4>Kinematics and Recoils for FF Antennae </h4> 
 
<modepick name="Vincia:kineMapFFemit" default="3" min="1" max="3"> 
Selects which method to use for choosing the Euler angle for the 
global orientation of the post-branching kinematics construction for 
gluon emissions. 
<option value="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> 
<option value="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> 
<option value="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 <ei>r=sij/(sij+sjk)</ei> in the 
definition of the map.)</option> 
</modepick> 
<modepick name="Vincia:kineMapFFsplit" default="2" min="0" max="3"> 
Selects which method to use for choosing the Euler angle for the 
global orientation of the post-branching kinematics construction for 
gluon splittings. 
<option value="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> 
<option value="2">LONGITUDINAL. For gluon splittings, this choice 
forces the recoiler to always recoil purely longitudinally (in the antenna 
CM) regardless of the size of the branching invariants.</option> 
<option value="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 <ei>r=sij/(sij+sjk)</ei> in the 
definition of the map.)</option> 
</modepick> 
 
<h4>Kinematics and Recoils for RF Antennae </h4> 
 
<modepick name="Vincia:kineMapRFemit" default="1" min="1" max="2"> 
There is only one choice of kinematics map for resonance emissions. 
However there is a freedom to choose the recoiler(s).  <option 
value="1">Takes all non-colour-connected daughters in the resonance 
decay system as the recoilers.</option> 
<option value="2">Takes the original non-colour-connected daughter of 
the resonance to always take the full recoil. E.g. in t->bW the 
recoiler is always the W. This is equivalent to setting 
TimeShower:recoilToColoured = off for Pythia.</option> 
</modepick> 
<modepick name="Vincia:kineMapRFsplit" default="1" min="1" max="2"> 
Same as above, but for R-g splittings. 
<option value="1">Takes all non-colour-connected daughters in the 
resonance decay system as the recoilers.</option> 
<option value="2">Takes the original non-colour-connected daughter of 
the resonance to always take the full recoil. E.g. in t->bW the 
recoiler is always the W. This is equivalent to setting 
TimeShower:recoilToColoured = off for Pythia.</option> 
</modepick> 
 
<h3>Interleaved Resonance Decays</h3> 
 
<p/> 
Decays of resonances produced in the hard process can be interleaved 
with the shower evolution (see also further comments under the 
equivalent option for Pythia's 
<aloc href="TimelikeShowers">TimelikeShowers</aloc>). In VINCIA, this 
setting is controlled by the following switch: 
 
<flag name="Vincia:interleaveResDec" default="on"> 
When this flag is set to <code>on</code>, decays of hard-process 
resonances are performed as part of the final-state shower when the 
event evolution reaches the <ei>pT</ei> scale defined by 
<code>VINCIA:resDecScaleChoice</code> below. An internal shower which 
preserves the invariant mass of the resonance is then performed inside 
the resonance-decay system (including nested sequential resonance 
decays) until that system reaches the same overall <ei>pT</ei> scale 
as the rest of the event, at which point the resonance-decay system is 
merged back into the upstream system and the event evolution 
continues.  When this flag is set to <code>off</code>, resonance 
decays are only performed after the shower off the production process 
has been completed. All of the subsequent FSR is carried out inside 
the resonance, with preserved resonance mass.</flag> 
 
<p/> 
<a name="ResonanceDecays"></a> When decays of hard-process resonances are 
interleaved with the shower evolution (and for any resonances produced 
by VINCIA's electroweak shower), the evolution scale at which these 
decays are performed (including showers and any nested resonance 
decays at higher scales) is controlled by the following choice: 
 
