The Antenna Shower Model (VINCIA)
- Overview
- Main Switches
- Shower Starting Scales
- Further Shower Settings
- Merging with VINCIA
- Interleaved Resonance Decays
- Evolution with Enhanced (Biased) Kernels
- VINCIA Tunes
- Expert Settings
- References
Overview
The cornerstone of VINCIA is a pT-ordered model for QCD + EW/QED
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.
QCD: 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.
QED:
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.
EW: 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, since
this module is based on collinar approximations, it does not exhibit
the full coherence of the pure-QED shower module.
Mass Effects: 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.
When VINCIA's option for interleaved resonance decays is selected (see below), 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 sequentially, depending on the
offshellnesses involved.
Interleaved Evolution and MPI: 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.
Main Switches
mode
Vincia:nFlavZeroMass
(default = 4
; minimum = 2
; maximum = 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→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).
flag
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
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.
parm
Vincia:sectorDamp
(default = 1.0
; minimum = 0.0
; maximum = 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
yijSym = yik = 1 - yij - yjk. If the swapped gluons are j and
k (and straightforwardly generalised if they are i and j) then the
collinear yjk→0 limit does not change by adding or
subtracting a term of order yjk. Therefore one could equally well use
yijSym = 1 - yij (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 yijSym 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
sectorDamp
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.
Shower Starting Scales
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
scale
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 SpaceShower:PTmaxMatch
option in PYTHIA.
mode
Vincia:pTmaxMatch
(default = 0
; minimum = 0
; maximum = 2
)
option
0 : Showers off processes that include at least one
final-state quark, gluon, or photon, are started at the factorisation
scale, while processes that do not include any such partons are
started at the phase-space maximum.
option
1 : Showers are always started at the factorisation
scale.
option
2 : Showers are always started at the phase-space
maximum. This option is not recommended for physics runs as it will
lead to unphysical double counting in many cases.
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
Vincia:pTmaxFudge
(default = 1.0
; minimum = 0.1
; maximum = 10.0
)
parm
Vincia:pTmaxFudgeMPI
(default = 1.0
; minimum = 0.1
; maximum = 10.0
)
Same as above but for MPI systems, affecting the underlying event.
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.
Further Shower Settings
- Settings for VINCIA's QCD antenna showers
are described here.
- Settings for VINCIA's QED and EW antenna showers
are described here.
- Settings for VINCIA's matrix-element corrections (MECs)
are described here.
Merging with VINCIA
VINCIA has its own dedicated approach to merging with
higher-multiplicity tree-level matrix elements, called sector
merging. Similar in spirit to the CKKW(-L) approaches, it exploits the
unique bijective property 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
sector merging are described here.
Interleaved Resonance Decays
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 TimelikeShowers).
In VINCIA, this setting is
controlled by the following switch:
flag
Vincia:interleaveResDec
(default = on
)
When this flag is set to on
, decays of hard-process
resonances are performed as part of the final-state shower when the
event evolution reaches the pT scale defined by
VINCIA:resDecScaleChoice
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 pT 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 off
, 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.
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:
mode
Vincia:resDecScaleChoice
(default = 1
; minimum = 0
; maximum = 2
)
option
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
1 : Resonance decays are assigned a dynamical scale
given by |m2 - m02|/m0. This implies,
e.g., that the decay of a resonance which has m = m0 ±
Γ will be performed at a scale pT ~ sqrt(2)
Γ.
option
2 : Resonance decays are assigned a dynamical scale
given by sqrt(|m2 - m02|). This
implies, e.g., that the decay of a resonance which has m = m0 ±
Γ will be performed at a scale pT ~ sqrt(2 Γ m0
).
Evolution with Enhanced (Biased) Kernels
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 g→bb and g→cc 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
[MS16]. 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
Vincia:enhanceInHardProcess
(default = on
)
This flag controls whether the enhancement factors are applied to
shower branchings in the hard-process system.
flag
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
Vincia:enhanceInMPIshowers
(default = off
)
This flag controls whether the enhancement factors are applied to
shower branchings in MPI systems.
parm
Vincia:enhanceFacAll
(default = 1.0
; minimum = 0.01
; maximum = 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
Vincia:enhanceFacBottom
(default = 1.0
; minimum = 1.0
; maximum = 100.0
)
This enhances the probability for all branchings that increase the
number of bottom quarks (i.e., FSR g→bb splittings and
the corresponding ISR flavour-excitation process). Note: this factor
is applied on top of Vincia:biasAll
.
parm
Vincia:enhanceFacCharm
(default = 1.0
; minimum = 1.0
; maximum = 100.0
)
Same as Vincia:enhanceFacBottom
but for charm quarks.
