Here, parameters specific to VINCIA's QCD antenna shower are collected. See the main VINCIA antenna shower page for more general parameters that are common to both the QCD and QED showers.
flag
Vincia:doII
(default = on
)PartonLevel
settings).
flag
Vincia:doIF
(default = on
)PartonLevel
settings).
Note: setting this to off will switch off both the initial- and final-state
ends of corresponding QCD antennae.
flag
Vincia:doFF
(default = on
)PartonLevel
).
flag
Vincia:doRF
(default = on
)PartonLevel
settings).
Note: setting this to off will switch off both the resonance- and
final-state ends of corresponding QCD antennae.
mode
Vincia:nGluonToQuark
(default = 5
; minimum = 0
; maximum = 6
)flag
Vincia:convertGluonToQuark
(default = on
)flag
Vincia:convertQuarkToGluon
(default = on
)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).
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.
VINCIA implements its own instance of PYTHIA's AlphaStrong
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.
The free parameter of the strong coupling constant is specified by
parm
Vincia:alphaSvalue
(default = 0.118
; minimum = 0.06
; maximum = 2.0
)mode
Vincia:alphaSorder
(default = 2
; minimum = 0
; maximum = 2
)option
0 : zeroth order, i.e. αs is kept
fixed.
option
1 : first order, i.e., one-loop running.
option
2 : second order, i.e., two-loop running.
Resummation arguments [Cat91] 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 Lambda_QCD, defined by
flag
Vincia:useCMW
(default = on
)
Note 1: 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.
Note 2: Tunes using this option need roughly 10% lower values of
alphas(mZ) than tunes that do not.
For both one- and two-loop running, the AlphaStrong
class
automatically switches from 3-, to 4-, and then to 5-flavour running as
one passes the s, c, and b thresholds,
respectively, with matching equations imposed at each flavour
treshold to ensure continuous values.
By default, a change to 6-flavour running is also included above the
t threshold, though this can be disabled using the following
parameter:
mode
Vincia:alphaSnfmax
(default = 6
; minimum = 5
; maximum = 6
)option
5 : Use 5-flavour running for all scales above the
b
flavour threshold (old default).
option
6 : Use 6-flavour running above the t
threshold (new default).
parm
Vincia:alphaSmuFreeze
(default = 0.75
; minimum = 0.0
; maximum = 10.0
)parm
Vincia:alphaSmax
(default = 2.0
; minimum = 0.1
; maximum = 10.0
)
When Vincia:alphaSorder
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
kR*Q2, at which the shower evaluates
αs, with Q2 the
Vincia evolution scale of the branching.
The multiplicative scale factor kR is given by
parm
Vincia:renormMultFacEmitF
(default = 0.66
; minimum = 0.1
; maximum = 10.0
)parm
Vincia:renormMultFacSplitF
(default = 0.8
; minimum = 0.1
; maximum = 10.0
)For initial-state branchings, the functional form of muR is given by the evolution variable and the scale factor kR is given by
parm
Vincia:renormMultFacEmitI
(default = 0.66
; minimum = 0.1
; maximum = 10.0
)parm
Vincia:renormMultFacSplitI
(default = 0.5
; minimum = 0.1
; maximum = 10.0
)parm
Vincia:renormMultFacConvI
(default = 0.5
; minimum = 0.1
; maximum = 10.0
)The normalisation of colour factors in VINCIA is chosen such that the coupling factor for all antenna functions is αS/4π. 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 → qgqbar antenna function is 2CF.)
