<chapter name="VINCIA QCD Antenna Shower Settings"> 
<h2>VINCIA QCD Antenna Shower Settings</h2> 
 
<p/> 
Here, parameters specific to VINCIA's QCD antenna 
shower are collected. See the <aloc href="Vincia">main VINCIA 
antenna shower page</aloc> for more general parameters that are 
common to both the QCD and QED showers. 
 
<h3>Main Switches</h3> 
 
<flag name="Vincia:doII" default="on"> 
Main switch for initial-initial (II) antennae on/off (subject to 
the <code>PartonLevel</code> settings).</flag> 
 
<flag name="Vincia:doIF" default="on"> 
Main switch for initial-final (IF) antennae on/off 
(subject to the <code>PartonLevel</code> settings). 
Note: setting this to off will switch off both the initial- and final-state 
ends of corresponding QCD antennae.</flag> 
 
<flag name="Vincia:doFF" default="on"> 
Main switch for final-final (FF) antennae on/off (subject to choices 
made at <code>PartonLevel</code>).</flag> 
 
<flag name="Vincia:doRF" default="on"> 
Main switch for resonance-final (RF) antennae on/off 
(subject to the <code>PartonLevel</code> settings). 
Note: setting this to off will switch off both the resonance- and 
final-state ends of corresponding QCD antennae.</flag> 
 
<modepick name="Vincia:nGluonToQuark" default="5" min="0" max="6"> 
Number of allowed quark flavours in final-state gluon splittings, 
<ei>g &rarr; q qbar</ei>, during the shower evolution, phase space 
permitting. E.g.,  a change to 4 would exclude <ei>g &rarr; b 
bbar</ei> but would include the lighter quarks, etc. Note that this 
parameter does not directly affect the running coupling.</modepick> 
 
<flag name="Vincia:convertGluonToQuark" default="on"> 
Allow incoming gluons to backwards-evolve into (anti)quarks 
during the initial-state shower evolution.</flag> 
 
<flag name="Vincia:convertQuarkToGluon" default="on"> 
Allow incoming (anti)quarks to backwards-evolve into (anti)quarks 
during the initial-state shower evolution.</flag> 
 
<a name="alphaS"></a> 
<h3>The QCD coupling in the Vincia Shower</h3> 
 
<p/> 
The strong coupling constant is specified by providing 
its reference value (interpreted as given at the Z pole in 
the MSbar scheme) and running properties (loop order, behaviour at top 
threshold, and any low-scale regularisation/dampening). 
 
<p/> 
Note that VINCIA only uses one global value for the definition of 
the strong coupling constant. The effective couplings used in shower 
branchings (renormalisation scheme and scale) are governed by separate 
parameters which are specified under initial- and final-state showers 
respectively. 
 
<p/> 
VINCIA implements its own instance of PYTHIA's <code>AlphaStrong</code> class 
for the strong coupling. You can find more documentation of the class in 
the section on Standard-Model Parameters in the PYTHIA documentation. 
Here, we list the specific parameters and switches governing its use 
in VINCIA. 
 
<a name="ref"></a> 
<p/> 
The free parameter of the strong coupling constant is specified by 
 
<parm name="Vincia:alphaSvalue" default="0.118" 
min="0.06" max="2.0"> 
The value of <ei>&alpha;<sub>s</sub></ei> at 
the scale <ei>m<sub>Z</sub></ei>, in the MSbar scheme. The default 
value is chosen to be in 
agreement with the current world average. The effective value used for 
showers may be further affected by translation to the CMW scheme 
(below) and by renormalisation-scale prefactors given for FSR and ISR 
showers separately.</parm> 
 
<a name="order"></a> 
<modepick name="Vincia:alphaSorder" default="2" min="0" max="2"> 
Order at which <ei>&alpha;<sub>s</sub></ei> runs, 
<option value="0">zeroth order, i.e. <ei>&alpha;<sub>s</sub></ei> is kept 
fixed.</option> 
<option value="1">first order, i.e., one-loop running. 
</option> 
<option value="2">second order, i.e., two-loop running. 
</option> 
</modepick> 
 