<modepick name="Vincia:resDecScaleChoice" default="1" min="0" max="2"> 
<option value="0">Resonance decays are assigned a fixed scale equal to 
the on-shell width of the given resonance. (This option is currently 
not implemented for resonances produced by VINCIA's EW 
shower.)</option> 
<option value="1">Resonance decays are assigned a dynamical scale 
given by <ei>|m<sup>2</sup> - m0<sup>2</sup>|/m0</ei>. This implies, 
e.g., that the decay of a resonance which has <ei>m = m0 &pm; 
&Gamma;</ei> will be performed at a scale <ei>pT ~ sqrt(2) 
&Gamma;</ei>.</option> 
<option value="2">Resonance decays are assigned a dynamical scale 
given by <ei>sqrt(|m<sup>2</sup> - m0<sup>2</sup>|)</ei>. This 
implies, e.g., that the decay of a resonance which has <ei>m = m0 &pm; 
&Gamma;</ei> will be performed at a scale <ei>pT ~ sqrt(2 &Gamma; m0 
)</ei>.</option> 
</modepick> 
 
<h3>Further Shower Settings</h3> 
 
<ul> 
<li>Settings for VINCIA's QCD antenna showers 
are described <aloc href="VinciaQCD">here</aloc>.</li> 
<li>Settings for VINCIA's QED and EW antenna showers 
are described <aloc href="VinciaEW">here</aloc>.</li> 
<li>Settings for VINCIA's matrix-element corrections (MECs) 
are described <aloc href="VinciaMECs">here</aloc>.</li> 
</ul> 
 
<h3>VINCIA Tunes</h3> 
 
<p/> 
<b>VINCIA has its own</b> set of dedicated tune presets, which can be 
specified by the user. Every tune here can then be equivalently loaded with 
the command <code>include = tunes/settings.cmnd</code> where 
<code>settings.cmnd</code> is the corresponding settings file. In the 
future the <code>Vincia:tune</code> setting will be removed in favor 
of the <code>include</code> syntax. 
 
<modepick name="Vincia:Tune" default="-1" min="-1" max="0"> 
<option value="-1">None. No VINCIA-specific tune parameter settings 
will be used during initialisation.</option> 
<option value="0">(<code>tunes/VinciaDefault.cmnd</code>) Hadronization 
and MPI parameters optimised for use with the VINCIA shower model, 
used as default VINCIA parameters since PYTHIA 8.302.</option> 
</modepick> 
 
<p/> 
<b>Note 2:</b> as with ordinary Pythia tune parameters, the tuned 
parameter values will be superseded by any user modifications made in 
the user's command file or main program. This should allow sufficient 
flexibility to explore user variations away from the tuned values. 
 
<h4>Advice on Tuning</h4> 
 
<p/> 
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 the tune file. This implies 
some responsibility on the part of the user. 
 
<p/> 
<b>As a guideline,</b> 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. 
 
<p/> 
The strategy used for the default tune of VINCIA is to take the 
reference value for alphaS from the current world average value in the 
MSbar scheme, and let the effective shower scheme tuning be done by 
first translating to the CMW scheme and then fine-tune by modifying 
the renormalisation-scale prefactors used for shower branchings. 
 
<p/> 
An alternative (but equivalent) strategy that is often used in PYTHIA 
tunes, is to perceive of the value of the strong coupling itself as a 
tuning parameter.  In this case the interpretation is that extracting 
alphaS from, e.g., event shapes, can be done equally well using a 
shower code as with more analytical approaches. The difference is that 
the alphaS value extracted with the shower code is in an a priori 
unknown scheme, defined by the shower algorithm. If the shower only 
includes LO/LL accuracy for the given observable(s), the extraction 
should be compared with other LO/LL extractions. This typically yields 
alphaS values ~ 0.13 - 0.14. When explicit NLO corrections are 
included for the relevant observable(s), values comparable to other 
NLO extractions should result, around 0.12. 
 
<h3>Expert Settings</h3> 
 
<b>Importante Note:</b> the parameters in this section are intended 
for use only by authors or expert users. 
 
<h4>Octet Partitioning</h4> 
 
<p/> 
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), 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.  For 
sector showers, this parameter has no effect, as the collinear 
singularities are combined within a single antenna function. 
 