Note: this factor is applied on top of Vincia:biasAll
.
parm
Vincia:enhanceCutoff
(default = 10.0
; minimum = 0.0
; maximum = 1000.0
)
Do not apply enhancement factors to branchings below this
scale. Intended to be used to focus on enhancements of hard branchings
only.
VINCIA Tunes
VINCIA has its own set of dedicated tune presets, which can be
specified by the user.
mode
Vincia:Tune
(default = 0
; minimum = -1
; maximum = 0
)
option
-1 : None. No VINCIA-specific tune parameter settings
will be used during initialisation.
option
0 : Hadronisation and MPI parameters optimised for use
with the VINCIA shower model, used as default VINCIA parameters since
PYTHIA 8.302.
Note: the requested tune parameters will only be activated when
VINCIA is switched on, in order not to interfere with the PYTHIA
settings when VINCIA is switched off.
Note 2: 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.
Advice on Tuning
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.
As a guideline, the main parameters that need to be properly
tuned are the non-perturbative hadronisation 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 hadronisation
model will not re-absorb automatically and which therefore only a
retuning can address.
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.
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.
Expert Settings
Importante Note: the parameters in this section are intended
for use only by authors or expert users.
Octet Partitioning
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
Vincia:octetPartitioning
(default = 0.0
; minimum = 0.0
; maximum = 1.0
)
Gluon-collinear α 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: α=0, which corresponds to the
Gehrmann-Gehrmann-de Ridder-Glover (GGG) partitioning, and α=1,
which corresponds to the Gustafson (ARIADNE) partitioning. For the
sector shower, the octetPartitioning
is forced to
α=1, to ensure positivity of the antenna function and as there
is no sensible meaning of the parameter anymore.
Verbose Level
mode
Vincia:verbose
(default = 1
; minimum = -1
; maximum = 3
)
Level of detail of information written to standard output on what goes
on inside VINCIA
.
option
-1 : No runtime output.
option
0 : Quiet. Errors and (important) warnings are
printed.
option
1 : Normal. All warnings and error messages are
printed, but no additional diagnostic output is given. Standard
initialisation and summary information is also printed.
option
2 : Report. Enhanced runtime output. As for =1, but
additional diagnostic information is printed, especially
when/where errors occur, and additional self-consistency checks may
be performed. Full initialisation and summary information is
printed.
option
3 : Debug. This level is solely intended to assist
authors for debugging purposes.
Numerical Checks
flag
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.
mode
Vincia:nPointsCheck
(default = 1000
; minimum = 0
; maximum = 1e6
)
Number of random points to check each antenna functions for
positivity.
parm
Vincia:deadZoneAvoidance
(default = 0.0001
; minimum = 0.0
; maximum = 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.
Shower Uncertainty Bands
Automated uncertainty bands are not available in this version of
the VINCIA code.
References
The main references for the current version of VINCIA are:
Authors and Contributions
The authors of the current version, taking active responsibility for
code development and day-to-day maintenance, are
- P. Skands: Main author
- C. T. Preuss: Sector Showers, Sector Merging, MG5 Interface.
- R. Verheyen: QED showers and EW showers.
In addition, the following people have made significant individual
contributions to the underlying formalism and code development of
VINCIA:
- H. Brooks: Resonance-decay showers and Sector Merging.
- W. T. Giele and D. A. Kosower: Basics of the antenna shower
formalism.
- L. Hartgring and E. Laenen: on NLO corrections to Z + 3 jets.
- H.-T. Li: on 2→4 branchings and one-loop corrections to FF
shower kernels.
- A. Larkoski: on helicity-based final-state showers and matrix
elements.
- A. Lifson: Analytical MHV amplitudes.
- N. Fischer: ISR and MECs for hadron collisions.
- J. J. Lopez-Villarejo: Sector showers.
- S. Prestel: Merging and strongly-ordered MECs.
- M. Ritzmann: Mass corrections and initial-state antenna showers.
- T. Sjöstrand: Integration with PYTHIA 8.
About VINCIA
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.