<a name="schemeFSR"></a> 
<p/> 
Resummation arguments <ref>Cat91</ref> indicate that a set of 
universal QCD corrections can be absorbed in coherent parton showers by 
applying the so-called CMW rescaling of the MSbar value 
of <ei>Lambda_QCD</ei>, defined by 
<center> 
  <ei>&alpha;<sub>s</sub>(CMW) = &alpha;<sub>s</sub>(MSbar) 
  * (1 + K * &alpha;<sub>s</sub>(MSbar) / 2&pi;)</ei> 
</center> 
with <ei>K = CA * (67/18 - &pi;<sup>2</sup>/6) - 5/9nf</ei>. The 
translation amounts to an <ei>NF</ei>-dependent 
rescaling of <ei>Lambda_QCD</ei>, relative to its MSbar value, by 
a factor 1.661 for NF=3, 1.618 for NF=4, 1.569 for NF=5, 
and 1.513 for NF=6. Although the original argument strictly concerned 
only the eikonal for soft-gluon emissions, the current version of 
VINCIA only offers the option of switching the rescaling of 
<ei>Lambda_QCD</ei> on or off. When on, the rescaling is applied to 
all branching types, not just gluon emissions. 
 
<p/> 
<flag name="Vincia:useCMW" default="on"> 
If set to on, the alphaS value used for shower branchings will be 
translated from the MSbar value to the CMW ("MC") scheme. If set to 
off, the MSbar value will be used.</flag> 
 
<p/> 
<b>Note 1:</b> If using VINCIA with an externally defined matching scheme, be 
aware 
that the CMW rescaling may need be taken into account in the context of 
matrix-element matching. Note also that this option has only been made 
available for timelike and spacelike showers, not for hard processes. <br/> 
<b>Note 2:</b> Tunes using this option need roughly 10% lower values of 
<ei>alpha<sub>s</sub>(m<sub>Z</sub>)</ei> than tunes that do not. 
 
<a name="thresholds"></a> 
<p/> 
For both one- and two-loop running, the <code>AlphaStrong</code> class 
automatically switches from 3-, to 4-, and then to 5-flavour running as 
one passes the <ei>s</ei>, <ei>c</ei>, and <ei>b</ei> thresholds, 
respectively, with matching equations imposed at each flavour 
threshold to ensure continuous values. 
By default, a change to 6-flavour running is also included above the 
<ei>t</ei> threshold, though this can be disabled using the following 
parameter: 
<modepick name="Vincia:alphaSnfmax" default="6" min="5" max="6"> 
<option value="5">Use 5-flavour running for all scales above the 
<ei>b</ei> 
flavour threshold (old default).</option> 
<option value="6">Use 6-flavour running above the <ei>t</ei> 
threshold (new default).</option> 
</modepick> 
 
<a name="ir"></a> 
<parm name="Vincia:alphaSmuFreeze" default="0.75" min="0.0" max="10.0"> 
  The behaviour of the running coupling in the far infrared is 
  regulated by a shift in the effective renormalisation scale, 
  to <ei>&mu;<sub>eff</sub> 
<sup>2</sup> = &mu;<sub>freeze</sub><sup>2</sup> 
   + &mu;<sub>R</sub><sup>2</sup></ei>.</parm> 
 
<a name="maxCoup"></a> 
<parm name="Vincia:alphaSmax" default="2.0" min="0.1" max="10.0"> 
Largest allowed numerical value for <ei>alphaS</ei>. I.e., the running 
is forced to plateau at this value.</parm> 
 
<a name="scaleFSR"></a> 
 
<h4>Choice of Renormalisation Scales for Shower Branchings</h4> 
<p/> 
When <code>Vincia:alphaSorder</code> is non-zero, 
the actual value of alphaS used for shower branchings is governed by 
the choice of scheme (MSbar or CMW, see the section on AlphaStrong 
and then by running to the scale 
<ei>kR*Q<sup>2</sup></ei>, at which the shower evaluates 
<ei>&alpha;<sub>s</sub></ei>, with <ei>Q<sup>2</sup></ei> the 
Vincia evolution scale of the branching. 
 