<parm name="Vincia:octetPartitioning" default="0.0" min="0.0" max="1.0"> 
Gluon-collinear &alpha; 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: &alpha;=0, which corresponds to the 
Gehrmann-Gehrmann-de Ridder-Glover (GGG) partitioning, and &alpha;=1, 
which corresponds to the Gustafson (ARIADNE) partitioning. For the 
sector shower, the <code>octetPartitioning</code> is forced to 
&alpha;=1, to ensure positivity of the antenna function and as there 
is no sensible meaning of the parameter anymore.</parm> 
 
<h4>Debugging</h4> 
<flag name="Vincia:debug" default="off"> 
This flag is solely intended to assist authors for debugging purposes. 
</flag> 
 
<h4>Numerical Checks</h4> 
 
<flag name="Vincia:CheckAntennae" default="on"> 
By default, VINCIA checks antenna functions for positivity and absence 
of dead zones. Switch to control whether to perform antenna 
self-consistency checks or not. 
</flag> 
 
<modeopen name="Vincia:nPointsCheck" default="1000" min="0" max="1e6"> 
Number of random points to check each antenna functions for 
positivity. 
</modeopen> 
 
<parm name="Vincia:deadZoneAvoidance" default="0.0001" min="0.0" max="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.</parm> 
 
<h4>Shower Uncertainty Bands</h4> 
 
Automated uncertainty bands are not available in this version of 
the VINCIA code. 
 
<h3>References</h3> 
 
<p/> 
<b>The main references</b> for the current version of VINCIA are: 
<ul> 
<li><a href="http://arxiv.org/abs/arXiv:2003.00702">Sector Showers 
for Hadron Collisions</a> (for Sector Showers).</li> 
<li><a href="http://arxiv.org/abs/arXiv:1907.08980">Coherent Showers 
in Decays of Coloured Resonances</a> (for showers in top quark 
decays).</li> 
<li><a href="http://arxiv.org/abs/arXiv:2002.04939">Multipole Photon 
Radiation in the VINCIA Parton Shower</a> (for QED showers).</li> 
<li><a href="https://arxiv.org/abs/2002.09248">Collinear electroweak 
radiation in antenna parton showers</a> (for EW showers).</li> 
<li><a href="http://arxiv.org/abs/arXiv:2008.09468">Efficient 
Multi-Jet Merging with the VINCIA Sector Shower</a> (for MESS 
Merging).</li> 
<li><a href="https://arxiv.org/abs/2106.10987">Coherent QCD Radiation 
in Vector-Boson-Fusion Higgs Production</a> (for POWHEG matching).</li> 
</ul> 
 
<h4>Authors and Contributions</h4> 
 
<p/> 
The authors of the current version, taking active responsibility for 
code development and day-to-day maintenance, are 
<ul> 
<li>P. Skands: Main author</li> 
<li>C. T. Preuss: Sector Showers, Matching and Merging, MG5 Interface.</li> 
<li>R. Verheyen: QED showers and EW showers.</li> 
</ul> 
 
<p/> 
In addition, the following people have made significant individual 
contributions to the underlying formalism and code development of 
VINCIA: 
<ul> 
<li>H. Brooks: Resonance-decay showers and Sector Merging.</li> 
<li>W. T. Giele and D. A. Kosower: Basics of the antenna shower 
formalism.</li> 
<li>L. Hartgring and E. Laenen: on NLO corrections to Z + 3 jets.</li> 
<li>H.-T. Li: on 2&rarr;4 branchings and one-loop corrections to FF 
shower kernels.</li> 
<li>A. Larkoski: on helicity-based final-state showers and matrix 
elements.</li> 
<li>A. Lifson: Analytical MHV amplitudes.</li> 
<li>N. Fischer: ISR and MECs for hadron collisions.</li> 
<li>J. J. Lopez-Villarejo: Sector showers.</li> 
<li>S. Prestel: Merging and strongly-ordered MECs.</li> 
<li>M. Ritzmann: Mass corrections and initial-state antenna showers.</li> 
<li>T. Sj&ouml;strand: Integration with PYTHIA 8.</li> 
</ul> 
 
</chapter> 
 
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