parm
Vincia:QQemitII:chargeFactor
(default = 2.66666667
)parm
Vincia:GQemitII:chargeFactor
(default = 2.83333333
)parm
Vincia:GGemitII:chargeFactor
(default = 3.0
)parm
Vincia:QXSplitII:chargeFactor
(default = 1.0
)parm
Vincia:GXConvII:chargeFactor
(default = 2.66666667
)parm
Vincia:QQemitIF:chargeFactor
(default = 2.66666667
)parm
Vincia:GQemitIF:chargeFactor
(default = 2.83333333
)parm
Vincia:QGemitIF:chargeFactor
(default = 2.83333333
)parm
Vincia:GGemitIF:chargeFactor
(default = 3.0
)parm
Vincia:QXSplitIF:chargeFactor
(default = 1.0
)parm
Vincia:GXConvIF:chargeFactor
(default = 2.66666667
)parm
Vincia:XGSplitIF:chargeFactor
(default = 1.0
)parm
Vincia:QQEmitFF:chargeFactor
(default = 2.66666667
)parm
Vincia:QGEmitFF:chargeFactor
(default = 2.85
)parm
Vincia:GGEmitFF:chargeFactor
(default = 3.0
)parm
Vincia:QGSplitFF:chargeFactor
(default = 1.0
)parm
Vincia:GGSplitFF:chargeFactor
(default = 1.0
)parm
Vincia:GXSplitFF:chargeFactor
(default = 1.0
)parm
Vincia:QQEmitRF:chargeFactor
(default = 2.66666667
)parm
Vincia:QGEmitRF:chargeFactor
(default = 2.85
)parm
Vincia:XGSplitRF:chargeFactor
(default = 1.0
)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 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).
The post-branching momenta are fixed by the following requirements:
1) The direction of the initial state partons is aligned with the beam axis
(z-axis).
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.
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.
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.
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 < 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, P(global) = (sAK - saj)^2/[ (sAK + sjk)^2 + (sAK - saj)^2 ] * Theta( sAK - saj ), with Theta the unit step function (since the momenta in the global map always become unphysical for saj > sAK).
mode
Vincia:kineMapIF
(default = 1
; minimum = 1
; maximum = 3
)option
1 : Local recoil map.
option
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
3 : Probabilistic selection between the global and
and local maps, for all IF branchings irrespective of their
singularity structure.
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 > 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
Vincia:kineMapIFretry
(default = off
)option
off : If the map selected according to the probabilistic
choice above returns unphysical momenta, the trial branching is
vetoed.
option
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.
mode
Vincia:kineMapFFemit
(default = 3
; minimum = 1
; maximum = 3
)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.)
mode
Vincia:kineMapFFsplit
(default = 2
; minimum = 0
; maximum = 3
)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. 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
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.)
mode
Vincia:kineMapRFemit
(default = 1
; minimum = 1
; maximum = 2
)option
1 : Takes all non-colour-connected daughters in the
resonance decay system as the recoilers.
option
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.
mode
Vincia:kineMapRFsplit
(default = 1
; minimum = 1
; maximum = 2
)option
1 : Takes all non-colour-connected daughters in the
resonance decay system as the recoilers.
option
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.
The hadronisation cutoff, 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 fundmanentally unresolvable and are in effect inclusively summed over.
Important Note: when hadronisation is switched on, there is a delicate interplay between the hadronisation scale used in the shower and the parameters of the hadronisation model. Ideally, the parameters of the hadronisation model should scale as a function of the shower cutoff. This scaling does not happen automatically in current hadronisation models, such as the string model employed by PYTHIA. Instead, the parameters of the hadronisation model are tuned for one specific shower setting at a time and should be retuned if changes are made to the shower cutoff.
parm
Vincia:cutoffScaleFF
(default = 0.75
; minimum = 0.1
; maximum = 10.0
)parm
Vincia:cutoffScaleII
(default = 1.25
; minimum = 0.1
; maximum = 10.0
)parm
Vincia:cutoffScaleIF
(default = 1.5
; minimum = 0.1
; maximum = 10.0
)parm
Vincia:ThresholdMB
(default = 4.8
)parm
Vincia:ThresholdMC
(default = 1.5
)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 CA 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 2CF, hence the difference is explicitly subleading in colour). A more realistic starting point is to use 2CF for quark-antiquark antennae, CA 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:
mode
Vincia:modeSLC
(default = 2
; minimum = 0
; maximum = 3
)option
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
1 : Simple Colour Factors. The chargeFactor
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
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
2CF * (1-yij)/(2-yij-yjk) + CA * (1-yjk)/(2-yij-yjk), 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
3 : Only used for development purposes.
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:
mode
Vincia:CRinheritMode
(default = 1
; minimum = -2
; maximum = 2
)option
0 : Random
option
1 : The daughter with the largest invariant mass has
a probability 1/(1 + r) to inherit the parent tag, with
r < 1 the ratio of the smallest to the largest daughter
invariant masses squared.
option
2 : The daughter with the largest invariant mass
always inherits the parent tag (winner-takes-all extreme variant of
option 1).
option
-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
-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.