The multiplicative scale factor <ei>kR</ei> is given by 
<parm name="Vincia:renormMultFacEmitF" default="0.66" min="0.1" max="10.0"> 
for gluon emission</parm> 
 
<p/> 
and 
<parm name="Vincia:renormMultFacSplitF" default="0.8" min="0.1" max="10.0"> 
for gluon splitting.</parm> 
 
<a name="scaleISR"></a> 
<p/> 
For initial-state branchings, the functional form of <ei>muR</ei> is given by 
the evolution variable and the scale factor <ei>kR</ei> is given by 
<parm name="Vincia:renormMultFacEmitI" default="0.66" min="0.1" max="10.0"> 
for gluon emission,</parm> 
<parm name="Vincia:renormMultFacSplitI" default="0.5" min="0.1" max="10.0"> 
for gluon splitting (quark in the initial state backwards evolving into a 
gluon),</parm> 
<parm name="Vincia:renormMultFacConvI" default="0.5" min="0.1" max="10.0"> 
for gluon conversion (gluon in the initial state backwards evolving into a 
(anti)quark)</parm> 
 
<h3>Colour Charges</h3> 
<p/> 
The normalisation of colour factors in VINCIA is chosen such that 
the coupling factor for all antenna functions is &alpha;S/4&pi;. With 
this normalisation, 
all gluon-emission colour factors tend to NC in the large-NC limit 
while all gluon-splitting colour factors tend to unity. (Thus, e.g., 
the default normalisation of the qqbar &rarr; qgqbar antenna function 
is 2CF.) 
 
<p/> 
<parm name="Vincia:QQEmitII:chargeFactor" default="2.66666667"> 
Emission of a final-state gluon from an initial-state qqbar pair. 
</parm> 
<parm name="Vincia:GQEmitII:chargeFactor" default="2.83333333"> 
Emission of a final-state gluon from an initial-state qg (or gqbar) 
pair.</parm> 
<parm name="Vincia:GGEmitII:chargeFactor" default="3.0"> 
Emission of a final-state gluon from an initial-state gg pair.</parm> 
<parm name="Vincia:QXConvII:chargeFactor" default="1.0"> 
Quark in the initial state backwards evolving into a gluon and emitting 
an antiquark in the final state.</parm> 
<parm name="Vincia:GXConvII:chargeFactor" default="2.66666667"> 
Gluon in the initial state backwards evolving into a quark and emitting 
a quark in the final state (gluon conversion).</parm> 
<parm name="Vincia:QQEmitIF:chargeFactor" default="2.66666667"> 
Gluon emission of an initial-final qq pair.</parm> 
<parm name="Vincia:GQEmitIF:chargeFactor" default="2.83333333"> 
Gluon emission off an initial-final gq pair.</parm> 
<parm name="Vincia:QGEmitIF:chargeFactor" default="2.83333333"> 
Gluon emission of an initial-final qg pair.</parm> 
<parm name="Vincia:GGEmitIF:chargeFactor" default="3.0"> 
Gluon emission of an initial-final gg pair.</parm> 
<parm name="Vincia:QXConvIF:chargeFactor" default="1.0"> 
Quark in the initial state evolving backwards into a gluon and emitting 
an antiquark in the final state.</parm> 
<parm name="Vincia:GXConvIF:chargeFactor" default="2.66666667"> 
Gluon in the initial state backwards evolving into a quark and emitting 
a quark into the final state (gluon conversion).</parm> 
<parm name="Vincia:XGSplitIF:chargeFactor" default="1.0"> 
Gluon splitting in the final state.</parm> 
<parm name="Vincia:QQEmitFF:chargeFactor" default="2.66666667"/> 
<parm name="Vincia:QGEmitFF:chargeFactor" default="2.85"/> 
<parm name="Vincia:GGEmitFF:chargeFactor" default="3.0"/> 
<parm name="Vincia:QGSplitFF:chargeFactor" default="1.0"/> 
<parm name="Vincia:GGSplitFF:chargeFactor" default="1.0"/> 
<parm name="Vincia:GXSplitFF:chargeFactor" default="1.0"/> 
<parm name="Vincia:QQEmitRF:chargeFactor" default="2.66666667"/> 
<parm name="Vincia:QGEmitRF:chargeFactor" default="2.85"> 
<parm name="Vincia:XGSplitRF:chargeFactor" default="1.0"/> 
Note: the two permutations g-g &rarr; g-q+qbar and g-g &rarr; qbar+q-g are 
explicitly summed over in the code (with  appropriate swapping of 
invariants in the latter case). 
</parm> 
 
<h3>Lower Cutoffs for the QCD evolution</h3> 
 
<p/> 
<b>The hadronization cutoff</b>, a.k.a. the infrared regularisation 
scale, defines the resolution scale at which the perturbative shower 
evolution is stopped. Thus, perturbative emissions below this scale 
are treated as fundamentally unresolvable and are in effect 
inclusively summed over. 
 
<p/> 
<b>Important Note:</b> 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 scaling does not happen automatically in current 
hadronization models, such as the string model employed by 
PYTHIA. Instead, the parameters of the hadronization model are tuned 
for one specific shower setting at a time and should be retuned if 
changes are made to the shower cutoff. 
 
<parm name="Vincia:cutoffScaleFF" default="0.75" min="0.1" max="10.0"> 
This parameter sets the value (in GeV) of the final-state shower 
cutoff.</parm> 
<parm name="Vincia:cutoffScaleII" default="1.25" min="0.1" max="10.0"> 
This parameter sets the value (in GeV) of the shower cutoff for 
initial-initial antennae.</parm> 
<parm name="Vincia:cutoffScaleIF" default="1.5" min="0.1" max="10.0"> 
This parameter sets the value (in GeV) of the shower cutoff for 
initial-final antennae.</parm> 
 
<parm name="Vincia:ThresholdMB" default="4.8">Minimum 
threshold (mass, in GeV) for bottom quark production.</parm> 
 
<parm  name="Vincia:ThresholdMC" default="1.5">Minimum 
threshold (mass, in GeV) for charm quark production.</parm> 
 
<h3>Other QCD Settings</h3> 
 
<h4>Subleading Colour</h4> 
<p/> 
During the perturbative shower evolution, the first aspect of 
subleading colour is simply what colour factors are used for the 
antenna functions. 
In a strict leading-colour limit, one would use <ei>CA</ei> for all antennae, 
thus overestimating the amount of radiation from quarks 
(note that we use a normalisation convention in which the colour factor 
for quarks is <ei>2CF</ei>, 
hence the difference is explicitly subleading in colour). 
A more realistic starting point is to use <ei>2CF</ei> for quark-antiquark 
antennae, <ei>CA</ei> for gluon-gluon ones, and something inbetween for 
quark-gluon ones.  The following switch 
determines whether and how subleading-colour corrections are treated 
in the evolution: 
 
<modepick name="Vincia:modeSLC" default="2" min="0" max="3"> 
<option value="0">Strict LC evolution. All gluon-emission colour 
factors are forced equal to CA thus overcounting the radiation from 
quarks. Note that matrix-element corrections will still generate 
corrections to the evolution up to the matched number of legs.</option> 
<option value="1">Simple Colour Factors. The <code>chargeFactor</code> 
parameters for each of the antenna functions are used to set the 
colour factor for each type of gluon-emission antenna; see the section 
on antenna functions. (Typically, 2CF for qqbar antennae, CA for gg 
antennae, and the average of 2CF and CA for qg antennae.)</option> 
<option value="2">Interpolating Colour Factors. The colour factor 
for quark-antiquark antennae is forced equal to 2CF. 
Gluon-gluon antennae are normalised to CA. 
The colour factor for QG antennae is 
<ei>2CF * (1-yij)/(2-yij-yjk) + CA * (1-yjk)/(2-yij-yjk)</ei>, which is 
just a simple interpolation between CA in the gluon-collinear limit 
and 2CF in the quark-collinear limit. More sophisticated choices 
could also be motivated and may be interesting to explore in future 
versions.</option> 
<option value="3">Only used for development purposes.</option> 
</modepick> 
 
<p/> 
Colour flow is traced using Les-Houches style colour tags, augmented 
by letting the last digit encode the "colour index", running from 1 to 
9, described further in the section below on antenna swing. One 
ambiguity arises in gluon emission as to which of the daughter 
antennae should inherit the "parent" colour tag/index, and which 
should be assigned a new one. This is controlled by the following parameter: 
 
<modepick name="Vincia:CRinheritMode" default="1" min="-2" max="2"> 
<option value="0">Random</option> 
<option value="1">The daughter with the largest invariant mass has 
a probability <em>1/(1 + r)</em> to inherit the parent tag, with 
<em>r &lt; 1</em> the ratio of the smallest to the largest daughter 
invariant masses squared.</option> 
<option value="2">The daughter with the largest invariant mass 
always inherits the parent tag (winner-takes-all extreme variant of 
option 1).</option> 
<option value="-1">(Unphysical, intended for theory-level studies 
only). Inverted variant of option 1, so that the 
daughter with the smallest invariant mass preferentially inherits 
the parent colour tag.</option> 
<option value="-2">(Unphysical, intended for theory-level studies 
only). Inverted variant of option 2, so that the daughter with the 
smallest invariant mas always inherits the parent colour tag.</option> 
</modepick> 
 
 
<h3>Evolution with Enhanced (Biased) Kernels</h3> 
 
<p/> 
VINCIA's shower evolution can be biased to populate the multi-jet 
phase space more efficiently and/or enhance the rate of rare processes 
such as <ei>g&rarr;bb</ei> and <ei>g&rarr;cc</ei> splittings. It is 
also possible to inhibit radiation (e.g., to focus on Sudakov 
regions), by choosing enhancement factors smaller than unity. When 
these options are used, it is important to note that the event weights 
will be modified, reflecting that some types of events (e.g., multijet 
events, or events with gluon splittings to heavy quarks) will be 
"overrepresented" statistically, and others (events with few jets, or 
events with no gluon splittings to heavy quarks) 
underrepresented. Averages and histograms will therefore only be 
correct if computed using the correct weight for each generated 
event. A description and proof of the algorithm can be found in 
<ref>MS16</ref>. Note that care has been taken to ensure that the 
weights remain positive definite; under normal circumstances, VINCIA's 
enhancement algorithm should not result in any negative weights. 
 
<flag name="Vincia:enhanceInHardProcess" default="on"> 
This flag controls whether the enhancement factors are applied to 
shower branchings in the hard-process system.</flag> 
 
<flag name="Vincia:enhanceInResonanceDecays" default="on"> 
This flag controls whether the enhancement factors are applied to 
shower branchings inside resonance-decay systems (like Z/W/H decays) 
that are treated as factorised from the hard process.</flag> 
 
<flag name="Vincia:enhanceInMPIshowers" default="off"> 
This flag controls whether the enhancement factors are applied to 
shower branchings in MPI systems.</flag> 
 
<parm name="Vincia:enhanceFacAll" default="1.0" min="0.01" max="100.0"> 
This enhancement factor is applied as a multiplicative factor common 
to all antenna functions, increasing the likelihood of all shower 
branchings by the same amount. Values greater than unity thus more 
frequently yields "busy" events, with many shower branchings. Values 
smaller than unity suppress additional branchings, yielding more 
Sudakov-like events.</parm> 
 
<parm name="Vincia:enhanceFacBottom" default="1.0" min="1.0" max="100.0"> 
This enhances the probability for all branchings that increase the 
number of bottom quarks (i.e., FSR <ei>g&rarr;bb</ei> splittings and 
the corresponding ISR flavour-excitation process). Note: this factor 
is applied on top of <code>Vincia:biasAll</code>.</parm> 
 
<parm name="Vincia:enhanceFacCharm" default="1.0" min="1.0" max="100.0"> 
Same as <code>Vincia:enhanceFacBottom</code> but for charm quarks. 
Note: this factor is applied on top of <code>Vincia:biasAll</code>.</parm> 
 
<parm name="Vincia:enhanceCutoff" default="10.0" min="0.0" max="1000.0"> 
Do not apply enhancement factors to branchings below this 
scale. Intended to be used to focus on enhancements of hard branchings 
only.</parm> 
 
</chapter> 
 
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